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

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Date:	Sat, 28 Feb 2009 09:42:17 +0900 (JST)
From:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
Cc:	"Ken Chen" <kenchen@...gle.com>,
	"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

KAMEZAWA Hiroyuki wrote：
> Ken Chen wrote：
>> 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.
>
Ignore above, I caught, at last. I'll add text to documenation.

BTW, current interface to reset cpuacct (write ops) just reset
specified level of cpuacct and will not clear other hierarchical levels.
Doesn't this behavior confuse software ?

Thanks,
-Kame





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