[mpich-discuss] beginner for remote mem access (sufeng)

Sufeng Niu sniu at hawk.iit.edu
Fri Jul 5 11:18:22 CDT 2013

Hi Pavan,

Thanks a lot!

Now I make sense for MPI_Win_fence.

for hybrid RMA issue, did you mean in each rank, remote get the data into a
shared memory, then threads in each rank load it by thread-level operation?

Thank you!

On Fri, Jul 5, 2013 at 10:36 AM, Pavan Balaji <balaji at mcs.anl.gov> wrote:

> On 07/05/2013 10:28 AM, Sufeng Niu wrote:
>> 1.
>> MPI_Win_fence is not similar to MPI_Barrier in some cases.  The
>> MPICH implementation can turn some calls into no-ops and others into
>> MPI_Reduce_scatter.
>> I am not quite sure about "turn some calls into no-ops and others into
>> MPI_Reduce_scatter" Could you please give an example if possible?
>> Another thing is if a process create a window for other process, but the
>> data access is available after some operations. Should I use
>> MPI_Win_fence or MPI_Barrier to sync? or other methods?
> MPI_WIN_FENCE will make sure that all PUT/GET/ACCUMULATE operations are
> complete locally and remotely.  This is probably what you need.
> MPI_BARRIER is useless in this case since PUT/GET/ACCUMULATE can be
> nonblocking.
>  2.
>> Regarding "use MPI window as thread level", I really don't
>> understand your thinking at all.  MPI RMA is not shared memory nor is
>> MPI a threading model.
>> Sorry for uncleared statements, right now I would like to do
>> multithreads and MPI hybrid programming. let me give an example: I have
>> 3 processes, each one has 8 threads. thread 0 in process 0 creates a RMA
>> window. if I would like all other threads to access it, should I use
>> thread 0 in process 1, thread 0 in process 2 to MPI_Get the data from
>> window, then use shared memory for internal threads to load the data? i
>> am not sure what is the proper way for RMA in hybrid model.
> All threads within the OS process are part of the same MPI rank.  Any of
> them can access the window as the same rank.
>  -- Pavan
> --
> Pavan Balaji
> http://www.mcs.anl.gov/~balaji

Best Regards,
Sufeng Niu
ECASP lab, ECE department, Illinois Institute of Technology
Tel: 312-731-7219
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