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Subject: Re: [OMPI users] openib btl slows down application
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2010-01-04 14:02:17


First, a small comment:  use the MPI standard timer MPI_Wtime() instead of gettimeofday.

I think the problem is that MPI_Sendrecv_replace needs a temporary buffer.  Unless the message is very small, the function uses MPI_Alloc_mem and MPI_Free_mem to allocate and free this temporary buffer.  When openib is available, I guess it uses mca_mpool_rdma_alloc and mca_mpool_rdma_register, which are relatively expensive.

Here is an experiment you can try.  Write a PMPI wrapper for MPI_Sendrecv_replace().  That is, provide your own function that's called MPI_Sendrecv_replace() and inside it call PMPI_Sendrecv_replace().  Then, play with alternative implementations of MPI_Sendrecv_replace.  E.g., allocate your own memory for the temporary buffer and then call PMPI_Sendrecv().  Try allocating your temporary memory with MPI_Alloc_mem/MPI_Free_mem and also try with malloc/free.  I expect you'll see the timings you're seeing now.

Maybe the experts on this list can comment on what *should* be happening inside OMPI.

Meanwhile, you should probably avoid MPI_Sendrecv_replace if you care about performance.  The function is mostly a convenience function and if you care about performance you'd be safest, if you're going to run with different MPIs, to use MPI_Sendrecv instead.  That means you need a send buffer and a receive buffer.  A little more hassle, perhaps, but it means you have better control over the performance characteristics.  E.g., you won't have all those extra allocs/frees, which is what you almost surely have with most MPI implementations.

Götz Waschk wrote:
Hi everyone,

I'm seeing a very strange effect with the openib btl. It seems to slow
down my application even if not used at all. For my test, I am running
a simple application with 8 processes on a single node, so openib
should not be used at all. Still, the result with the btl enabled is
much worse:

% /usr/lib64/openmpi/1.3.2-gcc/bin/mpirun -np 8 --mca btl
self,sm,openib ./a.out
11 tests with 2 x 2 x 2 processes: L0 = 32, L1xL2 = 256 DP spinors
Overlap over 8 processes: 271769.0 usec
Overlap over 8 processes: 298237.0 usec
Overlap over 8 processes: 261648.0 usec
Overlap over 8 processes: 369170.0 usec
Overlap over 8 processes: 383065.0 usec
Overlap over 8 processes: 280675.0 usec
Overlap over 8 processes: 270912.0 usec
Overlap over 8 processes: 198789.0 usec
Overlap over 8 processes: 339857.0 usec
Overlap over 8 processes: 192087.0 usec
Overlap over 8 processes: 209025.0 usec
Average of 10 measurements (skipped first) on 8 processes: 280.3 msec

% /usr/lib64/openmpi/1.3.2-gcc/bin/mpirun -np 8 --mca btl self,sm ./a.out
11 tests with 2 x 2 x 2 processes: L0 = 32, L1xL2 = 256 DP spinors
Overlap over 8 processes: 7445.0 usec
Overlap over 8 processes: 7355.0 usec
Overlap over 8 processes: 7311.0 usec
Overlap over 8 processes: 7473.0 usec
Overlap over 8 processes: 7409.0 usec
Overlap over 8 processes: 7449.0 usec
Overlap over 8 processes: 7261.0 usec
Overlap over 8 processes: 7451.0 usec
Overlap over 8 processes: 7430.0 usec
Overlap over 8 processes: 7320.0 usec
Overlap over 8 processes: 7384.0 usec
Average of 10 measurements (skipped first) on 8 processes: 7384.3 usec

This is the default openmpi as shipped with SL5.4 (based on RHEL5.4).
I have also tested openmpi 1.4, same result. The other mpi shipped by
Red Hat (mvapich2) does not show this problem. Any idea?

Regards, Götz Waschk

  

/* Benchmark of MPI_Sendrecv_replace as in get_overlaps_spinor_tslice of ddhqet hs 15.12.2009 */ #include <stdio.h> #include <stdlib.h> #include <time.h> #include <mpi.h> #define NMEAS 11 #define L0 32 #define L1 16 #define L2 16 int nproc1, nproc2, nproc3; int nrank(int n1, int n2, int n3) { while( n1 < 0 ) n1 += nproc1; while( n2 < 0 ) n2 += nproc2; while( n3 < 0 ) n3 += nproc3; while( n1 >= nproc1) n1 -= nproc1; while( n2 >= nproc2) n2 -= nproc2; while( n3 >= nproc3) n3 -= nproc3; return n1*nproc2*nproc3 + n2*nproc3 + n3; } /********************************************************************/ int main(int argc ,char **argv){ int rank, nproc, err, i, x0; MPI_Status status; struct timeval time; double t0, t1, tsum=0; double buffer[L1*L2*24]; int tcnt=0; int n1, n2, n3; int nup[3]; int ndn[3]; err=MPI_Init(&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &rank); MPI_Comm_size(MPI_COMM_WORLD, &nproc); /* Choose process grid */ i = nproc; nproc1 = nproc2 = nproc3 = 1; while ( i%2 == 0 ) { if ( i%2 == 0 ) { nproc1 *=2; i/=2; } if ( i%2 == 0 ) { nproc2 *=2; i/=2; } if ( i%2 == 0 ) { nproc3 *=2; i/=2; } } nproc3 *= i; /* Initialize lookup table for process grid */ for(n1=0; n1 < nproc1; n1++ ) { for(n2=0; n2 < nproc2; n2++ ) { for(n3=0; n3 < nproc3; n3++ ) { if ( rank == nrank(n1,n2,n3) ) { nup[0] = nrank(n1+1,n2,n3); nup[1] = nrank(n1,n2+1,n3); nup[2] = nrank(n1,n2,n3+1); ndn[0] = nrank(n1-1,n2,n3); ndn[1] = nrank(n1,n2-1,n3); ndn[2] = nrank(n1,n2,n3-1); #ifdef DEBUG printf("Rank %3d: nup = %3d %3d %3d ndn = %3d %3d %3d\n", rank, nup[0], nup[1], nup[2], ndn[0], ndn[1], ndn[2]); #endif } } } } if( rank == 0 ) printf("%d tests with %d x %d x %d processes: L0 = %d, L1xL2 = %d DP spinors\n", NMEAS, nproc1, nproc2, nproc3, L0, L1*L2); MPI_Barrier(MPI_COMM_WORLD); for(i=0; i<NMEAS; i++) { gettimeofday(&time,NULL); t0=time.tv_sec*1e6+time.tv_usec; for(x0=0; x0<L0; x0++) { MPI_Sendrecv_replace(buffer, L1*L2*24, MPI_DOUBLE, nup[0], 81, ndn[0], 81, MPI_COMM_WORLD, &status); MPI_Sendrecv_replace(buffer, L1*L2*24, MPI_DOUBLE, ndn[0], 82, nup[0], 82, MPI_COMM_WORLD, &status); MPI_Sendrecv_replace(buffer, L1*L2*24, MPI_DOUBLE, nup[1], 83, ndn[1], 83, MPI_COMM_WORLD, &status); MPI_Sendrecv_replace(buffer, L1*L2*24, MPI_DOUBLE, ndn[1], 84, nup[1], 84, MPI_COMM_WORLD, &status); MPI_Sendrecv_replace(buffer, L1*L2*24, MPI_DOUBLE, nup[2], 85, ndn[2], 85, MPI_COMM_WORLD, &status); MPI_Sendrecv_replace(buffer, L1*L2*24, MPI_DOUBLE, ndn[2], 86, nup[2], 86, MPI_COMM_WORLD, &status); } gettimeofday(&time,NULL); t1=time.tv_sec*1e6+time.tv_usec; if ( rank == 0 ) { // if ( t1-t0 > 10000 ) { if ( 0 ) { printf("Overlap over %d processes: %.1f msec\n", nproc, (t1-t0)/1000); } else { printf("Overlap over %d processes: %.1f usec\n", nproc, (t1-t0)); } if ( i>0 ) { tcnt++; tsum += (t1-t0); } } } if ( rank == 0 && tcnt > 0 ) { tsum = tsum/tcnt; if ( tsum > 10000 ) printf("Average of %d measurements (skipped first) on %d processes: %.1f msec\n", tcnt, nproc, tsum/1000); else printf("Average of %d measurements (skipped first) on %d processes: %.1f usec\n", tcnt, nproc, tsum); } MPI_Finalize(); return 0; }