Hi
I have the following small program where the rank-0 process does sleep
and then all the processes perform barrier().
#include "mpi.h"
#include <stdio.h>
int main(int argc, char *argv[])
{
int rank, nprocs;
MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD,&nprocs);
MPI_Comm_rank(MPI_COMM_WORLD,&rank);
if ( rank == 0) sleep(60);
MPI_Barrier(MPI_COMM_WORLD);
printf("Hello, world. I am %d of %d\n", rank,
nprocs);fflush(stdout);
MPI_Finalize();
return 0;
}
When I run this program on two nodes consuming 16 cores, I see that the
non rank-0 processes which are in wait mode for rank-0 process to
complete barrier() are consuming only user time. I was expecting this
behavior and there are no questions about it.
However if I initialize MPI threads by replacing MPI_Init() with
MPI_Init_thread(), I see quite a different behavior of this program.
While rank-0 process is sleeping, all non rank-0 processes seem to be
spending time in kernel mode (thus increasing system time) instead of
waiting in user mode.
Following is the sar output on the node where rank-0 process is running
Node1> sar 2 10
Linux 2.6.18-128.1.10.el5-perfctr (Node1) 10/29/2010
02:33:51 PM CPU %user %nice %system %iowait %steal
%idle
02:33:53 PM all 6.69 0.00 80.88 0.00 0.00
12.44
02:33:55 PM all 6.56 0.00 81.00 0.00 0.00
12.44
02:33:57 PM all 6.62 0.00 80.89 0.00 0.00
12.49
02:33:59 PM all 6.68 0.00 80.89 0.00 0.00
12.43
02:34:01 PM all 6.69 0.00 81.00 0.00 0.00
12.31
02:34:03 PM all 6.75 0.00 80.76 0.00 0.00
12.49
02:34:05 PM all 6.75 0.00 80.82 0.00 0.00
12.43
02:34:07 PM all 6.75 0.00 81.19 0.00 0.00
12.06
02:34:09 PM all 6.93 0.00 80.64 0.00 0.00
12.43
02:34:11 PM all 6.75 0.00 80.81 0.00 0.00
12.44
Average: all 6.72 0.00 80.89 0.00 0.00
12.40
And following is the sar output on the second node:
Node2> sar 2 10
Linux 2.6.18-128.1.10.el5-perfctr (Node2) 10/29/2010
02:33:48 PM CPU %user %nice %system %iowait %steal
%idle
02:33:50 PM all 6.37 0.00 93.63 0.00 0.00
0.00
02:33:52 PM all 6.19 0.00 93.81 0.00 0.00
0.00
02:33:54 PM all 6.31 0.00 93.69 0.00 0.00
0.00
02:33:56 PM all 6.50 0.00 93.50 0.00 0.00
0.00
02:33:58 PM all 6.81 0.00 93.19 0.00 0.00
0.00
02:34:00 PM all 6.56 0.00 93.44 0.00 0.00
0.00
02:34:02 PM all 6.50 0.00 93.50 0.00 0.00
0.00
02:34:04 PM all 6.50 0.00 93.50 0.00 0.00
0.00
02:34:06 PM all 6.56 0.00 93.44 0.00 0.00
0.00
02:34:08 PM all 6.68 0.00 93.32 0.00 0.00
0.00
Average: all 6.50 0.00 93.50 0.00 0.00
0.00
Can someone please explain the difference in behavior of barrier() call
when I use MPI_Init() vs MPI_Init_thread()?
Thanks
Ananda
Ananda B Mudar, PMP
Senior Technical Architect
Wipro Technologies
Ph: 972 765 8093
[email protected]
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