Re: performance decreased

[Tobias Burnus <burnus at net-b dot de>]

On 11/07/2011 07:32 PM, Francisco Llaryora wrote:
> With the purpose of measuring the SpedUp by changing the number of threads.
> I did run fourty times by changing the value OMP_NUM_THREAD   from 1 to 40.
> I run it in a node with 40 cores Xenon.4 Processors with 10 cores each one.
> The next is time in sec, no speedUP:

First, the email is more appropriate for gcc-help@ as gcc@ is the 
developer list.

Regarding the issue itself: Try OMP_WAIT_POLICY=PASSIVE - cf. 
http://gcc.gnu.org/onlinedocs/libgomp/OMP_005fWAIT_005fPOLICY.html or 
any OpenMP documentation.

One still expects that the performance decreases if one has more threads 
than cores, but it should not decrease as much as with ACTIVE.

The default value is something in between - it burns wait cycles as with 
ACTIVE but only for a very short time while with ACTIVE is does so for a 
much longer time before continuing to wait passively.

Note: The comments are for the current version; I am not sure about GCC 
4.2.1 - it might well be that the default wait policy wasn't falling 
back to a passive wait that quickly.

Tobias