linux-kernel - Re: [PATCH v3 0/7] x86: Rewrite switch

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Date:	Mon, 15 Aug 2016 07:10:02 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v3 0/7] x86: Rewrite switch_to()


* 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?)

Thanks,

	Ingo