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Subject: Re: [OMPI users] (no subject)
From: Alberto Giannetti (albertogiannetti_at_[hidden])
Date: 2008-04-23 12:19:43


Thanks Torje. I wonder what is the benefit of looping on the incoming
message-queue socket rather than using system I/O signals, like read
() or select().

On Apr 23, 2008, at 12:10 PM, Torje Henriksen wrote:
> Hi Alberto,
>
> The blocked processes are in fact spin-waiting. While they don't have
> anything better to do (waiting for that message), they will check
> their incoming message-queues in a loop.
>
> So the MPI_Recv()-operation is blocking, but it doesn't mean that the
> processes are blocked by the OS scheduler.
>
>
> I hope that made some sense :)
>
>
> Best regards,
>
> Torje
>
>
> On Apr 23, 2008, at 5:34 PM, Alberto Giannetti wrote:
>
>> I have simple MPI program that sends data to processor rank 0. The
>> communication works well but when I run the program on more than 2
>> processors (-np 4) the extra receivers waiting for data run on > 90%
>> CPU load. I understand MPI_Recv() is a blocking operation, but why
>> does it consume so much CPU compared to a regular system read()?
>>
>>
>>
>> #include <sys/types.h>
>> #include <unistd.h>
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <mpi.h>
>>
>> void process_sender(int);
>> void process_receiver(int);
>>
>>
>> int main(int argc, char* argv[])
>> {
>> int rank;
>>
>> MPI_Init(&argc, &argv);
>> MPI_Comm_rank(MPI_COMM_WORLD, &rank);
>>
>> printf("Processor %d (%d) initialized\n", rank, getpid());
>>
>> if( rank == 1 )
>> process_sender(rank);
>> else
>> process_receiver(rank);
>>
>> MPI_Finalize();
>> }
>>
>>
>> void process_sender(int rank)
>> {
>> int i, j, size;
>> float data[100];
>> MPI_Status status;
>>
>> printf("Processor %d initializing data...\n", rank);
>> for( i = 0; i < 100; ++i )
>> data[i] = i;
>>
>> MPI_Comm_size(MPI_COMM_WORLD, &size);
>>
>> printf("Processor %d sending data...\n", rank);
>> MPI_Send(data, 100, MPI_FLOAT, 0, 55, MPI_COMM_WORLD);
>> printf("Processor %d sent data\n", rank);
>> }
>>
>>
>> void process_receiver(int rank)
>> {
>> int count;
>> float value[200];
>> MPI_Status status;
>>
>> printf("Processor %d waiting for data...\n", rank);
>> MPI_Recv(value, 200, MPI_FLOAT, MPI_ANY_SOURCE, 55,
>> MPI_COMM_WORLD, &status);
>> printf("Processor %d Got data from processor %d\n", rank,
>> status.MPI_SOURCE);
>> MPI_Get_count(&status, MPI_FLOAT, &count);
>> printf("Processor %d, Got %d elements\n", rank, count);
>> }
>>
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