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Subject: Re: [OMPI users] (no subject)
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2008-04-23 16:52:07


On Apr 23, 2008, at 3:49 PM, Danesh Daroui wrote:

> 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.

It depends on what you're optimizing for. :-) We're optimizing for
minimum message passing latency on hosts that are not oversubscribed;
polling is very good at that. Polling is much better than blocking,
particularly if the blocking involves a system call (which will be
"slow"). Note that in a compute-heavy environment, they nodes are
going to be running at 100% CPU anyway.

Also keep in mind that you're only going to have "waste" spinning in
MPI if you have a loosely/poorly synchronized application. Granted,
some applications are this way by nature, but we have not chosen to
optimize spare CPU cycles for them. As I said in a prior mail, adding
a blocking strategy is on the to-do list, but it's fairly low in
priority right now. Someone may care / improve the message passing
engine to include blocking, but it hasn't happened yet. Want to work
on it? :-)

And for reference: almost all MPI's do busy polling to minimize
latency. Some of them will shift to blocking if nothing happens for a
"long" time. This second piece is what OMPI is lacking.

> Why
> don't you use blocking and/or signals instead of
> that?

FWIW: I mentioned this in my other mail -- latency is quite definitely
negatively impacted when you use such mechanisms. Blocking and
signals are "slow" (in comparison to polling).

> I think the priority of this task is very high because polling
> just wastes resources of the system.

In production HPC environments, the entire resource is dedicated to
the MPI app anyway, so there's nothing else that really needs it. So
we allow them to busy-spin.

There is a mode to call yield() in the middle of every OMPI progress
loop, but it's only helpful for loosely/poorly synchronized MPI apps
and ones that use TCP or shared memory. Low latency networks such as
IB or Myrinet won't be as friendly to this setting because they're
busy polling (i.e., they call yield() much less frequently, if at all).

> 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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>>>>>
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-- 
Jeff Squyres
Cisco Systems