[Beowulf] bizarre scaling behavior on a Nehalem

"C. Bergström" codestr0m at osunix.org
Sat Aug 15 23:13:42 EDT 2009


Tom Elken wrote:
>> On Behalf Of Christian Bell
>> On Aug 12, 2009, at 11:14 AM, Bill Broadley wrote:
>>
>>     
>>> Is it really necessary for dynamic arrays
>>>  to be substantially slower than static?
>>>       
>> Yes -- when pointers, the compiler assumes (by default) that the
>> pointers can alias each other, which can prevent aggressive
>> optimizations that are otherwise possible with arrays. 
>>     
>  ...
>   
>>  I remember stacking half a dozen pragmas over a
>> 3-line loop on a Cray C compiler years ago to ensure that accesses
>> where suitably optimized (or in this case, vectorized).
>>     
>
> To add some details to what Christian says, the HPC Challenge version of STREAM uses dynamic arrays and is hard to optimize.  I don't know what's best with current compiler versions, but you could try some of these that were used in past HPCC submissions with your program, Bill:
>
> PathScale 2.2.1 on Opteron:
> Base OPT flags: -O3 -OPT:Ofast:fold_reassociate=0 
> STREAMFLAGS=-O3 -OPT:Ofast:fold_reassociate=0 -OPT:alias=restrict:align_unsafe=on -CG:movnti=1
>
>   
I hope people don't mind me replying specifically to this PathScale 
related stuff, but last publicly released version was 3.2 (and a 
follow-up 3.3-beta)  If you're a PathScale customer and interested in 
this update please feel free to contact me off list.

./Christopher

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