Improved multiprocessor usage on amd64

Stephen Montgomery-Smith stephen at math.missouri.edu
Tue Sep 16 03:48:56 UTC 2008


Stephen Montgomery-Smith wrote:
> 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
>>

Let me say a little bit more.

I have this gut feeling that the problem has a lot to do with cache 
management.  My program has each thread doing, in effect, huge matrix 
multiplications, each one working on their own little bit.  If a CPU 
core changes from one thread to another, it then has to flush out the 
cache to RAM, and read in a whole bunch of other RAM into cache.

I have this sense that Linux and FreeBSD have something in its internals 
where it figures this out, and after a while starts changing the time 
between when it changes from one process to another.  But Linux has a 
faster learning curve than FreeBSD.

But this is all pure speculation on my part, because I have very little 
ideas as to how these internals work.

Stephen


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