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Subject: Re: [OMPI users] Is Iprobe fast when there is no message to recieve
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2009-10-03 07:05:30


Keep in mind that MPI says you do have to eventually receive the
message -- so just checking if it's there is not enough (eventually).
Iprobe is definitely one way. You could also post a non-blocking
receive (persistent or not) and MPI_TEST to see if it has completed.

However, if the message is long, MPI implementations like Open MPI
*may* require multiple invocations of the progression engine to
actually receive the entire message (e.g., it may get fragmented by
the sender and use a rendezvous protocol, therefore having multiple
states in the progression logic, each of which may only advance one or
two states in each call to MPI_TEST).

That being said, if you just want to send a quick "notify" that an
event has occurred, you might want to use a specific tag and/or
communicator for these extraordinary messages. Then, when the event
occurs, send a very short message on this special tag/communicator
(potentially even a 0-byte message). Open MPI will send short
messages eagerly and not require multiple states through a progression
machine (heck, just about all MPI's do this). You can MPI_TEST for
the completion of this short/0-byte receive very quickly. You can
then send the actual data of the event in a different non-blocking
receive that is only checked if the short "alert" message is received.

There are a small number of cases (e.g., resource exhaustion) where
Open MPI will have to fall back out of the eager send mode for short
messages, but in general, sending a short message with an alert and a
larger message with the actual data to be processed might be a good
choice.

On Oct 1, 2009, at 10:43 PM, Peter Lonjers wrote:

> I am not sure if this is the right place the ask this question but
> here
> it goes.
>
> Simplified abstract version of the question.
> I have 2 MPI processes and I want one to make an occasional signal to
> the other process. These signals will not happen at predictable
> times.
> I want the other process sitting in some kind of work loop to be
> able to
> make a very fast check to see if a signal has been sent to it.
>
> What is the best way to do this.
>
> Actual problem
> I am working on a realistic neural net simulator. The neurons are
> split
> into groups with one group to each processor to simulate them.
> Occasionally a neuron will spike and have to send that message to
> neurons on a different processor. This is a relatively rare event. The
> receiving neurons need to be able to make a very fast check to see if
> there is a message from neurons on another processor.
>
> The way I am doing it now is to use simple send and receive commands.
> The receiving cell does an iprobe check on every loop through the
> simulation for every cell that connects to it to see if there is a
> message(spike) from that cell. If the iprobe says there is a message
> is
> does a receive on that message.
>
> This seems convoluted though. I do not actually need to receive the
> message just know that a message is there. And it seems like depending
> on how Iprobe works there might be a faster method.
>
> Is Iprobe fast if there is no message to receive?
> Would persistent connections work better?
>
>
>
> Anyway any help would be greatly appreciated.
>
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>

-- 
Jeff Squyres
jsquyres_at_[hidden]