[Mpi-forum] MPI_DATATYPE_NULL when count=0
Schulz Martin
schulzm at llnl.gov
Wed Jan 13 14:54:22 CST 2016
Hi Jeff,
Unfortunately, I don¹t have the code on a public repo (it is part of our
internal PnMPI branch), but the critical macro is below. Nothing too
surprising it basically takes the existing datatype and adds one field,
creates a new datatype and then commits it. The following communication then
uses this datatype together with MPI_BOTTOM.
I agree, this code could be optimized for such special cases and adding an
if statement to check for zero counts is doable. However, my point was that
the proposed change does change the standard so that existing code breaks.
Also, it would seem strange that you then could use MPI_TYPE_NULL for some
operations with count zero and not for others. For this particular code, if
we would define what it means to have MPI_TYPE_NULL for MPI_Type_struct with
a zero count (it should probably just be ignored, although we would have to
think through if there are any consequences for the extent calculations),
the code would work just fine. I am not sure, if this would cover all cases,
though (it might, if we are carefully defining this for all routines that
take datatypes as arguments).
Martin
#define DT_GENERALALLOC(b,c,dt,pb,var) \
{ int err,lens[2]; MPI_Aint addr[2]; MPI_Datatype types[2]; \
types[0]=MPI_CHAR; lens[0]=piggyback_size; err=MPI_Address((void*) pb,
&(addr[0])); \
if (err!=MPI_SUCCESS) { return err; } \
types[1]=dt; lens[1]=c; err=MPI_Address((void*) b, &(addr[1])); \
if (err!=MPI_SUCCESS) { return err; } \
err=MPI_Type_struct(2,lens,addr,types,&var); \
if (err!=MPI_SUCCESS) { return err; } \
err=MPI_Type_commit(&var); \
if (err!=MPI_SUCCESS) { return err; } \
}
________________________________________________________________________
Martin Schulz, schulzm at llnl.gov, http://scalability.llnl.gov/
CASC @ Lawrence Livermore National Laboratory, Livermore, USA
From: mpi-forum <mpi-forum-bounces at lists.mpi-forum.org> on behalf of Jeff
Hammond <jeff.science at gmail.com>
Reply-To: Main mailing list <mpi-forum at lists.mpi-forum.org>
Date: Wednesday, January 13, 2016 at 12:09 PM
To: Main mailing list <mpi-forum at lists.mpi-forum.org>
Subject: Re: [Mpi-forum] MPI_DATATYPE_NULL when count=0
Can you provide the specifics of a tool that does this today? I'd like to
read the source code to see what changes are actually required.
How can you piggyback an MPI collective call by hacking the datatype if
count=0? You already have to check if count=0 and bump it to count=1 if you
are going to send a piggy for a null transfer, so how hard is it to replace
MPI_DATATYPE_NULL with MPI_BYTE? If you are not sending a piggy for a null
transfer, then the issue is moot.
If nothing else, the overhead of implementing the trivial branch that solves
this issue will be negligible compared to firing up the datatype engine.
Jeff
On Wed, Jan 13, 2016 at 11:41 AM, Schulz Martin <schulzm at llnl.gov> wrote:
> In general, I agree that allowing any argument for datatype if the count is
> zero makes sense. However, such a change (even if it is just allowing more)
> can cause backwards compatibility problems: e.g., any PMPI tool or other kind
> of internal wrapper library can currently have the assumption that it is only
> passed valid datatypes and hence can use the datatype without having to check.
> We, for example, use this to implement piggybacking (by creating a new struct
> datatype that contains the original one and adds a piggyback field). If we
> allowed arbitrary datatypes, we would brake such codes, unless we then also
> allowed it for datatype creation.
>
> If we wanted to change this, we probably need to be very careful and make sure
> we are consistent across the standard and touch all routines that take
> datatype arguments. Just adding the exception for P2P, or even for all
> communication routines, seems insufficient.
>
> Martin
>
>
> ________________________________________________________________________
>
> ________________________________________________________________________
> Martin Schulz, schulzm at llnl.gov, http://scalability.llnl.gov/
> CASC @ Lawrence Livermore National Laboratory, Livermore, USA
>
>
> From: mpi-forum <mpi-forum-bounces at lists.mpi-forum.org> on behalf of Jeff
> Hammond <jeff.science at gmail.com>
> Reply-To: Main mailing list <mpi-forum at lists.mpi-forum.org>
> Date: Wednesday, January 13, 2016 at 6:24 AM
> To: Main mailing list <mpi-forum at lists.mpi-forum.org>
>
> Subject: Re: [Mpi-forum] MPI_DATATYPE_NULL when count=0
>
> That is precisely what I want: when count=0, the buffer and the datatype
> arguments are ignored. There may be a good reason for MPI_DATATYPE_NULL to be
> invalid when it is actually relevant, but when count=0, the datatype argument
> is not relevant, because there are zero of them.
>
> We already state explicitly that arguments are ignored for MPI_IN_PLACE and
> MPI_NO_OP:
>
> * The ³in place² option for intracommunicators is specified by passing
> MPI_IN_PLACE as the value of recvbuf at the root. In such a case, recvcount
> and recvtype are ignored..."
>
> * "When MPI_NO_OP is specified as the operation, the origin_addr,
> origin_count, and origin_datatype arguments are ignored."
>
> I see no reason why count=0 is different from these cases.
>
> Jeff
>
> On Wed, Jan 13, 2016 at 5:47 AM, William Gropp <wgropp at illinois.edu> wrote:
>> >
>> > True, MPI_BOTTOM need not be null, but it might be, so the standard doesn¹t
>> prohibit it.
>> >
>> > A carefully worded exception to the general rule for NULL handles would be
>> ok. I would still make it narrow; for example, just for the use of
>> MPI_DATATYPE_NULL in communication operations with a zero count.
>> >
>> > Bill
>> >
>> > William Gropp
>> > Director, Parallel Computing Institute
>> > Thomas M. Siebel Chair in Computer Science
>> > Chief Scientist, NCSA
>> > University of Illinois Urbana-Champaign
>> >
>> >
>> >
>> >
>> >
>> > On Jan 12, 2016, at 11:22 PM, Fab Tillier <ftillier at microsoft.com> wrote:
>> >
>> > Don¹t datatypes with absolute addresses rely on MPI_BOTTOM, and a portable
>> program can¹t assume that MPI_BOTTOM == NULL?
>> >
>> > I think from an ease of use perspective, if I have a count of zero, having
>> to pick an arbitrary committed datatype is non-intuitive. If count is zero,
>> there is no buffer or datatype, and there should be no requirement such
>> non-existent datatype be committed.
>> >
>> > Cheers,
>> > -Fab
>> >
>> > From: mpi-forum [mailto:mpi-forum-bounces at lists.mpi-forum.org] On Behalf Of
>> William Gropp
>> > Sent: Wednesday, 13 January 2016 4:52 p.m.
>> > To: Main MPI Forum mailing list <mpi-forum at lists.mpi-forum.org>
>> > Subject: Re: [Mpi-forum] MPI_DATATYPE_NULL when count=0
>> >
>> > Yes, NULL buffers are valid - they aren¹t an MPI object. There is no
>> prohibition about passing them; only if they are used to form an invalid
>> address (note that a datatype with absolute addresses relies on NULL as the
>> buffer address).
>> >
>> > Bill
>> >
>> >
>> > William Gropp
>> > Director, Parallel Computing Institute
>> > Thomas M. Siebel Chair in Computer Science
>> > Chief Scientist, NCSA
>> > University of Illinois Urbana-Champaign
>> >
>> >
>> >
>> >
>> > On Jan 12, 2016, at 9:49 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
>> >
>> >
>> > Who wrote Example 4.23 of MPI 3.1?
>> >
>> > I propose to add the exception rather than delete that example. I do not
>> want to have to use real datatypes when sending nothing.
>> >
>> > Is NULL a valid buffer handle? Can I pass null pointers when count=0?
>> >
>> >
>> > Jeff
>> >
>> >
>> > On Tuesday, January 12, 2016, William Gropp <wgropp at illinois.edu> wrote:
>> >
>> > The standard says that the null handles are invalid for input unless
>> explicitly permitted. There¹s no exemption for a datatype argument where the
>> associated count is zero.
>> >
>> > Bill
>> >
>> >
>> > William Gropp
>> > Director, Parallel Computing Institute
>> > Thomas M. Siebel Chair in Computer Science
>> > Chief Scientist, NCSA
>> > University of Illinois Urbana-Champaign
>> >
>> >
>> >
>> >
>> > On Jan 12, 2016, at 9:26 PM, Jeff Hammond <jeff.science at gmail.com> wrote:
>> >
>> >
>> > Is it allowed to communicate messages with
>> (buffer=NULL,count=0,type=MPI_DATATYPE_NULL)?
>> >
>> > George thinks MPI_DATATYPE_NULL cannot be used even when count=0, which I
>> think is ridiculous, particularly when one makes the analogy to buffer=NULL.
>> >
>> > See
>> http://secure-web.cisco.com/1uIwjXiVB0UCnFFG4b2DNhy-WFSpBHoZVan-ksg1oo1XCSjcK
>> asJCWljyye8xZYsJX2A72ZewdK6RzVNcE2jWKnCqVVFTmp9Oprj6gbb1orgQeDMDduaWKuVGKXKbR
>> sPnX2cJZeGodMtDUQx5HPmA46Qmql4kBAvZDR2KnWoFOQKuotirK6FQIazgUFBlC9CPP8fkF0TFKw
>> 4OIzxQJ_Tp2Xe-mRb3swzYiungQAHNUVs53lxZSOLwP432qi3ovJ8cFw4G6XCXEkE37Xx2rGYhrsH
>> iZQy03zUFRmYy13QckBzIrtfY1zG5piXPVIOCfIoFfvm4o-tUjgyZSzspCUye23lLO2G1ic7fjL5O
>> sxTrokI/http%3A%2F%2Fwww.open-mpi.org%2Fcommunity%2Flists%2Fusers%2F2016%2F01
>> %2F28255.php for the thread in question.
>> >
>> > There is example code in MPI that uses this behavior. George and I agree
>> that example text is non-normative, but I cannot imagine how this example
>> came to exist without the Forum believing it to be valid.
>> >
>> > Jeff
>> >
>> > --
>> > Jeff Hammond
>> > jeff.science at gmail.com
>> > http://jeffhammond.github.io/
>> > _______________________________________________
>> > mpi-forum mailing list
>> > mpi-forum at lists.mpi-forum.org
>> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
>> >
>> >
>> >
>> >
>> >
>> > --
>> > Jeff Hammond
>> > jeff.science at gmail.com
>> > http://jeffhammond.github.io/
>> > _______________________________________________
>> > mpi-forum mailing list
>> > mpi-forum at lists.mpi-forum.org
>> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
>> >
>> >
>> > _______________________________________________
>> > mpi-forum mailing list
>> > mpi-forum at lists.mpi-forum.org
>> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
>> >
>> >
>> >
>> > _______________________________________________
>> > mpi-forum mailing list
>> > mpi-forum at lists.mpi-forum.org
>> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
>
>
>
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
>
> _______________________________________________
> mpi-forum mailing list
> mpi-forum at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi-forum
--
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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