linux-kernel - about definition of iowait

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Message-ID: <5756AEFE.60300@cn.fujitsu.com>
Date:	Tue, 7 Jun 2016 19:24:46 +0800
From:	Cao jin <caoj.fnst@...fujitsu.com>
To:	<linux-kernel@...r.kernel.org>
CC:	<peterz@...radead.org>,
	Izumi, Taku/泉 拓 
	<izumi.taku@...fujitsu.com>
Subject: about definition of iowait

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...)

-- 
Yours Sincerely,

Cao jin