[Beowulf] Woodcrest Memory bandwidth

Richard Walsh rbw at ahpcrc.org
Tue Aug 15 11:25:05 EDT 2006


Mark Hahn wrote:
>>> Good point which makes perfect sense to me.
>>> Given that the theoretical maximum is actually 21.3 GB/s
>>> the real maximum Triad number must be 21.3/3 = 7.1 GB/s.
>
> I don't get this - triad does two reads and one write.
> if you don't use store-through ('nt' versions of mov),
> then the write also implies a read for write-allocate
> (filling the cache line).
> without store-through, the peak theoretical number reported by
> stream should be 3*peak/4.  the 4 is because there are 3r+1w,
> and the 3 because stream doesn't give credit for write-allocate.
That looks right.  So, one socket, with write allocate, >>should<< show:

      10.5 GB/sec * .75 or 7.875 GBytes/sec

and two sockets 15.75 GBytes/sec.  The problem could be related
to  competitive/ineffective use of the shared L2 cache or a bottleneck
in the North bridge.  It would seem that a look at how the performance grows
as you add cores within versus across sockets should reveal this.

Two cores on separate sockets should show higher numbers if it's
an L2 cache issue.  If they are the same as those for 2 cores on one
socket then you have a problem with the North bridge or getting
full bandwidth from the FB-DIMMs. 

A complication in this test could be that in the one core per socket case
the whole L2 cache is allocated to a single core.  Watching performance
change as the array sizes grow should reveal this. 

rbw


>
>> Then how do you explain a dual opteron with two 6.4GB/sec (peak)
>> memory system, 12.8GB/sec total per node managing 9-10GB/sec?
>>
>> 12.8/3=4.26GB/sec.  People are seeing well over twice that.
>
> since pathscale does write-through, the peak really should be 12.8,
> so achieving 9-10 is decent but not paradoxical.  (the peak would 
> correspond to 1.07 Gflops, significantly below the peak theoretical
> pipeline rate of 2*clock flops...)
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-- 

Richard B. Walsh

Project Manager
Network Computing Services, Inc.
Army High Performance Computing Research Center (AHPCRC)
rbw at ahpcrc.org  |  612.337.3467

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