linux-kernel - Re: [PATCH] change cpuacct usage percpu format v2

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Date:	Fri, 27 Feb 2009 16:00:06 -0800
From:	Ken Chen <kenchen@...gle.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"menage@...gle.com" <menage@...gle.com>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	mingo@...e.hu,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] change cpuacct usage percpu format v2

On Fri, Feb 27, 2009 at 3:34 PM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@...fujitsu.com> wrote:
> "a lot of" ? I talking about cpu hotplug and reading another file as
> /sys/devices/system/cpu/present every time before reading this file
> gives much much much more overhead ;)

yes, really a lot.  CPU hotplug is an uncommon event.  It happens
perhaps once a day? maybe once an hour?

User monitoring process usually reads usage_percpu at fairly high
rate, say once a sec.  At each pass it will need to parse N number of
CPU index.  The overhead is N_CPU * T, where T is time in second
between cpu hotplug event.  Assume T = one day, on a moderate sized
64-CPU size machine, the overhead is:

64 * 86400 : 1, that's like 5.5 million to 1 ratio.  To me that is
*high* overhead.

- Ken
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