From: Cao jin <caoj.fnst@cn.fujitsu.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, "Izumi,
Taku/泉 拓" <izumi.taku@jp.fujitsu.com>
Subject: Re: about definition of iowait
Date: Mon, 27 Jun 2016 14:06:29 +0800 [thread overview]
Message-ID: <5770C265.1000109@cn.fujitsu.com> (raw)
In-Reply-To: <20160607120125.GH30154@twins.programming.kicks-ass.net>
Hi Peter
Sorry to bother you on this topic again.
On 06/07/2016 08:01 PM, Peter Zijlstra wrote:
> On Tue, Jun 07, 2016 at 07:24:46PM +0800, Cao jin wrote:
>> Hi,
>>
>> I noticed some problems about iowait entry of /proc/stat: it seems not
>> accurate, and sometimes will decrease in SMP.
>>
>> For UP, we have a simple definition about iowait, which is:
>> The time that the processor is idle, during which there is a task waiting
>> for I/O.
>>
>> This definition seems don't fit well when step into SMP era, so I think
>> maybe it can be extended like:
>> For a given CPU, the I/O wait time is the time during which that CPU was
>> idle, and there is at least one outstanding disk I/O operation requested by
>> a task scheduled on that CPU.
>>
>> But tasks can migrate among cpus, so this definition maybe not accurate.
>>
>> I also saw some suggestions that it should be a global value, not per cpu,
>> but don`t see clear definition, so anyone have suggestions about it?
>> (personally guessing, maybe: there is task waiting for outstanding I/O while
>> all cpus are idle? But I don't think it is good...)
>
> It is possible to come up with definition for a single global number;
> per-cpu iowait numbers are doomed.
>
> But I don't see it matters one way or the other. Personally I would just
> make the kernel return 0 and be done with it.
>
So I just want to make sure, is it acceptable if I send a patch make
iowait a constant 0? Because make it zero seems we are abandoning it(0
doesn'`t have any meanings for it). While maybe some user space tools
monitors this value.
--
Yours Sincerely,
Cao jin
prev parent reply other threads:[~2016-06-27 6:00 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-07 11:24 about definition of iowait Cao jin
2016-06-07 12:01 ` Peter Zijlstra
2016-06-08 7:14 ` Cao jin
2016-06-27 6:06 ` Cao jin [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=5770C265.1000109@cn.fujitsu.com \
--to=caoj.fnst@cn.fujitsu.com \
--cc=izumi.taku@jp.fujitsu.com \
--cc=linux-kernel@vger.kernel.org \
--cc=peterz@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).