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Date: Wed, 17 Aug 2016 14:23:04 -0700 From: Andy Lutomirski <luto@...capital.net> To: Ingo Molnar <mingo@...nel.org> Cc: Thomas Gleixner <tglx@...utronix.de>, Denys Vlasenko <dvlasenk@...hat.com>, Josh Poimboeuf <jpoimboe@...hat.com>, Borislav Petkov <bp@...e.de>, "the arch/x86 maintainers" <x86@...nel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Brian Gerst <brgerst@...il.com>, "H. Peter Anvin" <hpa@...or.com>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: [PATCH v3 0/7] x86: Rewrite switch_to() On Aug 15, 2016 8:10 AM, "Ingo Molnar" <mingo@...nel.org> wrote: > > > * Brian Gerst <brgerst@...il.com> wrote: > > > > Something like this: > > > > > > taskset 1 perf stat -a -e '{instructions,cycles}' --repeat 10 perf bench sched pipe > > > > > > ... will give a very good idea about the general impact of these changes on > > > context switch overhead. > > > > Before: > > Performance counter stats for 'system wide' (10 runs): > > > > 12,010,932,128 instructions # 1.03 insn per > > cycle ( +- 0.31% ) > > 11,691,797,513 cycles > > ( +- 0.76% ) > > > > 3.487329979 seconds time elapsed > > ( +- 0.78% ) > > > > After: > > Performance counter stats for 'system wide' (10 runs): > > > > 12,097,706,506 instructions # 1.04 insn per > > cycle ( +- 0.14% ) > > 11,612,167,742 cycles > > ( +- 0.81% ) > > > > 3.451278789 seconds time elapsed > > ( +- 0.82% ) > > > > The numbers with or without this patch series are roughly the same. > > There is noticeable variation in the numbers each time I run it, so > > I'm not sure how good of a benchmark this is. > > Weird, I get an order of magnitude lower noise: > > triton:~/tip> taskset 1 perf stat -a -e '{instructions,cycles}' --repeat 10 perf bench sched pipe >/dev/null > > Performance counter stats for 'system wide' (10 runs): > > 11,503,026,062 instructions # 1.23 insn per cycle ( +- 2.64% ) > 9,377,410,613 cycles ( +- 2.05% ) > > 1.669425407 seconds time elapsed ( +- 0.12% ) > > But note that I also had '--sync' for perf stat and did a >/dev/null at the end to > make sure no terminal output and subsequent Xorg activities interfere. Also, full > screen terminal. > > Maybe try 'taskset 4' as well to put the workload on another CPU, if the first CPU > is busier than the others? > > (Any Hyperthreading on your test system?) > I've never investigated for real, but I suspect that cgroups are a big part of it. If you do a regular perf recording, I think you'll find that nearly all of the time is in the scheduler.
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