[mpich-discuss] Better alternatives of MPI_Allreduce()

[Benson Muite]

On Tue, May 5, 2020, at 9:57 AM, Benson Muite wrote:
> 
> 
> 
> On Tue, May 5, 2020, at 8:38 AM, hritikesh semwal via discuss wrote:
>> Hello all,
>> 
>> I am working on the development of a parallel CFD solver and I am using MPI_Allreduce for the global summation of the local errors calculated on all processes of a group and the summation is to be used by all the processes. My concern is that MPI_Allreduce is taking almost 27-30% of the total time used, which is a significant amount. So, I want to ask if anyone can suggest me better alternative/s to replace MPI_Allreduce which can reduce the time consumption.
>> 
>> Thank you.
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> 
> Hi Hitesh,
> 
> What hardware are you running on and what is the interconnect?
> Have you tried changing any of the MPI settings?
> Can the reduction be done asynchronously?
> 
> Regards,
> Benson

Also, is your work load balanced? One way to check this might be to place a barrier just before the all-reduce call. If the barrier ends up taking most of your time, then it is likely you will need to determine a better way to distribute the computational work.
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