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

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Message-ID: <1477449688-19397-1-git-send-email-fanc.fnst@cn.fujitsu.com>
Date:   Wed, 26 Oct 2016 10:41:28 +0800
From:   Chao Fan <fanc.fnst@...fujitsu.com>
To:     <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:     <peterz@...radead.org>, <corbet@....net>,
        <akpm@...ux-foundation.org>, <izumi.taku@...fujitsu.com>,
        Chao Fan <fanc.fnst@...fujitsu.com>,
        Cao Jin <caoj.fnst@...fujitsu.com>
Subject: [PATCH] Change the document about iowait

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.

Signed-off-by: Cao Jin <caoj.fnst@...fujitsu.com>
Signed-off-by: Chao Fan <fanc.fnst@...fujitsu.com>
---
 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
 - softirq: servicing softirqs
 - steal: involuntary wait
-- 
2.7.4