Hi, Thank you for the reply.
However, using MPI_waitall instead of MPI_wait didn't solve the
problem. The code would hang at the MPI_waitall. Also, I'm not quit
understand why the code is inherently unsafe. Can the non-blocking
send or receive cause any deadlock?
Thanks!
Kong
On Mon, Feb 21, 2011 at 2:32 PM, Jeff Squyres <jsquy...@cisco.com> wrote:
> It's because you're waiting on the receive request to complete before the
> send request. This likely works locally because the message transfer is
> through shared memory and is fast, but it's still an inherently unsafe way to
> block waiting for completion (i.e., the receive might not complete if the
> send does not complete).
>
> What you probably want to do is build an array of 2 requests and then issue a
> single MPI_Waitall() on both of them. This will allow MPI to progress both
> requests simultaneously.
>
>
> On Feb 18, 2011, at 11:58 AM, Xianglong Kong wrote:
>
>> Hi, all,
>>
>> I’m an mpi newbie. I’m trying to connect two desktops in my office
>> with each other using a crossing cable and implement a parallel code
>> on them using MPI.
>>
>> Now, the two nodes can ssh to each other without password, and can
>> successfully run the MPI “Hello world” code. However, when I tried to
>> use multiple MPI non-blocking sends or receives, the job would hang.
>> The problem only showed up if the two processes are launched in the
>> different nodes, the code can run successfully if the two processes
>> are launched in the same node. Also, the code can run successfully if
>> there are only one send or/and one receive in each process.
>>
>> Here is the code that can run successfully:
>>
>> #include <stdlib.h>
>> #include <stdio.h>
>> #include <string.h>
>> #include <mpi.h>
>>
>> int main(int argc, char** argv) {
>>
>> int myrank, nprocs;
>>
>> MPI_Init(&argc, &argv);
>> MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
>> MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
>>
>> printf("Hello from processor %d of %d\n", myrank, nprocs);
>>
>> MPI_Request reqs1, reqs2;
>> MPI_Status stats1, stats2;
>>
>> int tag1=10;
>> int tag2=11;
>>
>> int buf;
>> int mesg;
>> int source=1-myrank;
>> int dest=1-myrank;
>>
>> if(myrank==0)
>> {
>> mesg=1;
>>
>> MPI_Irecv(&buf, 1, MPI_INT, source, tag1, MPI_COMM_WORLD,
>> &reqs1);
>> MPI_Isend(&mesg, 1, MPI_INT, dest, tag2, MPI_COMM_WORLD,
>> &reqs2);
>>
>>
>> }
>>
>> if(myrank==1)
>> {
>> mesg=2;
>>
>> MPI_Irecv(&buf, 1, MPI_INT, source, tag2, MPI_COMM_WORLD,
>> &reqs1);
>> MPI_Isend(&mesg, 1, MPI_INT, dest, tag1, MPI_COMM_WORLD,
>> &reqs2);
>> }
>>
>> MPI_Wait(&reqs1, &stats1);
>> printf("myrank=%d,received the message\n",myrank);
>>
>> MPI_Wait(&reqs2, &stats2);
>> printf("myrank=%d,sent the messages\n",myrank);
>>
>> printf("myrank=%d, buf=%d\n",myrank, buf);
>>
>> MPI_Finalize();
>> return 0;
>> }
>>
>> And here is the code that will hang
>>
>> #include <stdlib.h>
>> #include <stdio.h>
>> #include <string.h>
>> #include <mpi.h>
>>
>> int main(int argc, char** argv) {
>>
>> int myrank, nprocs;
>>
>> MPI_Init(&argc, &argv);
>> MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
>> MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
>>
>> printf("Hello from processor %d of %d\n", myrank, nprocs);
>>
>> MPI_Request reqs1, reqs2;
>> MPI_Status stats1, stats2;
>>
>> int tag1=10;
>> int tag2=11;
>>
>> int source=1-myrank;
>> int dest=1-myrank;
>>
>> if(myrank==0)
>> {
>> int buf1, buf2;
>>
>> MPI_Irecv(&buf1, 1, MPI_INT, source, tag1, MPI_COMM_WORLD,
>> &reqs1);
>> MPI_Irecv(&buf2, 1, MPI_INT, source, tag2, MPI_COMM_WORLD,
>> &reqs2);
>>
>> MPI_Wait(&reqs1, &stats1);
>> printf("received one message\n");
>>
>> MPI_Wait(&reqs2, &stats2);
>> printf("received two messages\n");
>>
>> printf("myrank=%d, buf1=%d, buf2=%d\n",myrank, buf1, buf2);
>> }
>>
>> if(myrank==1)
>> {
>> int mesg1=1;
>> int mesg2=2;
>>
>> MPI_Isend(&mesg1, 1, MPI_INT, dest, tag1, MPI_COMM_WORLD,
>> &reqs1);
>> MPI_Isend(&mesg2, 1, MPI_INT, dest, tag2, MPI_COMM_WORLD,
>> &reqs2);
>>
>> MPI_Wait(&reqs1, &stats1);
>> printf("sent one message\n");
>>
>> MPI_Wait(&reqs2, &stats2);
>> printf("sent two messages\n");
>> }
>>
>> MPI_Finalize();
>> return 0;
>> }
>>
>> And the output of the second failed code:
>> ***********************************************
>> Hello from processor 0 of 2
>>
>> Received one message
>>
>> Hello from processor 1 of 2
>>
>> Sent one message
>> *******************************************************
>>
>> Can anyone help to point out why the second code didn't work?
>>
>> Thanks!
>>
>> Kong
>>
>> _______________________________________________
>> users mailing list
>> us...@open-mpi.org
>> http://www.open-mpi.org/mailman/listinfo.cgi/users
>
>
> --
> Jeff Squyres
> jsquy...@cisco.com
> For corporate legal information go to:
> http://www.cisco.com/web/about/doing_business/legal/cri/
>
>
> _______________________________________________
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>
--
Xianglong Kong
Department of Mechanical Engineering
University of Rochester
Phone: (585)520-4412
MSN: dinosaur8...@hotmail.com
