Hi,
OpenMPI-1.7.1 is announce support MPI-3 functionality such as non-blocking
collectives.
I have test MPI_Iallgatherv on a 8-node cluster, however, I got bad
performance. The MPI_Iallgatherv block the program for even longer time
than traditional 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 = 811,630 us
comm_time = 342,284 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 = 817,397 us
comm_time = 1,183,511 us
wait_time = 1,294,330 us
==============================
>From the result, we can tell that MPI_Iallgatherv block the program for
1,183,511 us, much longer than that of MPI_Allgatherv, which is 342,284 us.
Even worse, it still take 1,294,330 us to wait for the non-blocking
MPI_Iallgatherv to finish.
- Zehan Cui
