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

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Date:	Sat, 28 Feb 2009 09:14:27 +0900 (JST)
From:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To:	"Ken Chen" <kenchen@...gle.com>
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"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

Ken Chen さんは書きました：
> 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?
>
Are you saying the software should have hotplug script and send SIGHUP or
some to reload the present map ?

> 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.
>
Sounds strange. I can't catch hat you want to say.

parsing overhead is
==
 1. read cpu.usage_perpu.
 2. parse the line to number (separated by " ")
 3. tie the number to cpu id which is loadead at startup
v.s.
 1. read cpu.usage_percpu.
 2. parse the line to number (sparated by ":" and "\n")
 3. record it
==
Hmm ? no big difference.

But okay, if you don't want this, I'll just add warning text to
Documentation/cgroup/cpuacct.txt as the user may have to prepare
cpu hotplug script if you write flight-recorder like "sar".

Thanks,
-Kame

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