[Beowulf] bizarre scaling behavior on a Nehalem

Mikhail Kuzminsky kus at free.net
Fri Aug 14 19:24:25 EDT 2009


In message from Bill Broadley <bill at cse.ucdavis.edu> (Fri, 14 Aug 2009 
16:13:21 -0700):
>Mikhail Kuzminsky wrote:
>>> Your results look excellent, so I wouldn't be surprised if they are
>>> running at 1333.
>> 
>> I have 12-18 GB/s on 4 threads of stream/ifort w/DDR3-1066 on dual 
>>E5520
>> server. But it works under "numa-bad" kernel w/o control of
>> numa-efficient allocation.
>
>Sounds pretty bad.
>
>Why 4 threads?  You need 8 cores to keep all 6 memory busses busy.

For comparison w/your tests: you have only 4 cores. On 8 threads I 
have 20-26 GB/s.
>
>Which compiler?
  
ifort pointed above means intel fortran 11.0.38.

Mikhail

> open64 does substantially better than gcc.
>
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