gomp slowness

[Biagio Lucini]

skaller wrote:
> Hi, I have just run and timed a couple of tutorial examples for
> openMP using gcc (GCC) 4.2.1 (Ubuntu 4.2.1-5ubuntu4) on a dual core
> Athlon amd64, with OMP_NUM_THREADS set to 1 and 2, and occasionally 
> 8 I found that 1 thread outperforms 2 by almost 2:1 on all the examples,
> and 8 is only fractionally slower than 2. The code was compiled
> with just -fopenmp, no optimisation switches. OS: Linux, Ubuntu
> gutsy (7.10) with Linux 2.26.22-14-rt (with real time patches).
> 
> I confirmed by observing system monitor 1 thread maxes out 1 CPU,
> and 2 maxes out both, also the observable behaviour was correct
> and as expected.. it was just SLOW.
> 
> I've briefly looked at the current SVN source for libgomp and 
> can't see anything wrong there.
> 
> Can anyone venture an explanation as to what might be going
> wrong? At least one of the tests has long independent tasks
> (many seconds) for each thread, so it doesn't seem to be
> a synchronisation issue. 
> 
> Differences like: 4.7 second for 1 thread, 13 seconds for two
> were regularly observed in ALL the tests. Note: these are
> the real time figures, the CPU times were even worse.
> 
> [BTW: I built current SVN for gcc as well, but the installed
> result didn't run properly due to a missing .spec file,
> so I couldn't check if it was any different]
> 

It would be interesting to try with another compiler. Do you have access 
to another OpenMP-enabled compiler?

Biagio