Re: [Mpiwg-large-counts] Large Count - the principles for counts, sizes, and byte and nonbyte displacements

29 Oct 2019

      Dear Jim,
...
(a3) Section 4.1.5 of MPI 3.1 states "To ensure portability, arithmetic on
absolute addresses should not be performed with the intrinsic operators \-"
and \+".
The major problem is, that we decided "should" and not "maust" or "shall",
because there is such many existing MPI-1 ... MPI-3.0 code that must have
used + or - operators.

The only objective, that is true from the beginning, that MPI addresses must be 
retrieved with MPI_Get_address.

And the second also Major Problem is the new assigment of an MPI_Aint value 
into an MPI_Count variable with MPI_Count larger than MPI_Aint.

Therefore, I would prefere, that we keep this "should" and design in long term
MPI_Get_address in a way that in principle MPI_Aint_diff and _add 
need not to do anythin else as the + or - operator.

And this depends on the meaning of the unsigned addresses, i.e.,
what is the sequence of addresses (i.e., is it really going from
0 to FFFF...FFFF) and than mapping these addreses to the mathematical sequence
of MPI_Aint which starts at -2**(n-1) and ends at 2**(n-1)-1.

Thats all. For the moment, as far as the web and some emails told us,
we are fare away from this contiguous 64-bit address space (0 to FFFF...FFFF).

But we should be correctly prepared.

Or in other words:
...
(a2) Should be solved by MPI_Aint_add/diff.
In my opinion no, it must be solved by MPI_Get_addr
and MPI_Aint_add/diff can stay normal + or - operators.
I should also mention, that of course all MPI routines that
accept MPI_BOOTOM must reverse the work of MPI_Get_address
to get back the real "unsigned" virtual addresses of the OS.

The same what we already had if an implementation has chosen
to use the address of an MPI common block as base for MPI_BOTTOM.
Here, the MPI lib had the freedom to revert the mapping
within MPI_Get_addr or within all functions called with MPI_BOTTOM.

Best regards
Rolf

----- Original Message -----
...
From: "Jim Dinan" <[email protected]>
To: "Rolf Rabenseifner" <[email protected]>
Cc: "mpiwg-large-counts" <[email protected]>
Sent: Tuesday, October 29, 2019 3:58:18 PM
Subject: Re: [Mpiwg-large-counts] Large Count - the principles for counts, sizes, and byte and nonbyte displacements
...
Hi Rolf,
(a1) seems to me like another artifact of storing an unsigned quantity in a
signed variable, i.e., the quantity in an MPI_Aint can be an unsigned
address or a signed displacement.  Since we don't have an unsigned type for
addresses, the user can't portably fix this above MPI.  We will need to add
functions to deal with combinations of MPI_Aint and MPI_Counts.  This is
essentially why we needed MPI_Aint_add/diff.  Or ... the golden (Au is
gold) int ... MPI_Auint.
(a2) Should be solved by MPI_Aint_add/diff.
(a3) Section 4.1.5 of MPI 3.1 states "To ensure portability, arithmetic on
absolute addresses should not be performed with the intrinsic operators \-"
and \+".  MPI_Aint_add was written carefully to indicate that the "base"
argument is treated as an unsigned address and the "disp" argument is
treated as a signed displacement.
~Jim.
On Tue, Oct 29, 2019 at 5:19 AM Rolf Rabenseifner <[email protected]>
wrote:
...
Dear Jim and all,
I'm not sure whether I'm really able to understand your email.
I take the MPI view:
(1) An absolute address can stored in an MPI_Aint variable
    with and only with MPI_Get_address or MPI_Aint_add.
(2) A positive or negative number of bytes or a relative address
    which is by definition the amount of bytes between two locations
    in a MPI "sequential storage" (MPI-3.1 page 115)
    can be assigned with any method to an MPI_Aint variable
    as long as the original value fits into MPI_Aint.
    In both languages automatic type cast (i.e., sign expansion)
    is done.
(3) If users misuse MPI_Aint for storing anything else into MPI_Aint
    variable then this is out of scope of MPI.
    If such values are used in a minus operation then it is
    out of the scope of MPI whether this makes sense.
    If the user is sure that the new value falls into category (2)
    then all is fine as long as the user is correct.
I expect that your => is not a "greater or equal than".
I expect that you noticed assignments.
...
intptr_t => MPI_Aint
"intptr_t:  integer type capable of holding a pointer."
...
uintptr_t => ??? (Anyone remember the MPI_Auint "golden Aint" proposal?)
"uintptr_t:  unsigned integer type capable of holding a pointer."
may fall exactly exactly into (3) when used for pointers.
Especially on a 64 bit system the user may have in the future exactly
the problems (a), (a1), (a2) and (b) as described below.
But here, the user is responsible, to for example implement (a3),
whereas for MPI_Get_address, the implementors of the MPI library
are responsible and the MPI Forum may be responsible for giving
the correct advices.
By the way, the golden MPI_Auint was never golden.
Such need was "resolved" by introducing MPI_Aint_diff and MPI_Aint_add
in MPI-3.1.
...
ptrdiff_t => MPI_Aint
"std::ptrdiff_t is the signed integer type of the result of subtracting
two pointers."
may perfectly fit to (2).
All of the following falls into category (2):
...
size_t (sizeof) => MPI_Count, int
"sizeof( type )  (1)
 sizeof expression   (2)
 Both versions are constant expressions of type std::size_t."
...
size_t (offsetof) => MPI_Aint, int
"Defined in header <cstddef>
 #define offsetof(type, member) /*implementation-defined*/
 The macro offsetof expands to an integral constant expression
 of type std::size_t, the value of which is the offset, in bytes,
 from the beginning of an object of specified type to ist
 specified member, including padding if any."
Note that this offsetof has nothing to do with MPI_Offset.
On a system with less than 2*31 byte and 4-byte int, it is guaranteed
that  size_t => int  works.
On a system with less than 2*63 byte and 8-byte MPI_Aint, it is guaranteed
that  size_t => MPI_Aint  works.
Problem: size_t is unsigned, int and MPI_Aint are signed.
MPI_Count should be defined in a way that on systems with more than
2**63 Bytes of disc space, that MPI_Count can hold such values,
because
  int .LE. {MPI_Aint, MPI_Offset} .LE. MPI_Count
Therefore  size_t => MPI_Count  should always work.
...
ssize_t => Mostly for error handling. Out of scope for MPI?
"In short, ssize_t is the same as size_t, but is a signed type -
 read ssize_t as “signed size_t”. ssize_t is able to represent
 the number -1, which is returned by several system calls
 and library functions as a way to indicate error.
 For example, the read and write system calls: ...
 ssize_t read(int fildes, void *buf, size_t nbyte); ..."
ssize_t fits therefore better to MPI_Aint, because both
are signed types that can hold byte counts, but
the value -1 in a MPI_Aint variable stands for a
byte displacement of -1 bytes and not for an error code -1.
All use of (2) is in principle no problem.
------------------------------------------
All the complex discussiuon of the last days is about (1):
(1) An absolute address can stored in an MPI_Aint variable
    with and only with MPI_Get_address or MPI_Aint_add.
In MPI-1 to MPI-3.0 and still in MPI-3.1 (here as may be not portable),
we also allow
 MPI_Aint variable := absolute address in MPI_Aint variable
                       + or -
                      a number of bytes (in any integer type).
The result is then still in category (1).
For the difference of two absolute addresses,
MPI_Aint_diff can be used. The result is than MPI_Aint of category (2)
In MPI-1 to MPI-3.0 and still in MPI-3.1 (here as may be not portable),
we also allow
 MPI_Aint variable := absolute address in MPI_Aint variable
                      - absolute address in MPI_Aint variable.
The result is then in category (2).
The problems we discuss the last days are about systems
that internally use unsigned addresses and the MPI library stores
these addresses into MPI_Aint variables and
(a) a sequential storage can have virtual addresses that
    are both in the area with highest bit =0 and other addresses
    in the same sequential storage (i.e., same array or structure)
    with highest bit =1.
or
(b) some higher bits contain segment addresses.
(b) is not a problem as long as a sequential storage resides
    always within one Segment.
Therefore, we only have to discuss (a).
The two problems that we have is
(a1) that for the minus operations an integer overflow will
     happen and must be ignored.
(a2) if such addresses are expanded to larger variables,
     e.g., MPI_Count with more bits in MPI_Count than in MPI_Aint,
     sign expansion will result in completely wring results.
And here, the most simple trick is,
(a3) that MPI_Get_address really shall
map the contiguous unsigned range from 0 to 2**64-1 to the
signed (and also contiguous) range from -2**63 to 2**63-1
by simple subtracting 2**63.
With this simple trick in MPI_Get_address, Problems
8a1) and (a2) are resolved.
It looks like that (a) and therefore (a1) and (a2)
may be far in the future.
But they may be less far in the future, if a system may
map the whole applications cluster address space
into virtual memory (not cache coherent, but accessible).
And all this is never or only partial written into the
MPI Standard, also all is (well) known by the MPI Forum,
with the following exceptions:
- (a2) is new.
- (a1) is solved in MPI-3.1 only for MPI_Aint_diff and
       MPI_Aint_add, but not for the operators - and +
       if a user will switch on integer overflow detection
       in the future when we will have such large systems.
- (a3) is new and in principle solves the problem also
       for + and - operators.
At lease (a1)+(a2) should be added as rationale to MPI-4.0
and (a3) as advice to implementors within the framework
of big count, because (a2) is newly coming with big count.
I hope this helps a bit if you took the time to read
this long email.
Best regards
Rolf
----- Original Message -----
...
From: "mpiwg-large-counts" <[email protected]>
To: "mpiwg-large-counts" <[email protected]>
Cc: "Jim Dinan" <[email protected]>, "James Dinan" <
[email protected]>
Sent: Monday, October 28, 2019 5:07:37 PM
Subject: Re: [Mpiwg-large-counts] Large Count - the principles for
counts, sizes, and byte and nonbyte displacements
...
Still not sure I see the issue. MPI's memory-related integers should map
to
types that serve the same function in C. If the base language is broken
for
segmented addressing, we won't be able to fix it in a library. Looking
at the
mapping below, I don't see where we would have broken it:
intptr_t => MPI_Aint
uintptr_t => ??? (Anyone remember the MPI_Auint "golden Aint" proposal?)
ptrdiff_t => MPI_Aint
size_t (sizeof) => MPI_Count, int
size_t (offsetof) => MPI_Aint, int
ssize_t => Mostly for error handling. Out of scope for MPI?
It sounds like there are some places where we used MPI_Aint in place of
size_t
for sizes. Not great, but MPI_Aint already needs to be at least as large
as
size_t, so this seems benign.
~Jim.
On Fri, Oct 25, 2019 at 8:25 PM Dinan, James via mpiwg-large-counts < [
mailto:[email protected] |
[email protected] ] > wrote:
Jeff, thanks so much for opening up these old wounds. I’m not sure I
have enough
context to contribute to the discussion. Where can I read up on the
issue with
MPI_Aint?
I’m glad to hear that C signed integers will finally have a well-defined
representation.
~Jim.
From: Jeff Hammond < [ mailto:[email protected] |
[email protected] ]
...
Date: Thursday, October 24, 2019 at 7:03 PM
To: "Jeff Squyres (jsquyres)" < [ mailto:[email protected] |
[email protected]
] >
Cc: MPI BigCount Working Group < [ mailto:
[email protected]
| [email protected] ] >, "Dinan, James" < [
mailto:[email protected] | [email protected] ] >
Subject: Re: [Mpiwg-large-counts] Large Count - the principles for
counts,
sizes, and byte and nonbyte displacements
Jim (cc) suffered the most in MPI 3.0 days because of AINT_DIFF and
AINT_SUM, so
maybe he wants to create this ticket.
Jeff
On Thu, Oct 24, 2019 at 2:41 PM Jeff Squyres (jsquyres) < [
mailto:[email protected] | [email protected] ] > wrote:
Not opposed to ditching segmented addressing at all. We'd need a ticket
for this
ASAP, though.
This whole conversation is predicated on:
- MPI supposedly supports segmented addressing
- MPI_Aint is not sufficient for modern segmented addressing (i.e.,
representing
an address that may not be in main RAM and is not mapped in to the
current
process' linear address space)
If we no longer care about segmented addressing, that makes a whole
bunch of
BigCount stuff a LOT easier. E.g., MPI_Aint can basically be a
non-segment-supporting address integer. AINT_DIFF and AINT_SUM can go
away,
too.
On Oct 24, 2019, at 5:35 PM, Jeff Hammond via mpiwg-large-counts < [
mailto:[email protected] |
[email protected] ] > wrote:
Rolf:
Before anybody spends any time analyzing how we handle segmented
addressing, I
want you to provide an example of a platform where this is relevant. What
system can you boot today that needs this and what MPI libraries have
expressed
an interest in supporting it?
For anyone who didn't hear, ISO C and C++ have finally committed to
twos-complement integers ( [
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0907r1.html |
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0907r1.html ]
, [
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2218.htm |
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2218.htm ] ) because
modern
programmers should not be limited by hardware designs from the 1960s. We
should
similarly not waste our time on obsolete features like segmentation.
Jeff
On Thu, Oct 24, 2019 at 10:13 AM Rolf Rabenseifner via
mpiwg-large-counts < [
mailto:[email protected] |
[email protected] ] > wrote:
...
I think that changes the conversation entirely, right?
Not the first part, the state-of-current-MPI.
It may change something for the future, or a new interface may be needed.
Please, can you describe how MPI_Get_address can work with the
different variables from different memory segments.
Or whether a completely new function or a set of functions is needed.
If we can still express variables from all memory segments as
input to MPI_Get_address, there may be still a way to flatten
the result of some internal address-iquiry into a flattened
signed integer with the same behavior as MPI_Aint today.
If this is impossible, then new way of thinking and solution
may be needed.
I really want to see examples for all current stuff as you
mentioned in your last email.
Best regards
Rolf
----- Original Message -----
...
From: "Jeff Squyres" < [ mailto:[email protected] | [email protected]
] >
To: "Rolf Rabenseifner" < [ mailto:[email protected] |
[email protected] ]
...
Cc: "mpiwg-large-counts" < [ mailto:
[email protected] |
[email protected] ] >
Sent: Thursday, October 24, 2019 5:27:31 PM
Subject: Re: [Mpiwg-large-counts] Large Count - the principles for
counts,
sizes, and byte and nonbyte displacements
...
On Oct 24, 2019, at 11:15 AM, Rolf Rabenseifner
< [ mailto:[email protected] | [email protected] ] <mailto: [
mailto:[email protected] | [email protected] ] >> wrote:
For me, it looked like that there was some misunderstanding
of the concept that absolute and relative addresses
and number of bytes that can be stored in MPI_Aint.
...with the caveat that MPI_Aint -- as it is right now -- does not
support
modern segmented memory systems (i.e., where you need more than a small
number
of bits to indicate the segment where the memory lives).
I think that changes the conversation entirely, right?
--
Jeff Squyres
[ mailto:[email protected] | [email protected] ] <mailto: [
mailto:[email protected] | [email protected] ] >
--
Dr. Rolf Rabenseifner . . . . . . . . . .. email [ mailto:
[email protected] |
[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 . . . [
http://www.hlrs.de/people/rabenseifner |
www.hlrs.de/people/rabenseifner ] .
Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) .
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--
Jeff Hammond
[ mailto:[email protected] | [email protected] ]
[ http://jeffhammond.github.io/ | http://jeffhammond.github.io/ ]
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--
Jeff Squyres
[ mailto:[email protected] | [email protected] ]
--
Jeff Hammond
[ mailto:[email protected] | [email protected] ]
[ http://jeffhammond.github.io/ | http://jeffhammond.github.io/ ]
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--
Dr. Rolf Rabenseifner . . . . . . . . . .. email [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 .
Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) .
-- 
Dr. Rolf Rabenseifner . . . . . . . . . .. email [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 .
Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) .