Thanks for your response. Yes, you are right. I have put barrier just before Allreduce and out of the total time consumed by Allreduce, 79% time is consumed by the barrier. But my computational work is balanced. Right now, I have distributed 97336 cells among 24 processors and maximum and minimum cell distribution among all processors is 4057 and 4055 respectively which is not too bad. Is there any solution to get rid of this. On Tue, May 5, 2020 at 12:30 PM Joachim Protze <[email protected]> wrote:

Hello,

it is important to understand, that most of the time you see is not the cost of the allreduce, but the cost of synchronization (caused by load imbalance).

You can do an easy experiment and add a barrier before the allreduce. Then you will see the actual cost of the allreduce, while the cost of synchronization will go into the barrier.

Now, think about dependencies in your algorithm: do you need the output value immediately? Is this the same time, where you have the input value ready? -> otherwise use non-blocking communication and perform independent work in between

In any case: fix your load imbalance (the root cause of synchronization cost).

Best Joachim

Am 05.05.20 um 07:38 schrieb hritikesh semwal via discuss:

...

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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-- Dipl.-Inf. Joachim Protze

IT Center Group: High Performance Computing Division: Computational Science and Engineering RWTH Aachen University Seffenter Weg 23 D 52074 Aachen (Germany) Tel: +49 241 80- 24765 Fax: +49 241 80-624765 [email protected] www.itc.rwth-aachen.de