linux-kernel - Re: [PATCH] Change the document about iowait

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Message-ID: <20161113084755.0eeb4ad4@lwn.net>
Date:   Sun, 13 Nov 2016 08:47:55 -0700
From:   Jonathan Corbet <corbet@....net>
To:     Chao Fan <fanc.fnst@...fujitsu.com>
Cc:     <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <peterz@...radead.org>, <akpm@...ux-foundation.org>,
        <izumi.taku@...fujitsu.com>, Cao Jin <caoj.fnst@...fujitsu.com>
Subject: Re: [PATCH] Change the document about iowait

On Wed, 26 Oct 2016 10:41:28 +0800
Chao Fan <fanc.fnst@...fujitsu.com> wrote:

> The iowait is not reliable by reading from /proc/stat, so this
> method to get iowait is not suggested. And we mark it in the
> document.

Sorry for the delay on this.  Life has been...challenging...

> Signed-off-by: Cao Jin <caoj.fnst@...fujitsu.com>
> Signed-off-by: Chao Fan <fanc.fnst@...fujitsu.com>

What does this signoff chain mean?  

> ---
>  Documentation/filesystems/proc.txt | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
> index 74329fd..71f5096 100644
> --- a/Documentation/filesystems/proc.txt
> +++ b/Documentation/filesystems/proc.txt
> @@ -1305,7 +1305,16 @@ second).  The meanings of the columns are as follows, from left to right:
>  - nice: niced processes executing in user mode
>  - system: processes executing in kernel mode
>  - idle: twiddling thumbs
> -- iowait: waiting for I/O to complete
> +- iowait: In a word, iowait stands for waiting for I/O to complete. But there
> +  are several problems:
> +  1. Cpu will not wait for I/O to complete, iowait is the time that a task is
> +     waiting for I/O to complete. When cpu goes into idle state for
> +     outstanding task io, another task will be scheduled on this CPU.
> +  2. In a multi-core CPU, the task waiting for I/O to complete is not running
> +     on any CPU, so the iowait of each CPU is difficult to calculate.
> +  3. The value of iowait field in /proc/stat will decrease in certain
> +     conditions.
> +  So, the iowait is not reliable by reading from /proc/stat.
>  - irq: servicing interrupts

So I suppose I can apply this.  But is there any chance of making it say
what iowait actually measures, rather than just saying that it's
unreliable? 

Thanks,

jon