linux-kernel - Re: [PATCH 0/4 v6] sched,time: reduce nohz

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Date:	Mon, 15 Feb 2016 10:01:29 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	riel@...hat.com, linux-kernel@...r.kernel.org
Cc:	fweisbec@...il.com, tglx@...utronix.de, mingo@...nel.org,
	luto@...capital.net, peterz@...radead.org, clark@...hat.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH 0/4 v6] sched,time: reduce nohz_full syscall overhead 40%

Hi Rik,

On Wed, 2016-02-10 at 20:08 -0500, riel@...hat.com wrote:

> I tested this series with a microbenchmark calling
> an invalid syscall number ten million times in a row,
> on a nohz_full cpu.
> 
>     Run times for the microbenchmark:
>     
> 4.4                             3.8 seconds
> 4.5-rc1                         3.7 seconds
> 4.5-rc1 + first patch           3.3 seconds
> 4.5-rc1 + first 3 patches       3.1 seconds
> 4.5-rc1 + all patches           2.3 seconds
> 
>    Same test on a non-NOHZ_FULL, non-housekeeping CPU:
> all kernels                     1.86 seconds

I tested 10M stat(".", &buf) calls, and saw a win of ~20% on a
nohz_full cpu.  Below are nopreempt vs nohz_full+patches overhead
numbers from my box.
                                                        avg
4.4.1-nopreempt        0m1.652s   0m1.633s   0m1.635s   1.640   1.000

nohz_full + patches
nohz_full inactive     0m1.642s   0m1.631s   0m1.629s   1.634    .996
housekeeper CPU        0m2.013s   0m2.012s   0m2.033s   2.019   1.231
nohz_full CPU          0m2.247s   0m2.233s   0m2.239s   2.239   1.365

It still ain't free ;-) but between this set, and all the other work
that has gone in ~recently, it looks one hell of a lot better.  That's
not too scary a pricetag.

	-Mike