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Date: Mon, 15 Sep 2008 22:41:35 -0500
From: Stephen Montgomery-Smith <stephen@math.missouri.edu>
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Cc: Martin Cracauer <cracauer@cons.org>, freebsd-current@freebsd.org
Subject: Re: Improved multiprocessor usage on amd64
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Steve Kargl wrote:
> On Mon, Sep 15, 2008 at 07:36:04PM -0500, Stephen Montgomery-Smith wrote:
>> ... and each thread is a loop of the form
>>
>> while (1) {
>>   wait until told to start;
>>   do massive amounts of floating point arithmetic (only additions and
>> multiplications) on large arrays;
>>   tell the master process that you are done;
>> }
>>
>>> Do you have about as many threads as processor or more?
>> Both ways.  The time difference between the two approaches is negligible.
>>
> 
> Are you using ULE?  With my MPI applications, if the number of
> launched processes exceeds the number of cpus by 1, ULE falls
> through the floor.  I have a nagging feeling that there is 
> a problem with cpu affinity.
> 
> http://lists.freebsd.org/pipermail/freebsd-current/2008-July/086917.html
> 


I get the same phenomenon with ULE and 4BSD.  I would say that they 
perform about the same.

But yes, I do have at least one more process running than the number of 
CPUs.  One of the processes is the master process, that controls the 
others, and it does comparatively little work compared to the others, 
but it is still there, and it does do some work.