linux-kernel - Re: about definition of iowait

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Message-ID: <5757C5CF.6090906@cn.fujitsu.com>
Date:	Wed, 8 Jun 2016 15:14:23 +0800
From:	Cao jin <caoj.fnst@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	<linux-kernel@...r.kernel.org>,
	Izumi, Taku/泉 拓 
	<izumi.taku@...fujitsu.com>
Subject: Re: about definition of iowait



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.
>

I see. Thanks peter

-- 
Yours Sincerely,

Cao jin