Do you really mean that Open-MPI uses busy loop in order to handle
incomming calls? It seems to be incorrect since
spinning is a very bad and inefficient technique for this purpose. Why
don't you use blocking and/or signals instead of
that? I think the priority of this task is very high because polling
just wastes resources of the system. On the other hand,
what Alberto claims is not reasonable to me.
Alberto,
- Are you oversubscribing one node which means that you are running your
code on a single processor machine, pretending
to have four CPUs?
- Did you compile Open-MPI or installed from RPM?
Receiving process shouldn't be that expensive.
Regards,
Danesh
Jeff Squyres skrev:
Because on-node communication typically uses shared memory, so we
currently have to poll. Additionally, when using mixed on/off-node
communication, we have to alternate between polling shared memory and
polling the network.
Additionally, we actively poll because it's the best way to lower
latency. MPI implementations are almost always first judged on their
latency, not [usually] their CPU utilization. Going to sleep in a
blocking system call will definitely negatively impact latency.
We have plans for implementing the "spin for a while and then block"
technique (as has been used in other MPI's and middleware layers), but
it hasn't been a high priority.
On Apr 23, 2008, at 12:19 PM, Alberto Giannetti wrote:
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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