linux-kernel - Re: [PATCH 2/2] sched,time: call __acct_update

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Date:	Fri, 29 Jan 2016 17:43:28 -0500
From:	Rik van Riel <riel@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	tglx@...utronix.de, mingo@...nel.org, peterz@...radead.org,
	luto@...capital.net, fweisbec@...il.com,
	Clark Williams <williams@...hat.com>
Subject: Re: [PATCH 2/2] sched,time: call __acct_update_integrals once a jiffy

On 01/29/2016 05:23 PM, riel@...hat.com wrote:
> From: Rik van Riel <riel@...hat.com>

> This speeds up

... ok, that changelog got truncated :(

Here is the full version:

Because __acct_update_integrals does nothing unless the time
interval in question exceeds a jiffy, there is no real reason
to call it more than once a jiffy from the syscall, irq, and
guest entry & exit paths.

If tasks get rescheduled frequently, the scheduler will still
update their time statistics normally.

However, longer running tasks with frequent syscall, irq,
or guest entry & exit see a difference with this patch.

A microbenchmark calling an invalid syscall number 10 million
times in a row speeds up an additional 30% over the numbers
with just the previous patch, for a total speedup of about 40%
over 4.4 and 4.5-rc1.

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 + both patches          2.3 seconds

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