Dear Rich, Thank you. I'm basically after making sure that we don't design something that does not work. From this discussion, it appears that the existing progress rules do cover the proposal well enough to proceed. Best regards. Alexander ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Richard Graham Sent: Friday, October 17, 2008 3:42 PM To: MPI-3 Collective Subgroup Discussions Subject: Re: [mpi3-coll] Non-blocking Collectives Proposal Draft Alexander, I am not sure what the really trying to get after. The example I gave is exactly what I intended for it to be, not a complete working MPI example, but the core as it relates to progress. The example that Torsten gave should progress and complete, whether one uses asynchronous progress mechanisms or the synchronous approach most of us take today. If the question is about the semantics of MPI_Ibarrier(), I think the only specification is that it starts when the ibarrier is posted (what ever that means, which I am sure is implementation specific) and ends with the wait - I don't think that there is any other specification here. Have I completely missed what your question is ? Rich On 10/17/08 8:51 AM, "Supalov, Alexander" <[email protected]> wrote: Dear Rich, Thanks. You probably meant Recv(0) in Proc 1. I also miss a Wait for req_r. So, a complete example would probably look like: Proc 0 Proc 1 Irecv(1,req_r) Irecv(0,req_r) Isend(1,req_s) Isend(0,req_s) Waitall(req_s, req_r) Recv(0) Send(1) Waitall(req_s,req_r) This would work. However, let's do one change, replacing Isend by Issend. Will this still work? Irecv(1,req_r) Irecv(0,req_r) Issend(1,req_s) Issend(0,req_s) Waitall(req_s, req_r) Recv(0) Send(1) Waitall(req_s,req_r) Another possible complication: let's swap the Send and Recv, and make Send synchronous (or rendezvous): Irecv(1,req_r) Irecv(0,req_r) Issend(1,req_s) Issend(0,req_s) Waitall(req_s, req_r) Ssend(0) Recv(1) Waitall(req_s,req_r) Here I have rather strong doubts that this will work. Do you? Best regards. Alexander -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Graham, Richard L. Sent: Friday, October 17, 2008 2:35 PM To: [email protected] Subject: Re: [mpi3-coll] Non-blocking Collectives Proposal Draft Alexander, Do you see this as different in principal from Proc 0. Proc 1 irecv(1,req_r) Irecv(0,req_r) Isend(1,req_s). Isend(0,req_s) Wait(req_s). Send(0) Send(1). Wait(req_s) I see these two cases as basically the same from a progress perspective. Rich ----- Original Message ----- From: [email protected] <[email protected]> To: MPI-3 Collective Subgroup Discussions <[email protected]> Sent: Fri Oct 17 07:22:02 2008 Subject: Re: [mpi3-coll] Non-blocking Collectives Proposal Draft Dear Rich, Thank you. Note that the first operation is an Ibarrier. According to Torsten's explanation, process 0 will block in the Wait, which it may well do if it is Wait in process 1 that actually completes its Ibarrier. Now, how can process 0 make progress in its Wait when there's no matching Wait on the other side yet? Do we assume that any messages that may have been sent by process 0's IBarrier will be handled as unexpected messages by process 1 that is busy doing its (also unmatched) part of the Bcast? Likewise, do we assume that all messages that process 1 has to send in its Ibarrier can leave process 1 before it enters its Wait? Aren't we assuming a bit too much? In other words, aren't we assuming that all message sends and receives will be started in Ibarrier, and will be able to proceed even though the Wait is some blocking collective ops away? Given the way most MPIs implement nonblocking pt2pt ops now, something looks fishy here. In any case, I'd like to see a detailed explanation how things will work, and moreover, a proof that this does not preclude some algorithms - both in the sense of the pt2pt operations deployed, and in the sense of collective exchange pattern, for the applicable message sizes - from being used. Best regards. Alexander -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Graham, Richard L. Sent: Friday, October 17, 2008 11:57 AM To: [email protected] Subject: Re: [mpi3-coll] Non-blocking Collectives Proposal Draft To me this does not look like a deadlock, proc o will make progress in the wait, and just like with point-to-point commuications process 1 will make general mpi progress while in the bcast. This includes progress on the ibcast so that process 0 can comete it's ibcast and move on to the bcast. Rich ----- Original Message ----- From: [email protected] <[email protected]> To: MPI-3 Collective Subgroup Discussions <[email protected]> Sent: Fri Oct 17 05:14:04 2008 Subject: Re: [mpi3-coll] Non-blocking Collectives Proposal Draft Dear Torsten, Thanks. I'm still unsure I understand how the example would work if processes blocked in the Wait calls as you said below. In this case process 0 would have no chance to reach the Bcast, while process 1 would be inside it and never reached the Wait. Here again is the original diagram: Process 0 Process 1 MPI_Ibarrier(req) MPI_Ibarrier(req) MPI_Wait(req)* MPI Bcast MPI_Bcast* MPI_Wait(req) The stars mark the point where the respective processes would block, as far as I understand. Looks like deadlock? Apart from this, stating that the nonblocking collectives can block in Wait presumes a certain implementation. What if Ibarrier was able to do all right away, and Wait were just a no-op? I think mixing blocking and nonblocking collectives in the proposed way should either be very carefully considered, or simply disallowed. Best regards. Alexander -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Torsten Hoefler Sent: Thursday, October 16, 2008 11:15 PM To: MPI-3 Collective Subgroup Discussions Subject: Re: [mpi3-coll] Non-blocking Collectives Proposal Draft Hello Alexander, > Thank you. The pt2pt progress comment below was taken into account when > you removed the nonblocking collective progress description. ah, ok - so I think a reference to the chapter should be sufficient. > As for the example, if it is legal, we should probably count the > blocking operations as well when we decide how many simultaneous ops are > to be supported. yes, maybe. I am not sure but this could be an issue. It's on the agenda https://svn.mpi-forum.org/trac/mpi-forum-web/wiki/forum201008 > Why "if" above: I'm not sure what a delayed nonblocking barrier would > mean for process 1. Won't one process possibly block somewhere? Can you > explain this please? the barriers will only block in the respective MPI_Wait() (or a tight MPI_Test() loop). But the operation can be initiated as soon as the critical region is left (and there mihgt be some other independent computation to do). > The format matter is complicated. I think it is worth discussing at the > Forum as we're likely to have more and more proposals approaching the > final stage, when editing them in PDF format will become an issue. yes, it's on the WG agenda - maybe we should discuss this in the whole forum? Best, Torsten -- bash$ :(){ :|:&};: --------------------- http://www.unixer.de/ ----- Torsten Hoefler | Postdoctoral Researcher Open Systems Lab | Indiana University 150 S. Woodlawn Ave. | Bloomington, IN, 474045, USA Lindley Hall Room 135 | +01 (812) 855-3608 _______________________________________________ mpi3-coll mailing list [email protected] http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-coll --------------------------------------------------------------------- Intel GmbH Dornacher Strasse 1 85622 Feldkirchen/Muenchen Germany Sitz der Gesellschaft: Feldkirchen bei Muenchen Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer Registergericht: Muenchen HRB 47456 Ust.-IdNr. 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