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Date: Tue, 24 Feb 2009 18:50:55 +0100 From: Ingo Molnar <mingo@...e.hu> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Dave Hansen <dave@...ux.vnet.ibm.com>, Nick Piggin <nickpiggin@...oo.com.au>, Salman Qazi <sqazi@...gle.com>, linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>, Dave Hansen <haveblue@...ibm.com> Subject: Re: Another Performance Regression in write() syscall * Linus Torvalds <torvalds@...ux-foundation.org> wrote: > On Tue, 24 Feb 2009, Dave Hansen wrote: > > > > Yeah, that's a good point. Are we sure that's what is > > happening here, though? That's one thing a profile would > > hopefully help with. > > One thing to note is that _if_ it's purely an issue of > nontemporal stores vs normal stores, then profiling is very > likely going to be almost entirely useless. You'll get > "results", but the results have nothing what-so-ever to do > with reality or anything interesting. > > The nontemporal stores may stand out in the profiles, but the > actual performance impact will be all about whether totally > unrelated code got cache misses or not. Quite often those > cache misses will also be in user mode, and very possibly in > other processes. > > So profiles can certainly be interesting, but if Salman says > that his patch makes a difference for his benchmark, then > profiling is almost certainly not interesting FOR THAT PATCH. > It's interesting mainly as a way to look at whether there are > then also _other_ issues that are worth addressing (ie the > whole atime thing is in a whole different dimension and an > independent issue). a 'perfstat' run would certainly be interesting (for cases where a pure /usr/bin/time run is inconclusive), comparing the unpatched and patched kernel. That way we can see summary counts for the whole workload, like: ----------------------------------------------- | Performance counter stats for './mmap-perf' | ----------------------------------------------- | | | x86-defconfig | PARAVIRT=y |------------------------------------------------------------------ | | 1311.554526 | 1360.624932 task clock ticks (msecs) +3.74% | | | 1 | 1 CPU migrations | 91 | 79 context switches | 55945 | 55943 pagefaults | ............................................ | 3781392474 | 3918777174 CPU cycles +3.63% | 1957153827 | 2161280486 instructions +10.43% | 50234816 | 51303520 cache references +2.12% | 5428258 | 5583728 cache misses +2.86% | | 437983499 | 478967061 branches +9.36% | 32486067 | 32336874 branch-misses -0.46% | | | 1314.782469 | 1363.694447 time elapsed (msecs) +3.72% | | ----------------------------------- Such a comparison of would certainly be more meaningful for such things than a profile. Salman, if you are interested in doing a perfstat comparison, just pick up a tip:master kernel [perfcounters are default-enabled in it]: http://people.redhat.com/mingo/tip.git/README and run perfstat on it (as root, to get the kernel-mode counts too): http://redhat.com/~mingo/perfcounters/perfstat.c Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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