[mpich-discuss] Active loop in MPI_Waitany?

[Jeff Hammond]

Everyone on this list doesn't know the guts of nemesis?!?!? :-)

Jeff

PS ch3:sock might give you different results ;-)

On Thursday, July 30, 2015, Dorier, Matthieu <mdorier at anl.gov> wrote:

> Not that obvious, a UNIX function like "select" would allow to block on a
> set of file descriptors (for instance sockets), so I was expecting
> MPI_Waitany to work kind of the same way. Pavan explained me (in person)
> why it does not: it cannot at the same time block on a read from shared
> memory (channels inside a node) and on network connections (outside the
> node). (I'm simplifying a bit here).
>
> So the answer is: yes, MPI_Waitany is actively looping.
>
> Matthieu
>
> ------------------------------
> *From:* Jeff Hammond [jeff.science at gmail.com
> <javascript:_e(%7B%7D,'cvml','jeff.science at gmail.com');>]
> *Sent:* Thursday, July 30, 2015 6:41 PM
> *To:* discuss at mpich.org
> <javascript:_e(%7B%7D,'cvml','discuss at mpich.org');>
> *Subject:* Re: [mpich-discuss] Active loop in MPI_Waitany?
>
> Seems obvious that Waitany spins on the array of requests until one
> completes. Is that an active loop by your definition?
>
> Jeff
>
> On Thursday, July 30, 2015, Dorier, Matthieu <mdorier at anl.gov
> <javascript:_e(%7B%7D,'cvml','mdorier at anl.gov');>> wrote:
>
>> Hi,
>>
>> I have a code that looks like this:
>>
>> while(true) {
>>    do some I/O (HDF5 POSIX output to a remote, parallel file system)
>>    wait for communication (MPI_Waitany) from other processes (in the same
>> node and outside the node)
>> }
>>
>> I'm measuring the energy consumption of the node that runs this process
>> for the same duration, as a function of the amount of data written in each
>> I/O operation.
>> Surprisingly, the larger the I/O in proposition to the communication, the
>> lower the energy consumption. In other words, the longer I wait in
>> MPI_Waitany, the more I consume.
>>
>> Does anyone have a good explanation for that? Is there an active loop in
>> MPI_Waitany? Another reason?
>>
>> Thanks!
>>
>> Matthieu
>>
>
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> <javascript:_e(%7B%7D,'cvml','jeff.science at gmail.com');>
> http://jeffhammond.github.io/
>


-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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