Date: Tue, 09 Nov 2004 14:11:58 -0500 From: Stephan Uphoff <ups@tree.com> To: Robert Watson <rwatson@FreeBSD.org> Cc: cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/i386/i386 pmap.c Message-ID: <1100027518.29384.87.camel@palm.tree.com> In-Reply-To: <Pine.NEB.3.96L.1041109130229.73102V-100000@fledge.watson.org>
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On Tue, 2004-11-09 at 08:03, Robert Watson wrote: > On Tue, 9 Nov 2004, Robert Watson wrote: > > > > I've tried changing the store_rel() to just do a simple store since writes are > > > ordered on x86, but benchmarks on SMP showed that it actually hurt. However, > > > it would probably be good to at least do that for UP. The current patch to > > > do it for all kernels is: > > Interestingly, I've now run through some more "macro" benchmarks. I saw a > couple of percent improvement on UP from the change, but indeed, I saw a > slight decrease in performance for the rapid packet send benchmark on SMP. > > So I guess my recommendation is to get this in the tree for UP, and see if > we can figure out why it's having the slow-down effect on SMP. We are probably talking cache line effects here. My guess is that we should: 1) Make sure that important spin mutexes are alone in a cache line. 2) Take care not to dirty the cache line unnecessarily. I think for 2 we need to change the spin mutex slightly (for SMP) to never call LOCK cmpxchgl before a simple load operation finds m->mtx_lock == MTX_UNOWNED since LOCK cmpxchgl always seems to dirty the cache line. I have a dual Xeon (p4) where I can run some tests. Please let me know if there are any tests that you can recommend - I don't want to reinvent the wheel here. Interestingly enough the linux spin locks implementation is mentioning some PPRO errata that seem to require a locked operation. Guess that means we should take a look at the errata of all SMP able processors out there :-( Intel also recommends a locked operation (or SFENCE) for future processors. Guess this means either non optimal code, lots of compile options or self modifying code. Stephanhome | help
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