[mvapich-discuss] Performance of MPI_Iallgatherv

[Akshay Venkatesh]

Zehan,

Can you provide a reproducer for your program (with relevant input
parameters) where you see the degradation with MPI_Iallgatherv() use? It'd
be helpful if you could provide details of the system and the configuration
(compilers used and MVAPICH2 library configuration flags) you used to run
the program.

Thanks


>
> ---------- Forwarded message ----------
> From: Zehan Cui <zehan.cui at gmail.com>
> Date: Tue, Apr 1, 2014 at 3:28 AM
> Subject: [mvapich-discuss] Performance of MPI_Iallgatherv
> To: mvapich-discuss at cse.ohio-state.edu
>
>
> Hi,
>
> I'm testing the non-blocking collective of MVAPICH2-2.0rc1.
>
> I have two nodes with Infiniband to perform allgather on totally 128MB
> data.
>
> I split the 128MB data into eight pieces, and perform computation and
> MPI_Iallgatherv() on one piece of data each iteration, hoping that the
> MPI_Iallgatherv() of last iteration can be overlapped with computation of
> current iteration. A MPI_Wait() is called at the end of last iteration.
>
> However, the total communication time (including the final wait time) even
> exceeds the traditional blocking MPI_Allgatherv.
>
>
> Following is the test pseudo-code and result.
>
> ===========================
>
> Using MPI_Allgatherv:
>
> for( i=0; i<8; i++ )
> {
>   // computation
>     mytime( t_begin );
>     computation;
>     mytime( t_end );
>     comp_time += (t_end - t_begin);
>
>   // communication
>     t_begin = t_end;
>     MPI_Allgatherv();
>     mytime( t_end );
>     comm_time += (t_end - t_begin);
> }
>
> result:
>     comp_time = 7,454,219 us
>     comm_time = 399,854 us
>
> --------------------------------------------
>
> Using MPI_Iallgatherv:
>
> for( i=0; i<8; i++ )
> {
>   // computation
>     mytime( t_begin );
>     computation;
>     mytime( t_end );
>     comp_time += (t_end - t_begin);
>
>   // communication
>     t_begin = t_end;
>     MPI_Iallgatherv();
>     mytime( t_end );
>     comm_time += (t_end - t_begin);
> }
>
> // wait for non-blocking allgather to complete
> mytime( t_begin );
> for( i=0; i<8; i++ )
>     MPI_Wait;
> mytime( t_end );
> wait_time = t_end - t_begin;
>
> result:
>     comp_time = 7,453,938 us
>     comm_time = 365,511 us
>     wait_time = 453,132 us
>
> ==============================
>
> It seems that MPI_Iallgatherv() is more blocking than MPI_Allgather().
>
> Does anyone have the same experience?
>
>
> Thanks,
> Zehan
>
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>
>


-- 
- Akshay
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