Re: [Fwd: performance with gcc -O0/-O2]

[Andrew Haley <aph@redhat.com>]

Andi Kleen writes:
 > Andrew Haley <aph@redhat.com> writes:
 > 
 > > Howard Chu writes:
 > >
 > >  > A bit of a minor mystery. Not a problem, just a curiosity. If
 > >  > someone knew off the top of their head a reason for it, that'd be
 > >  > cool, but otherwise no sweat.
 > >
 > > It's possible, although unlikley, that the optimized code has worse
 > > cache behaviour.  No way to know better without doing some profiling.
 > 
 > It's quite possible if he hits the conditional store "optimization"
 > (that actually adds unnecessary cache misses) that was recently discussed
 > in the load thread safety thread.

Possibly.  I'm guessing that what we are actually seeing is something
like an acutely timing-sensitive race condition, where making some
threads faster causes pessimal cache behaviour.  It's a really
interesting problem.  :-)

Andrew.

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