Hello again,
What is the default behaviour when two MPI processes try to perform an
MPI_Send at the same time?
Is this type of behaviour allowed in MPI, if so does it require the use
of non-blocking sends (or some type of buffer?).
I've managed to achieve deadlock with two MPI processes stuck in
MPI_Send calls to one another? After outr previous discussions I would
have thought the sends would have completed by placing the messages into
a buffer before sending? However, it seems both MPI_Sends are stuck in
the spinklock() in lam_ssi_rpi_usysv_low_fastsend()?
Thanks for all your help
Mike
Michael Lees wrote:
>
> Jeff Squyres wrote:
>
>>On Aug 12, 2005, at 11:22 AM, Michael Lees wrote:
>>
>>
>>
>>>>>I have a thread for performing asynchronous sending and receiving,
>>>>>something along the lines of...
>>>>>
>>>>>while(running){
>>>>>MPI_IRecv(&request)
>>>>>While(recvflag ==0){
>>>>> MPI_Test(request,recvflag)
>>>>> MPI_Send()
>>>>> yield()
>>>>>}
>>>>> yield()
>>>>>}
>>>>
>>>>
>>>>Wow, that's a lot of sending. :-)
>>>
>>>Is it though? As far as I understand it the blocking send will wait
>>>until a matching receive is posted? Am I wrong? So they'll only be one
>>>send per receive?
>>
>>
>>No, not necessarily. Check out the sidebar entitled "To block or not
>>to block" in this column for an explanation:
>>
>> http://cw.squyres.com/columns/2004-02-CW-MPI-Mechanic.pdf
>>
>>Short version: if you are sending a short message, you may actually be
>>sending many times before your receive completes (it's a parallel race
>>condition). If you're trying to simply match a single send and a
>>single receive, you might want to use MPI_SENDRECV or a pattern similar
>>to:
>>
>>MPI_Irecv(...)
>>MPI_Isend(...);
>>MPI_Waitall(...);
>>
>>
>
>
>
> I was thinking some more about the issue of sudden slow down in my
> application. After reading about the Buffered implementation of MPI_Send
> and MPI_Recv I was wondering if my performance could be reducing when
> the buffer fills?
> Each message is 132 bytes so the thread design above will allow a lots
> of sends to occur before the internal buffer is full(the buffer is
> default - which is 64k?). This could result in 496 messages being sent
> before the buffer fills up.
>
> What happens when a small message stored in the buffer is sent - does it
> empty the used buffer immediately? Or is the buffer reclaimed when
> needed ie., when full, or is there some type of garbage collection?
>
> I can't figure out what is causing the sudden massive drop in
> peformance. I've used gkrellm to monitor memory and cpu usage and both
> seem fairly constant and no paging is done at all. The other odd thing
> is that it doesn't matter if I allocate 11 processes to one cpu or 11
> processes to 3 cpus - the performance drop happens at about the same point.
>
> Is there a decent free tool for monitoring/profiling mpi programs?
>
> Cheers
>
> -Mike
>
>
>
>
>
>
>
>
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