[Bug libgomp/43706] scheduling two threads on one core leads to starvation

["johnfb at mail dot utexas dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #15 from johnfb at mail dot utexas dot edu  2010-07-30 14:00 -------
We have also had some trouble with this issue. We found that in general if we
where running on a machine with hardware threads (i.e., Intel's
Hyper-Threading) then performance was poor. Most of our runs where on a machine
with a Intel Xeon L5530 running RHEL 5. Profiling showed that our program was
spending 50% of its time inside libgomp. Setting either GOMP_SPINCOUNT or
OMP_WAIT_POLICY as discussed in this thread increased performance greatly.
Experiments with disabling and enabling cores with default OMP settings showed
that when the Hyper-Thread cores come on performance dipped below what we got
when we had only one core enabled on some runs.

A little thought as to how hardware threads are implemented makes it obvious
why spinning for more than a few cycles will cause performance problems. If one
hardware threads spins then all other threads on that core may be starved as
resources are shared between cores.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43706