Hi all, First regarding mutexes on Macs, I suppose a performance hit with mutexes beats no performance at all with missing spinlocks. And regarding "take more work," I believe spinlocks and mutexes were basically interchangeable in terms of functionality in our sort algorithm; the work probably will be little more than a few #ifdefs and a s/pthread_mutex_t/pthread_spinlock_t/
How difficult would it be to implement a basic spinlock in gnulib, though? Perhaps we can implement that and fall back to it on MacOS X? >> On a related note, I've been meaning to check >> if mutexes in coreutils/sort are more stable >> and/or fair to the system than spinlocks. > > They ought to be fairer, though I'd expect there to be a significant > performance price in some cases. I'm not sure what you mean > by "stable" (surely this has nothing to do with stable sorting :-). > An example of the stability issues is be the hyperthreaded processor issue. For example, when I was sorting on an i7 (4 cores, 8 logical), sort time with threads > 4 had very high variance with spinlocks, but not with mutexes. I do believe this particular issue is handled; I remember capping threads to the number of physical cores on the system. Of course, there might be other nasties that we just don't know about yet with spinlocks. >> perhaps we should cap the >> number of cores used to perhaps 8 given results like: >> http://debbugs.gnu.org/cgi/bugreport.cgi?msg=49;filename=sort-amd-24way.png;att=1;bug=6600 >> We would allow increasing up to the number of cores >> available on the system by user specified values >> (which currently only support decreasing from #cores). > > That result depends on the particular benchmark and platform, > of course. Still, it might make sense to default to at most 8 > for now, as you suggest; the documentation should say that that > default might change of course. > This is a good idea... I'm not sure how feasible this would be, but perhaps we can benchmark some common server systems or some performance outliers and define a custom max for these.
