[mpich-discuss] MPICH associating process with a core

[Dave Goodell]

As Nicolas mentioned, you can completely control affinity with the  
hydra process manager.  Please see the link he posted (http://wiki.mcs.anl.gov/mpich2/index.php/Using_the_Hydra_Process_Manager#Process-core_Binding 
).

-Dave

On Mar 2, 2010, at 11:30 AM, John Xu wrote:

> Thanks a lot, Nicolas.
>
> I am running MPI on my simulator and trying to evaluate MPI
> performance with core hardware
> changes. Therefore, I prefer to have deterministic simulation output
> for mapping multiple processes into the processor
> which may have several cores in one SMP processor.
>
> My initial interest is to query which process falls on what core and
> therefore I can correlate the performance
> events with the process.
>
> It will be nice to control which process going to a particular core as
> well. But according to you, that is not
> very easy to do. I will check out the Wiki pages as pointed out by you
> to see if there is a way to do so.
>
> I hope the process will not migrate between the cores, at least at
> this stage of my evaluation.
> Right now, I am not observing such behavior as it appears.
> If it does, what is the proper way to enforce strong core affinity?
> Can we do it on a core basis?
>
> thanks,
> john
>
> On Tue, Mar 2, 2010 at 2:10 AM, Nicolas Rosner <nrosner at gmail.com>  
> wrote:
>> Hi John,
>>
>> Assuming correct understanding of your question: no, I don't think  
>> you
>> can do that within the limits of the MPI standard itself. Also, you
>> seem to be assuming strong processor affinity as a fact (as in "each
>> process will always run on its initially assigned core"), which, as
>> far as I know, is not necessarily the case.
>>
>> (In fact, it looks to me like the precise meaning of such a concept
>> would depend rather heavily on the hardware architecture and
>> OS-related factors of each system -- which is what makes me guess it
>> would make sense for the Standard to stay above such lower-level
>> issues.)
>>
>> If all you need is the ability to *find out* which physical CPU  
>> within
>> an SMP machine is running a process: assuming your system supports
>> this, it probably provides a way for you to find out by querying it
>> directly (e.g. a syscall).
>>
>> If you need to *control* how processes are mapped to processors,  
>> maybe
>> the more OS-specific layers/wrappers/utils of an MPI implementation
>> might provide some higher-level assistant, e.g. check out
>>
>> http://wiki.mcs.anl.gov/mpich2/index.php/Using_the_Hydra_Process_Manager#Process-core_Binding
>>
>> (Also try googling "hwloc", and perhaps PLPA if you use Linux.)
>>
>> Perhaps you'd be willing to explain a bit more about what you're
>> trying to use this for?  Do you really need a physical processor ID,
>> or are you just trying to enforce a certain distribution of processes
>> and/or behaviors?  Such needs can often be addressed at a higher  
>> level
>> (without such a strong commitment to a certain flavor of scheduling,
>> etc).
>>
>> Hope this helps,
>>
>> Nicolás
>>
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