[mpich-discuss] Poor scaling of MPI_WIN_CREATE?

[Jim Dinan]

Hi Tim,

How often are you creating windows?  As Jed mentioned, this is expected 
to be fairly expensive and synchronizing on most systems.  The Cray XE 
has some special sauce that can make this cheap if you go through DMAPP 
directly, but if you want your performance tuning to be portable, taking 
window creation off the critical path would be a good change to make.

  ~Jim.

On 5/30/12 10:48 AM, Timothy Stitt wrote:
> Thanks Jeff...you provided some good suggestions. I'll consult the DMAPP
> documentation and also go back to the code to see if I can reuse window
> buffers in some way.
>
> Would you happen to have links to the DMAPP docs on-hand? I couldn't
> seem to find any tutorials etc. after a quick browse.
>
> Cheers,
>
> Tim.
>
> On May 30, 2012, at 11:40 AM, Jeff Hammond wrote:
>
>> If you don't care about portability, translating from MPI-2 RMA to
>> DMAPP is mostly trivial and you can eliminate collective window
>> creation altogether. However, I will note that my experience getting
>> MPI and DMAPP to inter-operate properly on XE6 (Hopper, in fact) was
>> terrible. And yes, I did everything the NERSC documentation and Cray
>> told me to do.
>>
>> I wonder if you can reduce the time spent in MPI_WIN_CREATE by calling
>> it less often. Can you not allocate the window once and keep reusing
>> it? You might need to restructure your code to reuse the underlying
>> local buffers but that isn't that complicated in some cases.
>>
>> Best,
>>
>> Jeff
>>
>> On Wed, May 30, 2012 at 10:36 AM, Jed Brown <jedbrown at mcs.anl.gov
>> <mailto:jedbrown at mcs.anl.gov>> wrote:
>>> On Wed, May 30, 2012 at 10:29 AM, Timothy Stitt
>>> <Timothy.Stitt.9 at nd.edu <mailto:Timothy.Stitt.9 at nd.edu>>
>>> wrote:
>>>>
>>>> Hi all,
>>>>
>>>> I am currently trying to improve the scaling of a CFD code on some Cray
>>>> machines at NERSC (I believe Cray systems leverage mpich2 for their MPI
>>>> communications, hence the posting to this list) and I am running
>>>> into some
>>>> scalability issues with the MPI_WIN_CREATE() routine.
>>>>
>>>> To cut a long story short, the CFD code requires each process to receive
>>>> values from some neighborhood processes. Unfortunately, each process
>>>> doesn't
>>>> know who its neighbors should be in advance.
>>>
>>>
>>> How often do the neighbors change? By what mechanism?
>>>
>>>>
>>>> To overcome this we exploit the one-sided MPI_PUT() routine to
>>>> communicate
>>>> data from neighbors directly.
>>>>
>>>> Recent profiling at 256, 512 and 1024 processes shows that the
>>>> MPI_WIN_CREATE routine is starting to dominate the walltime and
>>>> reduce our
>>>> scalability quite rapidly. For instance the %walltime for MPI_WIN_CREATE
>>>> over various process sizes increases as follows:
>>>>
>>>> 256 cores - 4.0%
>>>> 512 cores - 9.8%
>>>> 1024 cores - 24.3%
>>>
>>>
>>> The current implementation of MPI_Win_create uses an Allgather which is
>>> synchronizing and relatively expensive.
>>>
>>>>
>>>>
>>>> I was wondering if anyone in the MPICH2 community had any advice on how
>>>> one can improve the performance of MPI_WIN_CREATE? Or maybe someone
>>>> has a
>>>> better strategy for communicating the data that bypasses the (poorly
>>>> scaling?) MPI_WIN_CREATE routine.
>>>>
>>>> Thanks in advance for any help you can provide.
>>>>
>>>> Regards,
>>>>
>>>> Tim.
>>>> _______________________________________________
>>>> mpich-discuss mailing list mpich-discuss at mcs.anl.gov
>>>> <mailto:mpich-discuss at mcs.anl.gov>
>>>> To manage subscription options or unsubscribe:
>>>> https://lists.mcs.anl.gov/mailman/listinfo/mpich-discuss
>>>
>>>
>>>
>>> _______________________________________________
>>> mpich-discuss mailing list mpich-discuss at mcs.anl.gov
>>> <mailto:mpich-discuss at mcs.anl.gov>
>>> To manage subscription options or unsubscribe:
>>> https://lists.mcs.anl.gov/mailman/listinfo/mpich-discuss
>>>
>>
>>
>>
>> --
>> Jeff Hammond
>> Argonne Leadership Computing Facility
>> University of Chicago Computation Institute
>> jhammond at alcf.anl.gov <mailto:jhammond at alcf.anl.gov> / (630) 252-5381
>> http://www.linkedin.com/in/jeffhammond
>> https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond
>> _______________________________________________
>> mpich-discuss mailing list mpich-discuss at mcs.anl.gov
>> To manage subscription options or unsubscribe:
>> https://lists.mcs.anl.gov/mailman/listinfo/mpich-discuss
>
> *Tim Stitt*PhD(User Support Manager).
> Center for Research Computing | University of Notre Dame |
> P.O. Box 539, Notre Dame, IN 46556 | Phone: 574-631-5287 | Email:
> tstitt at nd.edu <mailto:tstitt at nd.edu>
>
>
>
> _______________________________________________
> mpich-discuss mailing list     mpich-discuss at mcs.anl.gov
> To manage subscription options or unsubscribe:
> https://lists.mcs.anl.gov/mailman/listinfo/mpich-discuss