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Date: Sat, 28 Feb 2009 08:34:02 +0900 (JST)
Subject: Re: [PATCH] change cpuacct usage percpu format v2
From: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>
To: "Ken Chen" <kenchen@google.com>
Cc: "KAMEZAWA Hiroyuki" <kamezawa.hiroyu@jp.fujitsu.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "menage@google.com" <menage@google.com>,
       "lizf@cn.fujitsu.com" <lizf@cn.fujitsu.com>,
       "balbir@linux.vnet.ibm.com" <balbir@linux.vnet.ibm.com>, mingo@elte.hu,
       "akpm@linux-foundation.org" <akpm@linux-foundation.org>
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Ken Chen さんは書きました：
> On Thu, Feb 26, 2009 at 11:29 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@jp.fujitsu.com> wrote:
>> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>>
>> cpuacct.usage_percpu shows per cpu usage information via cgroupfs.
>> It shows all "present" cpus information and "whose information ?"
>> is explained just by index of data implicitly.
>>
>> IIRC, there is physical cpu hotplug system and it can remove
>> cpus and change map. Considering a software which records
>> this information pediodically(like sar), its better to show id of cpu
>> in explict way.
>
> The new format carries a lot of overhead.  I would think user space
> software can record /sys/devices/system/cpu/present and use that to
> map cpu index.
>
"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 ;)

A program which handles this kind of information tend to ignore hotplug
event and continue to use informaton which it gathers at start-up and
will not see updated information....as libnuma etc.

Thanks,
-Kame