[gomp] omp performance question

[Daniel Franke]

2006/10/30, François-Xavier Coudert <fxcoudert@gmail.com>:
> > single threaded ( FCFLAGS=-O1) timings on x64_64, dual CPU (dual core
> > each), gave:
> > real    64m36.502s
> > user    64m36.886s
> > sys     0m0.040s
> >
> > same machine, OMP enabled  (FCFLAGS="-O1 -fopenmp"):
> > real    67m16.611s
> > user    112m22.885s
> > sys     25m44.641s
>
> What is OMP_NUM_THREADS set to?

The numbers above use the default implementation without setting
OMP_NUM_THREADS explicitely. Letting OMP_NUM_THREADS=4, i.e. the
number of cores available, results are:

OMP_NUM_THREADS=4, OMP_DYNAMIC=[DEFAULT]
real    59m47.223s
user    91m33.127s
sys     24m35.856s

OMP_NUM_THREADS=4, OMP_DYNAMIC=FALSE
real    57m36.233s
user    91m59.685s
sys     26m31.735s


Some more timings (same machine):
OMP_NUM_THREADS=4, OMP_DYNAMIC=TRUE
IN PROGRESS
(ps -efL shows four threads running)

OMP_NUM_THREADS=8, OMP_DYNAMIC=TRUE
real    168m46.983s
user    97m53.903s
sys     113m0.108s
(ps -efL showed three threads actually running)

OMP_NUM_THREADS=8, OMP_DYNAMIC=FALSE
real    85m54.649s
user    125m20.442s
sys     48m15.253s

I'll try to add some numbers from ia64 (8CPU) if I can get exclusive hold of it.