October 2021 - mpi-comments - mailman3.cels.anl.gov

Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL)
by Rolf Rabenseifner 22 Oct '21

22 Oct '21

Dan and Nils, I agree to you both - therefore no objection from my side. ----- Original Message ----- > From: "Nils Smeds" <nils.smeds(a)gmail.com> > To: "Comments on the MPI Standard" <mpi-comments(a)lists.mpi-forum.org> > Cc: "Rolf Rabenseifner" <rabenseifner(a)hlrs.de> > Sent: Friday, October 22, 2021 3:25:52 PM > Subject: Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL) > No I certainly do not hope so, > > I would suggest that only an MPI implementation with a "pedantic" mode > would check for this. For any production level implementation the check for > MPI_PROC_NULL (if present in the implementation) will be sufficient. If > MPI_TAG_NULL is provided with a valid processor argument (against the > suggested standard wording) the probe code will run through all pending > messages and not find any matching tag since MPI_TAG_NULL is not inside the > 0..MPI_TAG_UB range. Just as a probe for MPI_PROC_NULL today could > (should?) be scanning all messages. > > Probes with either MPI_PROC_NULL or MPI_TAG_NONE should not need fast > lanes. Let them suffer by checking every message and find that there is > nothing for them. Then, if you are nice, you can raise an error before > returning. > > If tags are implemented as signed integers MPI_TAG_NULL would most likely > be implemented as a negative value (which by the standard is not in the > valid tag range). If tags are implemented using unsigned int values, the > implementation could set MPI_TAG_NULL to "huge(unsigned 0)" and MPI_TAB_UB > to MPI_TAG_NULL-1. Either way it should not break existing implementations > (famous last words, I know...). > > > > On Fri, Oct 22, 2021 at 3:02 PM Rolf Rabenseifner <rabenseifner(a)hlrs.de> > wrote: > >> Do I understand correctly, that this proposal must add an additional >> lateny (internal if statement) to MPI_PROBE for *all* messages >> and also in most implementations (that do not want to ignore the >> error-checking >> when MPI_TAG_NULL is passed) for *all* point-to-point messages, >> but >> it this new MPI_TAG_NULL feature may be rearely used and extremly rarely >> used >> in MPI_PROBE? >> >> ----- Original Message ----- >> > From: "Comments on the MPI Standard" <mpi-comments(a)lists.mpi-forum.org> >> > To: "Comments on the MPI Standard" <mpi-comments(a)lists.mpi-forum.org> >> > Cc: "Nils Smeds" <nils.smeds(a)gmail.com> >> > Sent: Friday, October 22, 2021 2:33:36 PM >> > Subject: Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and >> MPI_TAG_NULL, MPI_ADDRESS_NULL) >> >> > Dan, >> > >> > Thank you for your encouraging response. >> > >> > Firstly, let me state that I have no strong feelings for either >> MPI_ADDRESS_NULL >> > nor MPI_COMM_NULL other than they would satisfy my sense of "symmetry" or >> > "closure". In my own programming I rarely find myself unclear what buffer >> > address or communicator a specific code section is targeting. Most often >> it is >> > the pt2pt matching triad (proc,tag,comm) that needs to be tailored to >> express >> > the "intent" of the code so that an unimplemented fringe case does not >> pass by >> > unnoticed. (Merely causes a dead-lock - as if that is any help - I >> know...) >> > >> > My view of the MPI_*_NULL entities in the standard is that they are >> comparable >> > to NaN in floating point. MPI_TAG_NULL is not a member of the set of >> valid >> > tags, and can not be matched in any tag comparison operation, not even >> itself >> > or as "not a member of". MPI_TAG_ANY should not match a message with >> > MPI_TAG_NULL, and there should indeed be no way to create a "real" >> message with >> > MPI_TAG_NULL. So a statement as you suggested >> > >> > >> > >> > " W hen the tag argument is MPI_TAG_NULL the only allowed processor >> argument is >> > MPI_PROC_NULL" >> > >> > preferably in section 3.2.4 just before the last sentence describing the >> use of >> > dest=MPI_PROC_NULL would be all that is needed. A probe for such a >> message >> > (that can never be created as a "real" message) will thus return a status >> > object with a count of 0 and a tag of MPI_TAG_ANY and no change in >> behaviour to >> > existing implementations. >> > >> > So the updated suggested changes to the standards are: >> > >> > 1. >> > Insert into appendix "A1 Assorted constants": MPI_TAG_NULL >> > >> > 2. >> > Insert into section "9.1.2 Environmental Inquiries" >> > >> > >> > >> > (Line 34, end of first paragraph) [... is also a valid value for >> MPITAG_UB]. The >> > constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB >> range. >> > >> > 3. >> > Insert into section 3.2.4 before the last sentence (line 42.5): >> > >> > " W hen the tag argument is MPI_TAG_NULL the only allowed processor >> argument is >> > MPI_PROC_NULL" >> > >> > 4. >> > Add to section 3.10 "Null processes", last in the section: >> > >> > " Advice to users: Note that MPI_TAG_NULL is not a valid tag in the >> > 0..MPI_TAG_UB range. Thus MPI_TAG_NULL is only allowed when the >> associated >> > processor argument is MPI_PROC_NULL" >> > >> > To avoid a lengthy discussion into what probes of non-existing messages >> should >> > return as a result, I suggest that the committee sees any triad >> (proc,tag,comm) >> > where either component has a "NULL" value as defined in the standard >> (currently >> > only MPI_PROC_NULL exists) is in essence a "NaN"-triad, and returns the >> already >> > defined result of 0 elements of (MPI_PROC_NULL,MPI_ANY_TAG) as of >> section 3.10. >> > Also, just by chance, this would not conflict with an introduction of an >> > MPI_COMM_NULL item in the future. >> > >> > >> > >> > On Thu, Oct 21, 2021 at 7:14 PM Dan Holmes <danholmes(a)chi.scot> wrote: >> > >> > >> > >> > Hi Nils, >> > >> > In general, I like the idea of having default/invalid values for these >> > MPI-related user variables. >> > >> > Would MPI_TAG_NULL be valid for a point-to-point send operation that >> does NOT >> > use MPI_PROC_NULL but is matched with receive operation at the >> destination MPI >> > process that uses MPI_ANY_TAG? Or should this be an >> MPI_ERR_ARG/MPI_ERR_TAG >> > error condition? Commonly, codes that use MPI_ANY_TAG in receive >> operations >> > must choose an arbitrary valid tag value to use in the send operations >> at the >> > source MPI process(es). Permitting the use of MPI_TAG_NULL in >> combination with >> > MPI_ANY_TAG might aid debugging when the matching does not go the way >> the user >> > expected. However, it would add complexity to the matching >> > rules/implementation. There is an interaction here with INFO key >> assertions >> > such as `mpi_assert_no_any_tag` — which means the error could be >> discovered at >> > the sender process -- and `mpi_assert_only_any_tag` (if that one ever >> gets >> > accepted into MPI). >> > >> > There is already an MPI constant that is commonly set to `(void*)0` (in >> C), >> > which is MPI_BOTTOM. Defining MPI_ADDRESS_NULL to be the same value is >> not a >> > good idea because MPI_BOTTOM is a valid address; although attempting to >> > read/write at that address is likely to cause trouble, using it as a >> base for a >> > user-defined datatype or a window offset is common/accepted practice. >> Would it >> > be sufficient for your purposes to use MPI_BOTTOM as your “not valid” >> value? >> > >> > IMHO, there would need to be a statement that, “when the comm argument is >> > MPI_COMM_NULL, the only acceptable value for the source/dest argument is >> > MPI_PROC_NULL” because asking for the 3rd item from an empty set (for >> example) >> > is clearly an error. Your proposal tries to treat MPI_COMM_NULL as if it >> is a >> > valid MPI intra-communicator that has size zero, or a valid >> inter-communicator >> > that has a remote group of size zero. I think I would prefer to create >> > MPI_COMM_EMPTY for that purpose, and continue to use MPI_COMM_NULL for >> > situations that call for an invalid communicator, i.e. something that >> can never >> > be used as a communicator without causing an error condition. >> > >> > The phrase used elsewhere in the MPI Standard is “XYZ parameter is >> significant >> > only <when condition>” >> > >> > Cheers, >> > Dan. >> > — >> > Dr Daniel Holmes PhD >> > Executive Director >> > Chief Technology Officer >> > CHI Ltd >> > [ mailto:[email protected] | danholmes(a)chi.scot ] >> > >> > >> > >> > >> > >> > >> > On 21 Oct 2021, at 11:35, Nils Smeds via mpi-comments < [ >> > mailto:[email protected] | >> mpi-comments(a)lists.mpi-forum.org ] > >> > wrote: >> > >> > Summary: >> > - Suggestion to introduce MPI_TAG_NULL in the standard with an obviously >> illegal >> > value (i.e. <0) >> > - Explicitly allow Point-to-Point messages to have one of more of: tag >> > MPI_TAG_NULL, datatype MPI_DATATYPE_NULL, invalid buffer addresses >> (including >> > MPI_ADDRESS_NULL) and communicator MPI_COMM_NULL, if (and only if) the >> > corresponding send or receive argument is MPI_PROC_NULL. >> > >> > Argument: >> > When writing code it is useful if it goes off with a bang when you do >> something >> > wrong. To catch matching errors in sends and receives I looked for a >> > MPI_TAG_NULL to use as a default value to catch cases where the tag was >> not set >> > correctly. I found that there was no such item. Referring to the >> standard I >> > found that tag values are restricted to the values 0...MPI_TAG_UB, where >> > MPI_TAG_UB (>=32767) can be found using the MPI_Comm_get_attr() function. >> > >> > So I used my own value for MPI_TAG_NULL as -1 and coded away. However, >> the MPI >> > implementation I was using blew up when I used it in a MPI_Sendrecv with >> the >> > remote task being MPI_PROC_NULL throwing an error message "illegal tag >> value", >> > which was no surprise to me since I had set it to this value explicitly. >> > >> > The workaround is of course to put a if( PartnerTask != MPI_PROC_NULL ) >> { ... } >> > around every point-to-point call, but I find it unnecessary and makes >> the code >> > look more complex. I think that MPI_PROC_NULL was invented for exactly >> this >> > purpose, to be a transparent if(...) clause for the operation. Is there >> really >> > a need to be restrictive on datat ype, tag or any other argument when >> the P2P >> > operation is with MPI_PROC_NULL? >> > >> > I would expect implementations to choose MPI_ADDRESS_NULL be equal to >> (*) 0 in C >> > and 0_MPI_ADDRESS_KIND in Fortran and MPI_TAG_NULL to be default integer >> kind >> > -1. >> > >> > Implementation: >> > Append to section "3.10 Null processes" text along the lines of: >> > >> > >> > >> > A call to any point-to-point messaging function with a process argument >> of >> > MPI_PROC_NULL is allowed to have any value for buffer address, data >> type, tag >> > and communicator including MPI_ADDRESS_NULL, MPI_DATATYPE_NULL, >> MPI_TAG_NULL >> > and MPI_COMM_NULL without causing an error condition in the execution. >> > >> > Insert into appendix "A1 Assorted constants" >> > >> > >> > >> > MPI_TAG_NULL and MPI_ADDRESS_NULL >> > >> > Insert into section "9.1.2 Environmental Inquiries" (which is an odd >> place where >> > Tag values are specified in the standard): >> > >> > >> > >> > (Line 34, end of first paragraph) [... is also a valid value for >> MPITAG_UB]. The >> > constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB >> range. >> > The value MPI_TAG_NULL is erroneous to use in any communication unless >> the >> > associated process argument is MPI_PROC_NULL. >> > >> > Add to section "15.3.3 Convention for Returning Strings" >> > >> > >> > >> > The constant MPI_ADDRESS_NULL can be used as an equivalent to the null >> pointer. >> > >> > >> > >> > /Nils Smeds >> > >> > _______________________________________________ >> > mpi-comments mailing list >> > [ mailto:[email protected] | >> mpi-comments(a)lists.mpi-forum.org ] >> > [ https://lists.mpi-forum.org/mailman/listinfo/mpi-comments | >> > https://lists.mpi-forum.org/mailman/listinfo/mpi-comments ] >> > >> > >> > _______________________________________________ >> > mpi-comments mailing list >> > mpi-comments(a)lists.mpi-forum.org >> > https://lists.mpi-forum.org/mailman/listinfo/mpi-comments >> >> -- >> Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner(a)hlrs.de . >> High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 . >> University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 . >> Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner . >> Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) . -- Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner(a)hlrs.de . High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 . University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 . Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner . Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) .

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Suggestion for MPI_COMM_NULL
by Nils Smeds 22 Oct '21

22 Oct '21

[This is a branch-off from thread "Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL,MPI_ADDRESS_NULL)" ] It turns out that MPI_COMM_NULL opens some questions that I am sure can be resolved by people on the list. It is enlightening that you see my suggested MPI_COMM_NULL as a set and not an item. I did not think about that, but it is of course the more natural approach and MPI_COMM_NULL can then be seen as problematic. I do indeed have the view that the MPI_xxxx_NULL items are to be compared to NaN in that they are items in themselves. In particular they are items that are not members of any set of similar items. Thus, MPI_COMM_NULL is a process communicator that is not a member of any set of (valid) process communicators and it is not MPI_COMM_EMPTY. I am not sure that there is a need for an explicit MPI_COMM_EMPTY, but if there was it is allowed and can be created as a sub-communicator to any communicator by MPI_Comm_create, eg for MPI_COMM_WORLD: MPI_Comm_create(MPI_COMM_WORLD,MPI_GROUP_EMPTY,&mpi_comm_empty) but its usefulness I find debatable. The set operations in MPI are, I believe, restricted to the MPI_GROUP_xxx set of items and do not directly involve communicators, but I am on thin ice here. This is for the intra-communicator people, MPMD and multi-physics scientists to have opinions on. In my view, the group members of MPI_COMM_NULL should be MPI_GROUP_NONE (not MPI_GROUP_EMPTY) if it should return any members at all. My suggestion would rather be that MPI_COMM_NULL is an illegal communicator argument in any call to MPI_Group_*() or MPI_Comm_*() that takes a communicator as an argument, eg MPI_Comm_dup(), MPI_Comm_free(), MPI_Comm_group(). So, I think that you and I are arguing for the same definition of MPI_COMM_NULL . /Nils On Thu, Oct 21, 2021 at 7:14 PM Dan Holmes <danholmes(a)chi.scot> wrote: > Hi Nils, > > In general, I like the idea of having default/invalid values for these > MPI-related user variables. > [...] > IMHO, there would need to be a statement that, “when the comm argument is > MPI_COMM_NULL, the only acceptable value for the source/dest argument is > MPI_PROC_NULL” because asking for the 3rd item from an empty set (for > example) is clearly an error. Your proposal tries to treat MPI_COMM_NULL as > if it is a valid MPI intra-communicator that has size zero, or a valid > inter-communicator that has a remote group of size zero. I think I would > prefer to create MPI_COMM_EMPTY for that purpose, and continue to use > MPI_COMM_NULL for situations that call for an invalid communicator, i.e. > something that can never be used as a communicator without causing an error > condition. > > The phrase used elsewhere in the MPI Standard is “XYZ parameter is > significant only <when condition>” > >

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Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL)
by Nils Smeds 22 Oct '21

22 Oct '21

No I certainly do not hope so, I would suggest that only an MPI implementation with a "pedantic" mode would check for this. For any production level implementation the check for MPI_PROC_NULL (if present in the implementation) will be sufficient. If MPI_TAG_NULL is provided with a valid processor argument (against the suggested standard wording) the probe code will run through all pending messages and not find any matching tag since MPI_TAG_NULL is not inside the 0..MPI_TAG_UB range. Just as a probe for MPI_PROC_NULL today could (should?) be scanning all messages. Probes with either MPI_PROC_NULL or MPI_TAG_NONE should not need fast lanes. Let them suffer by checking every message and find that there is nothing for them. Then, if you are nice, you can raise an error before returning. If tags are implemented as signed integers MPI_TAG_NULL would most likely be implemented as a negative value (which by the standard is not in the valid tag range). If tags are implemented using unsigned int values, the implementation could set MPI_TAG_NULL to "huge(unsigned 0)" and MPI_TAB_UB to MPI_TAG_NULL-1. Either way it should not break existing implementations (famous last words, I know...). On Fri, Oct 22, 2021 at 3:02 PM Rolf Rabenseifner <rabenseifner(a)hlrs.de> wrote: > Do I understand correctly, that this proposal must add an additional > lateny (internal if statement) to MPI_PROBE for *all* messages > and also in most implementations (that do not want to ignore the > error-checking > when MPI_TAG_NULL is passed) for *all* point-to-point messages, > but > it this new MPI_TAG_NULL feature may be rearely used and extremly rarely > used > in MPI_PROBE? > > ----- Original Message ----- > > From: "Comments on the MPI Standard" <mpi-comments(a)lists.mpi-forum.org> > > To: "Comments on the MPI Standard" <mpi-comments(a)lists.mpi-forum.org> > > Cc: "Nils Smeds" <nils.smeds(a)gmail.com> > > Sent: Friday, October 22, 2021 2:33:36 PM > > Subject: Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and > MPI_TAG_NULL, MPI_ADDRESS_NULL) > > > Dan, > > > > Thank you for your encouraging response. > > > > Firstly, let me state that I have no strong feelings for either > MPI_ADDRESS_NULL > > nor MPI_COMM_NULL other than they would satisfy my sense of "symmetry" or > > "closure". In my own programming I rarely find myself unclear what buffer > > address or communicator a specific code section is targeting. Most often > it is > > the pt2pt matching triad (proc,tag,comm) that needs to be tailored to > express > > the "intent" of the code so that an unimplemented fringe case does not > pass by > > unnoticed. (Merely causes a dead-lock - as if that is any help - I > know...) > > > > My view of the MPI_*_NULL entities in the standard is that they are > comparable > > to NaN in floating point. MPI_TAG_NULL is not a member of the set of > valid > > tags, and can not be matched in any tag comparison operation, not even > itself > > or as "not a member of". MPI_TAG_ANY should not match a message with > > MPI_TAG_NULL, and there should indeed be no way to create a "real" > message with > > MPI_TAG_NULL. So a statement as you suggested > > > > > > > > " W hen the tag argument is MPI_TAG_NULL the only allowed processor > argument is > > MPI_PROC_NULL" > > > > preferably in section 3.2.4 just before the last sentence describing the > use of > > dest=MPI_PROC_NULL would be all that is needed. A probe for such a > message > > (that can never be created as a "real" message) will thus return a status > > object with a count of 0 and a tag of MPI_TAG_ANY and no change in > behaviour to > > existing implementations. > > > > So the updated suggested changes to the standards are: > > > > 1. > > Insert into appendix "A1 Assorted constants": MPI_TAG_NULL > > > > 2. > > Insert into section "9.1.2 Environmental Inquiries" > > > > > > > > (Line 34, end of first paragraph) [... is also a valid value for > MPITAG_UB]. The > > constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB > range. > > > > 3. > > Insert into section 3.2.4 before the last sentence (line 42.5): > > > > " W hen the tag argument is MPI_TAG_NULL the only allowed processor > argument is > > MPI_PROC_NULL" > > > > 4. > > Add to section 3.10 "Null processes", last in the section: > > > > " Advice to users: Note that MPI_TAG_NULL is not a valid tag in the > > 0..MPI_TAG_UB range. Thus MPI_TAG_NULL is only allowed when the > associated > > processor argument is MPI_PROC_NULL" > > > > To avoid a lengthy discussion into what probes of non-existing messages > should > > return as a result, I suggest that the committee sees any triad > (proc,tag,comm) > > where either component has a "NULL" value as defined in the standard > (currently > > only MPI_PROC_NULL exists) is in essence a "NaN"-triad, and returns the > already > > defined result of 0 elements of (MPI_PROC_NULL,MPI_ANY_TAG) as of > section 3.10. > > Also, just by chance, this would not conflict with an introduction of an > > MPI_COMM_NULL item in the future. > > > > > > > > On Thu, Oct 21, 2021 at 7:14 PM Dan Holmes <danholmes(a)chi.scot> wrote: > > > > > > > > Hi Nils, > > > > In general, I like the idea of having default/invalid values for these > > MPI-related user variables. > > > > Would MPI_TAG_NULL be valid for a point-to-point send operation that > does NOT > > use MPI_PROC_NULL but is matched with receive operation at the > destination MPI > > process that uses MPI_ANY_TAG? Or should this be an > MPI_ERR_ARG/MPI_ERR_TAG > > error condition? Commonly, codes that use MPI_ANY_TAG in receive > operations > > must choose an arbitrary valid tag value to use in the send operations > at the > > source MPI process(es). Permitting the use of MPI_TAG_NULL in > combination with > > MPI_ANY_TAG might aid debugging when the matching does not go the way > the user > > expected. However, it would add complexity to the matching > > rules/implementation. There is an interaction here with INFO key > assertions > > such as `mpi_assert_no_any_tag` — which means the error could be > discovered at > > the sender process -- and `mpi_assert_only_any_tag` (if that one ever > gets > > accepted into MPI). > > > > There is already an MPI constant that is commonly set to `(void*)0` (in > C), > > which is MPI_BOTTOM. Defining MPI_ADDRESS_NULL to be the same value is > not a > > good idea because MPI_BOTTOM is a valid address; although attempting to > > read/write at that address is likely to cause trouble, using it as a > base for a > > user-defined datatype or a window offset is common/accepted practice. > Would it > > be sufficient for your purposes to use MPI_BOTTOM as your “not valid” > value? > > > > IMHO, there would need to be a statement that, “when the comm argument is > > MPI_COMM_NULL, the only acceptable value for the source/dest argument is > > MPI_PROC_NULL” because asking for the 3rd item from an empty set (for > example) > > is clearly an error. Your proposal tries to treat MPI_COMM_NULL as if it > is a > > valid MPI intra-communicator that has size zero, or a valid > inter-communicator > > that has a remote group of size zero. I think I would prefer to create > > MPI_COMM_EMPTY for that purpose, and continue to use MPI_COMM_NULL for > > situations that call for an invalid communicator, i.e. something that > can never > > be used as a communicator without causing an error condition. > > > > The phrase used elsewhere in the MPI Standard is “XYZ parameter is > significant > > only <when condition>” > > > > Cheers, > > Dan. > > — > > Dr Daniel Holmes PhD > > Executive Director > > Chief Technology Officer > > CHI Ltd > > [ mailto:[email protected] | danholmes(a)chi.scot ] > > > > > > > > > > > > > > On 21 Oct 2021, at 11:35, Nils Smeds via mpi-comments < [ > > mailto:[email protected] | > mpi-comments(a)lists.mpi-forum.org ] > > > wrote: > > > > Summary: > > - Suggestion to introduce MPI_TAG_NULL in the standard with an obviously > illegal > > value (i.e. <0) > > - Explicitly allow Point-to-Point messages to have one of more of: tag > > MPI_TAG_NULL, datatype MPI_DATATYPE_NULL, invalid buffer addresses > (including > > MPI_ADDRESS_NULL) and communicator MPI_COMM_NULL, if (and only if) the > > corresponding send or receive argument is MPI_PROC_NULL. > > > > Argument: > > When writing code it is useful if it goes off with a bang when you do > something > > wrong. To catch matching errors in sends and receives I looked for a > > MPI_TAG_NULL to use as a default value to catch cases where the tag was > not set > > correctly. I found that there was no such item. Referring to the > standard I > > found that tag values are restricted to the values 0...MPI_TAG_UB, where > > MPI_TAG_UB (>=32767) can be found using the MPI_Comm_get_attr() function. > > > > So I used my own value for MPI_TAG_NULL as -1 and coded away. However, > the MPI > > implementation I was using blew up when I used it in a MPI_Sendrecv with > the > > remote task being MPI_PROC_NULL throwing an error message "illegal tag > value", > > which was no surprise to me since I had set it to this value explicitly. > > > > The workaround is of course to put a if( PartnerTask != MPI_PROC_NULL ) > { ... } > > around every point-to-point call, but I find it unnecessary and makes > the code > > look more complex. I think that MPI_PROC_NULL was invented for exactly > this > > purpose, to be a transparent if(...) clause for the operation. Is there > really > > a need to be restrictive on datat ype, tag or any other argument when > the P2P > > operation is with MPI_PROC_NULL? > > > > I would expect implementations to choose MPI_ADDRESS_NULL be equal to > (*) 0 in C > > and 0_MPI_ADDRESS_KIND in Fortran and MPI_TAG_NULL to be default integer > kind > > -1. > > > > Implementation: > > Append to section "3.10 Null processes" text along the lines of: > > > > > > > > A call to any point-to-point messaging function with a process argument > of > > MPI_PROC_NULL is allowed to have any value for buffer address, data > type, tag > > and communicator including MPI_ADDRESS_NULL, MPI_DATATYPE_NULL, > MPI_TAG_NULL > > and MPI_COMM_NULL without causing an error condition in the execution. > > > > Insert into appendix "A1 Assorted constants" > > > > > > > > MPI_TAG_NULL and MPI_ADDRESS_NULL > > > > Insert into section "9.1.2 Environmental Inquiries" (which is an odd > place where > > Tag values are specified in the standard): > > > > > > > > (Line 34, end of first paragraph) [... is also a valid value for > MPITAG_UB]. The > > constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB > range. > > The value MPI_TAG_NULL is erroneous to use in any communication unless > the > > associated process argument is MPI_PROC_NULL. > > > > Add to section "15.3.3 Convention for Returning Strings" > > > > > > > > The constant MPI_ADDRESS_NULL can be used as an equivalent to the null > pointer. > > > > > > > > /Nils Smeds > > > > _______________________________________________ > > mpi-comments mailing list > > [ mailto:[email protected] | > mpi-comments(a)lists.mpi-forum.org ] > > [ https://lists.mpi-forum.org/mailman/listinfo/mpi-comments | > > https://lists.mpi-forum.org/mailman/listinfo/mpi-comments ] > > > > > > _______________________________________________ > > mpi-comments mailing list > > mpi-comments(a)lists.mpi-forum.org > > https://lists.mpi-forum.org/mailman/listinfo/mpi-comments > > -- > Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner(a)hlrs.de . > High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 . > University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 . > Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner . > Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) . >

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Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL)
by Dan Holmes 22 Oct '21

22 Oct '21

Hi Rolf, I don’t think checking for MPI_TAG_NULL during MPI_PROBE (or during MPI_RECV, etc) is required to add latency that is not already present. — If, with today’s MPI libraries, I write an erroneous program that uses a tag value that is not in the permitted range (because, for example, I coded an integer literal of -101 in the argument list of an MPI_SEND procedure call), the MPI process that executes that send initialization procedure call can discover the error immediately/directly and call an appropriate error handler (fatal, or whatever) instead of transmitting the erroneous value to the destination process. MPI_PROBE will never see the erroneous value and is not required to check for, or handle, its presence. — If, with today’s MPI libraries, I write an erroneous program that transmits a tag value that is not in the permitted range (because, for example, I coded an integer literal of -101 in the argument list of an MPI_SEND procedure call and MPI chooses to transmit that bad value to the destination process), then the destination MPI process for that message envelope will discover the error (probably during a receive or probe procedure call) and call an appropriate error handler (fatal, or whatever). — If I replace the integer literal with a named constant set to the same value, the compiler will replace the named constant with the integer literal and no runtime code (at the source or the destination) will see any difference. If the MPI Standard codifies the named constant and the definition of it is added to the MPI header file(s) & Fortran module(s), again no difference will be seen at runtime. Existing MPI libraries must already be able to handle this problem. — A debug build of the MPI library might do more error checking of input arguments and data packets received from the network than a production build (which might assume that no such errors remain in the application code and consequently do little or no checking of this kind). Cheers, Dan. — Dr Daniel Holmes PhD Executive Director Chief Technology Officer CHI Ltd danholmes(a)chi.scot > On 22 Oct 2021, at 14:02, Rolf Rabenseifner via mpi-comments <mpi-comments(a)lists.mpi-forum.org> wrote: > > Do I understand correctly, that this proposal must add an additional > lateny (internal if statement) to MPI_PROBE for *all* messages > and also in most implementations (that do not want to ignore the error-checking > when MPI_TAG_NULL is passed) for *all* point-to-point messages, > but > it this new MPI_TAG_NULL feature may be rearely used and extremly rarely used > in MPI_PROBE? > > ----- Original Message ----- >> From: "Comments on the MPI Standard" <mpi-comments(a)lists.mpi-forum.org> >> To: "Comments on the MPI Standard" <mpi-comments(a)lists.mpi-forum.org> >> Cc: "Nils Smeds" <nils.smeds(a)gmail.com> >> Sent: Friday, October 22, 2021 2:33:36 PM >> Subject: Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL) > >> Dan, >> >> Thank you for your encouraging response. >> >> Firstly, let me state that I have no strong feelings for either MPI_ADDRESS_NULL >> nor MPI_COMM_NULL other than they would satisfy my sense of "symmetry" or >> "closure". In my own programming I rarely find myself unclear what buffer >> address or communicator a specific code section is targeting. Most often it is >> the pt2pt matching triad (proc,tag,comm) that needs to be tailored to express >> the "intent" of the code so that an unimplemented fringe case does not pass by >> unnoticed. (Merely causes a dead-lock - as if that is any help - I know...) >> >> My view of the MPI_*_NULL entities in the standard is that they are comparable >> to NaN in floating point. MPI_TAG_NULL is not a member of the set of valid >> tags, and can not be matched in any tag comparison operation, not even itself >> or as "not a member of". MPI_TAG_ANY should not match a message with >> MPI_TAG_NULL, and there should indeed be no way to create a "real" message with >> MPI_TAG_NULL. So a statement as you suggested >> >> >> >> " W hen the tag argument is MPI_TAG_NULL the only allowed processor argument is >> MPI_PROC_NULL" >> >> preferably in section 3.2.4 just before the last sentence describing the use of >> dest=MPI_PROC_NULL would be all that is needed. A probe for such a message >> (that can never be created as a "real" message) will thus return a status >> object with a count of 0 and a tag of MPI_TAG_ANY and no change in behaviour to >> existing implementations. >> >> So the updated suggested changes to the standards are: >> >> 1. >> Insert into appendix "A1 Assorted constants": MPI_TAG_NULL >> >> 2. >> Insert into section "9.1.2 Environmental Inquiries" >> >> >> >> (Line 34, end of first paragraph) [... is also a valid value for MPITAG_UB]. The >> constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB range. >> >> 3. >> Insert into section 3.2.4 before the last sentence (line 42.5): >> >> " W hen the tag argument is MPI_TAG_NULL the only allowed processor argument is >> MPI_PROC_NULL" >> >> 4. >> Add to section 3.10 "Null processes", last in the section: >> >> " Advice to users: Note that MPI_TAG_NULL is not a valid tag in the >> 0..MPI_TAG_UB range. Thus MPI_TAG_NULL is only allowed when the associated >> processor argument is MPI_PROC_NULL" >> >> To avoid a lengthy discussion into what probes of non-existing messages should >> return as a result, I suggest that the committee sees any triad (proc,tag,comm) >> where either component has a "NULL" value as defined in the standard (currently >> only MPI_PROC_NULL exists) is in essence a "NaN"-triad, and returns the already >> defined result of 0 elements of (MPI_PROC_NULL,MPI_ANY_TAG) as of section 3.10. >> Also, just by chance, this would not conflict with an introduction of an >> MPI_COMM_NULL item in the future. >> >> >> >> On Thu, Oct 21, 2021 at 7:14 PM Dan Holmes <danholmes(a)chi.scot <mailto:[email protected]>> wrote: >> >> >> >> Hi Nils, >> >> In general, I like the idea of having default/invalid values for these >> MPI-related user variables. >> >> Would MPI_TAG_NULL be valid for a point-to-point send operation that does NOT >> use MPI_PROC_NULL but is matched with receive operation at the destination MPI >> process that uses MPI_ANY_TAG? Or should this be an MPI_ERR_ARG/MPI_ERR_TAG >> error condition? Commonly, codes that use MPI_ANY_TAG in receive operations >> must choose an arbitrary valid tag value to use in the send operations at the >> source MPI process(es). Permitting the use of MPI_TAG_NULL in combination with >> MPI_ANY_TAG might aid debugging when the matching does not go the way the user >> expected. However, it would add complexity to the matching >> rules/implementation. There is an interaction here with INFO key assertions >> such as `mpi_assert_no_any_tag` — which means the error could be discovered at >> the sender process -- and `mpi_assert_only_any_tag` (if that one ever gets >> accepted into MPI). >> >> There is already an MPI constant that is commonly set to `(void*)0` (in C), >> which is MPI_BOTTOM. Defining MPI_ADDRESS_NULL to be the same value is not a >> good idea because MPI_BOTTOM is a valid address; although attempting to >> read/write at that address is likely to cause trouble, using it as a base for a >> user-defined datatype or a window offset is common/accepted practice. Would it >> be sufficient for your purposes to use MPI_BOTTOM as your “not valid” value? >> >> IMHO, there would need to be a statement that, “when the comm argument is >> MPI_COMM_NULL, the only acceptable value for the source/dest argument is >> MPI_PROC_NULL” because asking for the 3rd item from an empty set (for example) >> is clearly an error. Your proposal tries to treat MPI_COMM_NULL as if it is a >> valid MPI intra-communicator that has size zero, or a valid inter-communicator >> that has a remote group of size zero. I think I would prefer to create >> MPI_COMM_EMPTY for that purpose, and continue to use MPI_COMM_NULL for >> situations that call for an invalid communicator, i.e. something that can never >> be used as a communicator without causing an error condition. >> >> The phrase used elsewhere in the MPI Standard is “XYZ parameter is significant >> only <when condition>” >> >> Cheers, >> Dan. >> — >> Dr Daniel Holmes PhD >> Executive Director >> Chief Technology Officer >> CHI Ltd >> [ mailto:[email protected] <mailto:[email protected]> | danholmes(a)chi.scot <mailto:[email protected]> ] >> >> >> >> >> >> >> On 21 Oct 2021, at 11:35, Nils Smeds via mpi-comments < [ >> mailto:[email protected] <mailto:[email protected]> | mpi-comments(a)lists.mpi-forum.org <mailto:[email protected]> ] > >> wrote: >> >> Summary: >> - Suggestion to introduce MPI_TAG_NULL in the standard with an obviously illegal >> value (i.e. <0) >> - Explicitly allow Point-to-Point messages to have one of more of: tag >> MPI_TAG_NULL, datatype MPI_DATATYPE_NULL, invalid buffer addresses (including >> MPI_ADDRESS_NULL) and communicator MPI_COMM_NULL, if (and only if) the >> corresponding send or receive argument is MPI_PROC_NULL. >> >> Argument: >> When writing code it is useful if it goes off with a bang when you do something >> wrong. To catch matching errors in sends and receives I looked for a >> MPI_TAG_NULL to use as a default value to catch cases where the tag was not set >> correctly. I found that there was no such item. Referring to the standard I >> found that tag values are restricted to the values 0...MPI_TAG_UB, where >> MPI_TAG_UB (>=32767) can be found using the MPI_Comm_get_attr() function. >> >> So I used my own value for MPI_TAG_NULL as -1 and coded away. However, the MPI >> implementation I was using blew up when I used it in a MPI_Sendrecv with the >> remote task being MPI_PROC_NULL throwing an error message "illegal tag value", >> which was no surprise to me since I had set it to this value explicitly. >> >> The workaround is of course to put a if( PartnerTask != MPI_PROC_NULL ) { ... } >> around every point-to-point call, but I find it unnecessary and makes the code >> look more complex. I think that MPI_PROC_NULL was invented for exactly this >> purpose, to be a transparent if(...) clause for the operation. Is there really >> a need to be restrictive on datat ype, tag or any other argument when the P2P >> operation is with MPI_PROC_NULL? >> >> I would expect implementations to choose MPI_ADDRESS_NULL be equal to (*) 0 in C >> and 0_MPI_ADDRESS_KIND in Fortran and MPI_TAG_NULL to be default integer kind >> -1. >> >> Implementation: >> Append to section "3.10 Null processes" text along the lines of: >> >> >> >> A call to any point-to-point messaging function with a process argument of >> MPI_PROC_NULL is allowed to have any value for buffer address, data type, tag >> and communicator including MPI_ADDRESS_NULL, MPI_DATATYPE_NULL, MPI_TAG_NULL >> and MPI_COMM_NULL without causing an error condition in the execution. >> >> Insert into appendix "A1 Assorted constants" >> >> >> >> MPI_TAG_NULL and MPI_ADDRESS_NULL >> >> Insert into section "9.1.2 Environmental Inquiries" (which is an odd place where >> Tag values are specified in the standard): >> >> >> >> (Line 34, end of first paragraph) [... is also a valid value for MPITAG_UB]. The >> constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB range. >> The value MPI_TAG_NULL is erroneous to use in any communication unless the >> associated process argument is MPI_PROC_NULL. >> >> Add to section "15.3.3 Convention for Returning Strings" >> >> >> >> The constant MPI_ADDRESS_NULL can be used as an equivalent to the null pointer. >> >> >> >> /Nils Smeds >> >> _______________________________________________ >> mpi-comments mailing list >> [ mailto:[email protected] <mailto:[email protected]> | mpi-comments(a)lists.mpi-forum.org <mailto:[email protected]> ] >> [ https://lists.mpi-forum.org/mailman/listinfo/mpi-comments <https://lists.mpi-forum.org/mailman/listinfo/mpi-comments> | >> https://lists.mpi-forum.org/mailman/listinfo/mpi-comments <https://lists.mpi-forum.org/mailman/listinfo/mpi-comments> ] >> >> >> _______________________________________________ >> mpi-comments mailing list >> mpi-comments(a)lists.mpi-forum.org <mailto:[email protected]> >> https://lists.mpi-forum.org/mailman/listinfo/mpi-comments <https://lists.mpi-forum.org/mailman/listinfo/mpi-comments> > > -- > Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner(a)hlrs.de <mailto:[email protected]> . > High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 . > University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 . > Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner <http://www.hlrs.de/people/rabenseifner> . > Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) . > _______________________________________________ > mpi-comments mailing list > mpi-comments(a)lists.mpi-forum.org <mailto:[email protected]> > https://lists.mpi-forum.org/mailman/listinfo/mpi-comments <https://lists.mpi-forum.org/mailman/listinfo/mpi-comments>

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Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL)
by Rolf Rabenseifner 22 Oct '21

22 Oct '21

Do I understand correctly, that this proposal must add an additional lateny (internal if statement) to MPI_PROBE for *all* messages and also in most implementations (that do not want to ignore the error-checking when MPI_TAG_NULL is passed) for *all* point-to-point messages, but it this new MPI_TAG_NULL feature may be rearely used and extremly rarely used in MPI_PROBE? ----- Original Message ----- > From: "Comments on the MPI Standard" <mpi-comments(a)lists.mpi-forum.org> > To: "Comments on the MPI Standard" <mpi-comments(a)lists.mpi-forum.org> > Cc: "Nils Smeds" <nils.smeds(a)gmail.com> > Sent: Friday, October 22, 2021 2:33:36 PM > Subject: Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL) > Dan, > > Thank you for your encouraging response. > > Firstly, let me state that I have no strong feelings for either MPI_ADDRESS_NULL > nor MPI_COMM_NULL other than they would satisfy my sense of "symmetry" or > "closure". In my own programming I rarely find myself unclear what buffer > address or communicator a specific code section is targeting. Most often it is > the pt2pt matching triad (proc,tag,comm) that needs to be tailored to express > the "intent" of the code so that an unimplemented fringe case does not pass by > unnoticed. (Merely causes a dead-lock - as if that is any help - I know...) > > My view of the MPI_*_NULL entities in the standard is that they are comparable > to NaN in floating point. MPI_TAG_NULL is not a member of the set of valid > tags, and can not be matched in any tag comparison operation, not even itself > or as "not a member of". MPI_TAG_ANY should not match a message with > MPI_TAG_NULL, and there should indeed be no way to create a "real" message with > MPI_TAG_NULL. So a statement as you suggested > > > > " W hen the tag argument is MPI_TAG_NULL the only allowed processor argument is > MPI_PROC_NULL" > > preferably in section 3.2.4 just before the last sentence describing the use of > dest=MPI_PROC_NULL would be all that is needed. A probe for such a message > (that can never be created as a "real" message) will thus return a status > object with a count of 0 and a tag of MPI_TAG_ANY and no change in behaviour to > existing implementations. > > So the updated suggested changes to the standards are: > > 1. > Insert into appendix "A1 Assorted constants": MPI_TAG_NULL > > 2. > Insert into section "9.1.2 Environmental Inquiries" > > > > (Line 34, end of first paragraph) [... is also a valid value for MPITAG_UB]. The > constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB range. > > 3. > Insert into section 3.2.4 before the last sentence (line 42.5): > > " W hen the tag argument is MPI_TAG_NULL the only allowed processor argument is > MPI_PROC_NULL" > > 4. > Add to section 3.10 "Null processes", last in the section: > > " Advice to users: Note that MPI_TAG_NULL is not a valid tag in the > 0..MPI_TAG_UB range. Thus MPI_TAG_NULL is only allowed when the associated > processor argument is MPI_PROC_NULL" > > To avoid a lengthy discussion into what probes of non-existing messages should > return as a result, I suggest that the committee sees any triad (proc,tag,comm) > where either component has a "NULL" value as defined in the standard (currently > only MPI_PROC_NULL exists) is in essence a "NaN"-triad, and returns the already > defined result of 0 elements of (MPI_PROC_NULL,MPI_ANY_TAG) as of section 3.10. > Also, just by chance, this would not conflict with an introduction of an > MPI_COMM_NULL item in the future. > > > > On Thu, Oct 21, 2021 at 7:14 PM Dan Holmes <danholmes(a)chi.scot> wrote: > > > > Hi Nils, > > In general, I like the idea of having default/invalid values for these > MPI-related user variables. > > Would MPI_TAG_NULL be valid for a point-to-point send operation that does NOT > use MPI_PROC_NULL but is matched with receive operation at the destination MPI > process that uses MPI_ANY_TAG? Or should this be an MPI_ERR_ARG/MPI_ERR_TAG > error condition? Commonly, codes that use MPI_ANY_TAG in receive operations > must choose an arbitrary valid tag value to use in the send operations at the > source MPI process(es). Permitting the use of MPI_TAG_NULL in combination with > MPI_ANY_TAG might aid debugging when the matching does not go the way the user > expected. However, it would add complexity to the matching > rules/implementation. There is an interaction here with INFO key assertions > such as `mpi_assert_no_any_tag` — which means the error could be discovered at > the sender process -- and `mpi_assert_only_any_tag` (if that one ever gets > accepted into MPI). > > There is already an MPI constant that is commonly set to `(void*)0` (in C), > which is MPI_BOTTOM. Defining MPI_ADDRESS_NULL to be the same value is not a > good idea because MPI_BOTTOM is a valid address; although attempting to > read/write at that address is likely to cause trouble, using it as a base for a > user-defined datatype or a window offset is common/accepted practice. Would it > be sufficient for your purposes to use MPI_BOTTOM as your “not valid” value? > > IMHO, there would need to be a statement that, “when the comm argument is > MPI_COMM_NULL, the only acceptable value for the source/dest argument is > MPI_PROC_NULL” because asking for the 3rd item from an empty set (for example) > is clearly an error. Your proposal tries to treat MPI_COMM_NULL as if it is a > valid MPI intra-communicator that has size zero, or a valid inter-communicator > that has a remote group of size zero. I think I would prefer to create > MPI_COMM_EMPTY for that purpose, and continue to use MPI_COMM_NULL for > situations that call for an invalid communicator, i.e. something that can never > be used as a communicator without causing an error condition. > > The phrase used elsewhere in the MPI Standard is “XYZ parameter is significant > only <when condition>” > > Cheers, > Dan. > — > Dr Daniel Holmes PhD > Executive Director > Chief Technology Officer > CHI Ltd > [ mailto:[email protected] | danholmes(a)chi.scot ] > > > > > > > On 21 Oct 2021, at 11:35, Nils Smeds via mpi-comments < [ > mailto:[email protected] | mpi-comments(a)lists.mpi-forum.org ] > > wrote: > > Summary: > - Suggestion to introduce MPI_TAG_NULL in the standard with an obviously illegal > value (i.e. <0) > - Explicitly allow Point-to-Point messages to have one of more of: tag > MPI_TAG_NULL, datatype MPI_DATATYPE_NULL, invalid buffer addresses (including > MPI_ADDRESS_NULL) and communicator MPI_COMM_NULL, if (and only if) the > corresponding send or receive argument is MPI_PROC_NULL. > > Argument: > When writing code it is useful if it goes off with a bang when you do something > wrong. To catch matching errors in sends and receives I looked for a > MPI_TAG_NULL to use as a default value to catch cases where the tag was not set > correctly. I found that there was no such item. Referring to the standard I > found that tag values are restricted to the values 0...MPI_TAG_UB, where > MPI_TAG_UB (>=32767) can be found using the MPI_Comm_get_attr() function. > > So I used my own value for MPI_TAG_NULL as -1 and coded away. However, the MPI > implementation I was using blew up when I used it in a MPI_Sendrecv with the > remote task being MPI_PROC_NULL throwing an error message "illegal tag value", > which was no surprise to me since I had set it to this value explicitly. > > The workaround is of course to put a if( PartnerTask != MPI_PROC_NULL ) { ... } > around every point-to-point call, but I find it unnecessary and makes the code > look more complex. I think that MPI_PROC_NULL was invented for exactly this > purpose, to be a transparent if(...) clause for the operation. Is there really > a need to be restrictive on datat ype, tag or any other argument when the P2P > operation is with MPI_PROC_NULL? > > I would expect implementations to choose MPI_ADDRESS_NULL be equal to (*) 0 in C > and 0_MPI_ADDRESS_KIND in Fortran and MPI_TAG_NULL to be default integer kind > -1. > > Implementation: > Append to section "3.10 Null processes" text along the lines of: > > > > A call to any point-to-point messaging function with a process argument of > MPI_PROC_NULL is allowed to have any value for buffer address, data type, tag > and communicator including MPI_ADDRESS_NULL, MPI_DATATYPE_NULL, MPI_TAG_NULL > and MPI_COMM_NULL without causing an error condition in the execution. > > Insert into appendix "A1 Assorted constants" > > > > MPI_TAG_NULL and MPI_ADDRESS_NULL > > Insert into section "9.1.2 Environmental Inquiries" (which is an odd place where > Tag values are specified in the standard): > > > > (Line 34, end of first paragraph) [... is also a valid value for MPITAG_UB]. The > constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB range. > The value MPI_TAG_NULL is erroneous to use in any communication unless the > associated process argument is MPI_PROC_NULL. > > Add to section "15.3.3 Convention for Returning Strings" > > > > The constant MPI_ADDRESS_NULL can be used as an equivalent to the null pointer. > > > > /Nils Smeds > > _______________________________________________ > mpi-comments mailing list > [ mailto:[email protected] | mpi-comments(a)lists.mpi-forum.org ] > [ https://lists.mpi-forum.org/mailman/listinfo/mpi-comments | > https://lists.mpi-forum.org/mailman/listinfo/mpi-comments ] > > > _______________________________________________ > mpi-comments mailing list > mpi-comments(a)lists.mpi-forum.org > https://lists.mpi-forum.org/mailman/listinfo/mpi-comments -- Dr. Rolf Rabenseifner . . . . . . . . . .. email rabenseifner(a)hlrs.de . High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 . University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 . Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner . Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) .

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Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL)
by Nils Smeds 22 Oct '21

22 Oct '21

Dan, Thank you for your encouraging response. Firstly, let me state that I have no strong feelings for either MPI_ADDRESS_NULL nor MPI_COMM_NULL other than they would satisfy my sense of "symmetry" or "closure". In my own programming I rarely find myself unclear what buffer address or communicator a specific code section is targeting. Most often it is the pt2pt matching triad (proc,tag,comm) that needs to be tailored to express the "intent" of the code so that an unimplemented fringe case does not pass by unnoticed. (Merely causes a dead-lock - as if that is any help - I know...) My view of the MPI_*_NULL entities in the standard is that they are comparable to NaN in floating point. MPI_TAG_NULL is not a member of the set of valid tags, and can not be matched in any tag comparison operation, not even itself or as "not a member of". MPI_TAG_ANY should not match a message with MPI_TAG_NULL, and there should indeed be no way to create a "real" message with MPI_TAG_NULL. So a statement as you suggested "When the tag argument is MPI_TAG_NULL the only allowed processor argument > is MPI_PROC_NULL" preferably in section 3.2.4 just before the last sentence describing the use of dest=MPI_PROC_NULL would be all that is needed. A probe for such a message (that can never be created as a "real" message) will thus return a status object with a count of 0 and a tag of MPI_TAG_ANY and no change in behaviour to existing implementations. So the updated suggested changes to the standards are: 1. Insert into appendix "A1 Assorted constants": MPI_TAG_NULL 2. Insert into section "9.1.2 Environmental Inquiries" (Line 34, end of first paragraph) [... is also a valid value for MPITAG_UB]. The constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB range. 3. Insert into section 3.2.4 before the last sentence (line 42.5): "When the tag argument is MPI_TAG_NULL the only allowed processor argument is MPI_PROC_NULL" 4. Add to section 3.10 "Null processes", last in the section: "*Advice to users:* Note that MPI_TAG_NULL is not a valid tag in the 0..MPI_TAG_UB range. Thus MPI_TAG_NULL is only allowed when the associated processor argument is MPI_PROC_NULL" To avoid a lengthy discussion into what probes of non-existing messages should return as a result, I suggest that the committee sees any triad (proc,tag,comm) where either component has a "NULL" value as defined in the standard (currently only MPI_PROC_NULL exists) is in essence a "NaN"-triad, and returns the already defined result of 0 elements of (MPI_PROC_NULL,MPI_ANY_TAG) as of section 3.10. Also, just by chance, this would not conflict with an introduction of an MPI_COMM_NULL item in the future. On Thu, Oct 21, 2021 at 7:14 PM Dan Holmes <danholmes(a)chi.scot> wrote: > Hi Nils, > > In general, I like the idea of having default/invalid values for these > MPI-related user variables. > > Would MPI_TAG_NULL be valid for a point-to-point send operation that does > NOT use MPI_PROC_NULL but is matched with receive operation at the > destination MPI process that uses MPI_ANY_TAG? Or should this be an > MPI_ERR_ARG/MPI_ERR_TAG error condition? Commonly, codes that use > MPI_ANY_TAG in receive operations must choose an arbitrary valid tag value > to use in the send operations at the source MPI process(es). Permitting the > use of MPI_TAG_NULL in combination with MPI_ANY_TAG might aid debugging > when the matching does not go the way the user expected. However, it would > add complexity to the matching rules/implementation. There is an > interaction here with INFO key assertions such as `mpi_assert_no_any_tag` — > which means the error could be discovered at the sender process -- and > `mpi_assert_only_any_tag` (if that one ever gets accepted into MPI). > > There is already an MPI constant that is commonly set to `(void*)0` (in > C), which is MPI_BOTTOM. Defining MPI_ADDRESS_NULL to be the same value is > not a good idea because MPI_BOTTOM is a valid address; although attempting > to read/write at that address is likely to cause trouble, using it as a > base for a user-defined datatype or a window offset is common/accepted > practice. Would it be sufficient for your purposes to use MPI_BOTTOM as > your “not valid” value? > > IMHO, there would need to be a statement that, “when the comm argument is > MPI_COMM_NULL, the only acceptable value for the source/dest argument is > MPI_PROC_NULL” because asking for the 3rd item from an empty set (for > example) is clearly an error. Your proposal tries to treat MPI_COMM_NULL as > if it is a valid MPI intra-communicator that has size zero, or a valid > inter-communicator that has a remote group of size zero. I think I would > prefer to create MPI_COMM_EMPTY for that purpose, and continue to use > MPI_COMM_NULL for situations that call for an invalid communicator, i.e. > something that can never be used as a communicator without causing an error > condition. > > The phrase used elsewhere in the MPI Standard is “XYZ parameter is > significant only <when condition>” > > Cheers, > Dan. > — > Dr Daniel Holmes PhD > Executive Director > Chief Technology Officer > CHI Ltd > danholmes(a)chi.scot > > > > On 21 Oct 2021, at 11:35, Nils Smeds via mpi-comments < > mpi-comments(a)lists.mpi-forum.org> wrote: > > *Summary:* > - Suggestion to introduce MPI_TAG_NULL in the standard with an obviously > illegal value (i.e. <0) > - Explicitly allow Point-to-Point messages to have one of more of: tag > MPI_TAG_NULL, datatype MPI_DATATYPE_NULL, invalid buffer addresses > (including MPI_ADDRESS_NULL) and communicator MPI_COMM_NULL, if (and only > if) the corresponding send or receive argument is MPI_PROC_NULL. > > *Argument:* > When writing code it is useful if it goes off with a bang when you do > something wrong. To catch matching errors in sends and receives I looked > for a MPI_TAG_NULL to use as a default value to catch cases where the tag > was not set correctly. I found that there was no such item. Referring to > the standard I found that tag values are restricted to the values > 0...MPI_TAG_UB, where MPI_TAG_UB (>=32767) can be found using the > MPI_Comm_get_attr() function. > > So I used my own value for MPI_TAG_NULL as -1 and coded away. However, the > MPI implementation I was using blew up when I used it in a MPI_Sendrecv > with the remote task being MPI_PROC_NULL throwing an error message "illegal > tag value", which was no surprise to me since I had set it to this value > explicitly. > > The workaround is of course to put a if( PartnerTask != MPI_PROC_NULL ) { > ... } around every point-to-point call, but I find it unnecessary and > makes the code look more complex. I think that MPI_PROC_NULL was invented > for exactly this purpose, to be a transparent if(...) clause for the > operation. Is there really a need to be restrictive on datat ype, tag or > any other argument when the P2P operation is with MPI_PROC_NULL? > > I would expect implementations to choose MPI_ADDRESS_NULL be equal to > (*) 0 in C and 0_MPI_ADDRESS_KIND in Fortran and MPI_TAG_NULL to be > default integer kind -1. > > *Implementation:* > Append to section "3.10 Null processes" text along the lines of: > > A call to any point-to-point messaging function with a process argument of > MPI_PROC_NULL is allowed to have any value for buffer address, data type, > tag and communicator including MPI_ADDRESS_NULL, MPI_DATATYPE_NULL, > MPI_TAG_NULL and MPI_COMM_NULL without causing an error condition in the > execution. > > > Insert into appendix "A1 Assorted constants" > > MPI_TAG_NULL and MPI_ADDRESS_NULL > > > Insert into section "9.1.2 Environmental Inquiries" (which is an odd > place where Tag values are specified in the standard): > > (Line 34, end of first paragraph) [... is also a valid value for > MPITAG_UB]. The constant MPI_TAG_NULL shall have a value outside of the > 0...MPI_TAG_UB range. The value MPI_TAG_NULL is erroneous to use in any > communication unless the associated process argument is MPI_PROC_NULL. > > > Add to section "15.3.3 Convention for Returning Strings" > > The constant MPI_ADDRESS_NULL can be used as an equivalent to the null > pointer. > > > > > /Nils Smeds > > _______________________________________________ > mpi-comments mailing list > mpi-comments(a)lists.mpi-forum.org > https://lists.mpi-forum.org/mailman/listinfo/mpi-comments > > >

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Re: [Mpi-comments] Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL)
by Dan Holmes 21 Oct '21

21 Oct '21

Hi Nils, In general, I like the idea of having default/invalid values for these MPI-related user variables. Would MPI_TAG_NULL be valid for a point-to-point send operation that does NOT use MPI_PROC_NULL but is matched with receive operation at the destination MPI process that uses MPI_ANY_TAG? Or should this be an MPI_ERR_ARG/MPI_ERR_TAG error condition? Commonly, codes that use MPI_ANY_TAG in receive operations must choose an arbitrary valid tag value to use in the send operations at the source MPI process(es). Permitting the use of MPI_TAG_NULL in combination with MPI_ANY_TAG might aid debugging when the matching does not go the way the user expected. However, it would add complexity to the matching rules/implementation. There is an interaction here with INFO key assertions such as `mpi_assert_no_any_tag` — which means the error could be discovered at the sender process -- and `mpi_assert_only_any_tag` (if that one ever gets accepted into MPI). There is already an MPI constant that is commonly set to `(void*)0` (in C), which is MPI_BOTTOM. Defining MPI_ADDRESS_NULL to be the same value is not a good idea because MPI_BOTTOM is a valid address; although attempting to read/write at that address is likely to cause trouble, using it as a base for a user-defined datatype or a window offset is common/accepted practice. Would it be sufficient for your purposes to use MPI_BOTTOM as your “not valid” value? IMHO, there would need to be a statement that, “when the comm argument is MPI_COMM_NULL, the only acceptable value for the source/dest argument is MPI_PROC_NULL” because asking for the 3rd item from an empty set (for example) is clearly an error. Your proposal tries to treat MPI_COMM_NULL as if it is a valid MPI intra-communicator that has size zero, or a valid inter-communicator that has a remote group of size zero. I think I would prefer to create MPI_COMM_EMPTY for that purpose, and continue to use MPI_COMM_NULL for situations that call for an invalid communicator, i.e. something that can never be used as a communicator without causing an error condition. The phrase used elsewhere in the MPI Standard is “XYZ parameter is significant only <when condition>” Cheers, Dan. — Dr Daniel Holmes PhD Executive Director Chief Technology Officer CHI Ltd danholmes(a)chi.scot > On 21 Oct 2021, at 11:35, Nils Smeds via mpi-comments <mpi-comments(a)lists.mpi-forum.org> wrote: > > Summary: > - Suggestion to introduce MPI_TAG_NULL in the standard with an obviously illegal value (i.e. <0) > - Explicitly allow Point-to-Point messages to have one of more of: tag MPI_TAG_NULL, datatype MPI_DATATYPE_NULL, invalid buffer addresses (including MPI_ADDRESS_NULL) and communicator MPI_COMM_NULL, if (and only if) the corresponding send or receive argument is MPI_PROC_NULL. > > Argument: > When writing code it is useful if it goes off with a bang when you do something wrong. To catch matching errors in sends and receives I looked for a MPI_TAG_NULL to use as a default value to catch cases where the tag was not set correctly. I found that there was no such item. Referring to the standard I found that tag values are restricted to the values 0...MPI_TAG_UB, where MPI_TAG_UB (>=32767) can be found using the MPI_Comm_get_attr() function. > > So I used my own value for MPI_TAG_NULL as -1 and coded away. However, the MPI implementation I was using blew up when I used it in a MPI_Sendrecv with the remote task being MPI_PROC_NULL throwing an error message "illegal tag value", which was no surprise to me since I had set it to this value explicitly. > > The workaround is of course to put a if( PartnerTask != MPI_PROC_NULL ) { ... } around every point-to-point call, but I find it unnecessary and makes the code look more complex. I think that MPI_PROC_NULL was invented for exactly this purpose, to be a transparent if(...) clause for the operation. Is there really a need to be restrictive on datat ype, tag or any other argument when the P2P operation is with MPI_PROC_NULL? > > I would expect implementations to choose MPI_ADDRESS_NULL be equal to (*) 0 in C and 0_MPI_ADDRESS_KIND in Fortran and MPI_TAG_NULL to be default integer kind -1. > > Implementation: > Append to section "3.10 Null processes" text along the lines of: > A call to any point-to-point messaging function with a process argument of MPI_PROC_NULL is allowed to have any value for buffer address, data type, tag and communicator including MPI_ADDRESS_NULL, MPI_DATATYPE_NULL, MPI_TAG_NULL and MPI_COMM_NULL without causing an error condition in the execution. > > Insert into appendix "A1 Assorted constants" > MPI_TAG_NULL and MPI_ADDRESS_NULL > > Insert into section "9.1.2 Environmental Inquiries" (which is an odd place where Tag values are specified in the standard): > (Line 34, end of first paragraph) [... is also a valid value for MPITAG_UB]. The constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB range. The value MPI_TAG_NULL is erroneous to use in any communication unless the associated process argument is MPI_PROC_NULL. > > Add to section "15.3.3 Convention for Returning Strings" > The constant MPI_ADDRESS_NULL can be used as an equivalent to the null pointer. > > > > /Nils Smeds > > _______________________________________________ > mpi-comments mailing list > mpi-comments(a)lists.mpi-forum.org > https://lists.mpi-forum.org/mailman/listinfo/mpi-comments

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Pt2pt messaging with MPI_PROC_NULL (and MPI_TAG_NULL, MPI_ADDRESS_NULL)
by Nils Smeds 21 Oct '21

21 Oct '21

*Summary:* - Suggestion to introduce MPI_TAG_NULL in the standard with an obviously illegal value (i.e. <0) - Explicitly allow Point-to-Point messages to have one of more of: tag MPI_TAG_NULL, datatype MPI_DATATYPE_NULL, invalid buffer addresses (including MPI_ADDRESS_NULL) and communicator MPI_COMM_NULL, if (and only if) the corresponding send or receive argument is MPI_PROC_NULL. *Argument:* When writing code it is useful if it goes off with a bang when you do something wrong. To catch matching errors in sends and receives I looked for a MPI_TAG_NULL to use as a default value to catch cases where the tag was not set correctly. I found that there was no such item. Referring to the standard I found that tag values are restricted to the values 0...MPI_TAG_UB, where MPI_TAG_UB (>=32767) can be found using the MPI_Comm_get_attr() function. So I used my own value for MPI_TAG_NULL as -1 and coded away. However, the MPI implementation I was using blew up when I used it in a MPI_Sendrecv with the remote task being MPI_PROC_NULL throwing an error message "illegal tag value", which was no surprise to me since I had set it to this value explicitly. The workaround is of course to put a if( PartnerTask != MPI_PROC_NULL ) { ... } around every point-to-point call, but I find it unnecessary and makes the code look more complex. I think that MPI_PROC_NULL was invented for exactly this purpose, to be a transparent if(...) clause for the operation. Is there really a need to be restrictive on datat ype, tag or any other argument when the P2P operation is with MPI_PROC_NULL? I would expect implementations to choose MPI_ADDRESS_NULL be equal to (*) 0 in C and 0_MPI_ADDRESS_KIND in Fortran and MPI_TAG_NULL to be default integer kind -1. *Implementation:* Append to section "3.10 Null processes" text along the lines of: A call to any point-to-point messaging function with a process argument of MPI_PROC_NULL is allowed to have any value for buffer address, data type, tag and communicator including MPI_ADDRESS_NULL, MPI_DATATYPE_NULL, MPI_TAG_NULL and MPI_COMM_NULL without causing an error condition in the execution. Insert into appendix "A1 Assorted constants" MPI_TAG_NULL and MPI_ADDRESS_NULL Insert into section "9.1.2 Environmental Inquiries" (which is an odd place where Tag values are specified in the standard): (Line 34, end of first paragraph) [... is also a valid value for MPITAG_UB]. The constant MPI_TAG_NULL shall have a value outside of the 0...MPI_TAG_UB range. The value MPI_TAG_NULL is erroneous to use in any communication unless the associated process argument is MPI_PROC_NULL. Add to section "15.3.3 Convention for Returning Strings" The constant MPI_ADDRESS_NULL can be used as an equivalent to the null pointer. /Nils Smeds

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Make MPI_Precv_init to return an array of MPI_Request to avoid MPI_Wait* MPI_Test* routines duplication
by BRELLE, EMMANUEL 15 Oct '21

15 Oct '21

Dear MPI-forum members, This comment applies to MPI-4.0 standard (https://www.mpi-forum.org/docs/mpi-4.0/mpi40-report.pdf) Having partitioned communication looks very promising to speed up applications, but I am wondering why did you introduce "MPI_Parrived"(page 151) routine instead of making "MPI_Precv_init"(page 147) returning an array of MPI_Request. To my understanding, one of the need for partitioned communications is runtimes that exchange set of "tasks" : computations do not immediately need all the partitions to continue. Partitioned communication enabled message aggregation but a single partition could be sufficient to unlock computation. In this case, processes will have to search over each partition to catch a completed ones that not have been already found. This is what MPI_Waitany (page 116) does with an array of MPI_Request. It would be have been therefore more interesting to handle explicit MPI_Request objects. I would appreciate that MPI_Precv_init returns an array of MPI_request in addition of the main MPI_request (p147 line 39). MPI_Parrived could be deprecated for MPI_Test on these sub-requests. In extension this would also avoid to duplicate MPI_Testany(p117), MPI_Waitall(p118), MPI_Testall(p119), MPI_Waitsome (p 120) and MPI_Testsome(p121). In a first idea, MPI users would provide this array of the size of the number of partitions as an extra parmeter to MPI_Precv_init. If the array is valid (for example not a special value "MPI_NO_REQUESTS") these MPI_Request would be created by MPI_Precv_init and freed by a call of MPI_Request_free on the main request. Waiting all the sub-requests would be equivalent to wait the main request. Like requests from MPI_Irecv, it would be invalid to call MPI_Start on these sub-requests. Regards, Emmanuel Brelle

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