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Date: Tue, 20 Jan 2009 21:56:53 +0100 From: Ingo Molnar <mingo@...e.hu> To: Jeremy Fitzhardinge <jeremy@...p.org> Cc: Nick Piggin <npiggin@...e.de>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, hpa@...or.com, jeremy@...source.com, chrisw@...s-sol.org, zach@...are.com, rusty@...tcorp.com.au, Andrew Morton <akpm@...ux-foundation.org>, Xen-devel <xen-devel@...ts.xensource.com> Subject: Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT * Jeremy Fitzhardinge <jeremy@...p.org> wrote: > Ingo Molnar wrote: >> * Ingo Molnar <mingo@...e.hu> wrote: >> >> >>>> Times I believe are in nanoseconds for lmbench, anyway lower is >>>> better. >>>> >>>> non pv AVG=464.22 STD=5.56 >>>> paravirt AVG=502.87 STD=7.36 >>>> >>>> Nearly 10% performance drop here, which is quite a bit... hopefully >>>> people are testing the speed of their PV implementations against >>>> non-PV bare metal :) >>>> >>> Ouch, that looks unacceptably expensive. All the major distros turn >>> CONFIG_PARAVIRT on. paravirt_ops was introduced in x86 with the >>> express promise to have no measurable runtime overhead. >>> >> >> Here are some more precise stats done via hw counters on a perfcounters >> kernel using 'timec', running a modified version of the 'mmap >> performance stress-test' app i made years ago. >> >> The MM benchmark app can be downloaded from: >> >> http://redhat.com/~mingo/misc/mmap-perf.c >> >> timec.c can be picked up from: >> >> http://redhat.com/~mingo/perfcounters/timec.c >> >> mmap-perf conducts 1 million mmap()/munmap()/mremap() calls, and >> touches the mapped area as well with a certain chance. The patterns are >> pseudo-random and the random seed is initialized to the same value so >> repeated runs produce the exact same mmap sequence. >> >> I ran the test with a single thread and bound to a single core: >> >> # taskset 2 timec -e -5,-4,-3,0,1,2,3 ./mmap-perf 1 >> >> [ I ran it as root - so that kernel-space hardware-counter statistics >> are included as well. ] >> >> The results are quite surprisingly candid about the true costs of >> paravirt_ops on the native kernel's overhead (CONFIG_PARAVIRT=y): >> >> ----------------------------------------------- >> | 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% >> > > Is this I or D, or combined? That's last-level-cache references+misses (L2 cache): Bit Position Event Name UMask Event Select CPUID.AH.EBX 3 LLC Reference 4FH 2EH 4 LLC Misses 41H 2EH 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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