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From: Brock Palen (brockp_at_[hidden])
Date: 2006-12-01 09:35:09


On Dec 1, 2006, at 9:23 AM, Gleb Natapov wrote:

> On Fri, Dec 01, 2006 at 04:14:31PM +0200, Gleb Natapov wrote:
>> On Fri, Dec 01, 2006 at 11:51:24AM +0100, Peter Kjellstrom wrote:
>>> On Saturday 25 November 2006 15:31, shaposh_at_[hidden] wrote:
>>>> Hello,
>>>> i cant figure out, is there a way with open-mpi to bind all
>>>> threads on a given node to a specified subset of CPUs.
>>>> For example, on a multi-socket multi-core machine, i want to use
>>>> only a single core on each CPU.
>>>> Thank You.
>>>
>>> This might be a bit naive but, if you spawn two procs on a dual
>>> core dual
>>> socket system then the linux kernel should automagically schedule
>>> them this
>>> way.
>>>
>>> I actually think this applies to most of the situations discussed
>>> in this
>>> thread. Explicitly assigning processes to cores may actually get
>>> it wrong
>>> more often than the normal linux scheduler.
>>>
>> If you run two single threaded ranks on the dual core dual socket
>> node
>> you better be placing them on the same core. Shared memory
>> communication
Isn't this only valid with NUMA systems? (large systems or AMD
Opteron) The intel multicores each must communicate along the bus to
the north-bridge and back again. So all cores have the same path to
memory. Correct me if im wrong. Though working on this would be
good, i dont expect all systems to stick with bus, and more and more
will be NUMA in the future.

On another note for systems that use pbs (and maybe other resource
managers) It gives out the cpus in the hostlist (hostname/0
hostname/1 etc) Why cant OMPI read that info if its available?

Im prob totally off on these comments.

Brock

> I mean "same socket" here and not "same core" of cause.
>
>> will be much faster (especially if two cores shares cache).
>>
>>> /Peter (who may be putting a bit too much faith in the linux
>>> scheduler...)
>> Linux scheduler does its best assuming the processes are
>> unrelated. This is
>> not the case with MPI ranks.
>>
>> --
>> Gleb.
>> _______________________________________________
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>
> --
> Gleb.
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