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

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Date:	Wed, 24 Feb 2016 12:16:19 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	riel@...hat.com
cc:	linux-kernel@...r.kernel.org, fweisbec@...il.com, 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%

On Wed, 10 Feb 2016, riel@...hat.com wrote:

> (v6: make VIRT_CPU_ACCOUNTING_GEN jiffy granularity)
> 
> Running with nohz_full introduces a fair amount of overhead.
> Specifically, various things that are usually done from the
> timer interrupt are now done at syscall, irq, and guest
> entry and exit times.
> 
> However, some of the code that is called every single time
> has only ever worked at jiffy resolution. The code in
> __acct_update_integrals was also doing some unnecessary
> calculations.
> 
> Getting rid of the unnecessary calculations, without
> changing any of the functionality in __acct_update_integrals
> gets us about an 11% win.
> 
> Not calling the time statistics updating code more than
> once per jiffy, like is done on housekeeping CPUs and on
> all the CPUs of a non-nohz_full system, shaves off a
> further 30%.

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>