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

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Message-ID: <20090228011115.GC28769@linux.vnet.ibm.com>
Date:	Sat, 28 Feb 2009 06:41:15 +0530
From:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Ken Chen <kenchen@...gle.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

On Sat, Feb 28, 2009 at 09:42:17AM +0900, KAMEZAWA Hiroyuki wrote:
> 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 ?
> 

hmmm. This got missed when we introduced hierarchy. But I wonder if it
is needed?

-- 
regards,
Dhaval
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