From 9c240d757658a3ae9968dd309e674c61f07c7f48 Mon Sep 17 00:00:00 2001 From: Chao Fan <fanc.fnst@cn.fujitsu.com> Date: Wed, 26 Oct 2016 10:41:28 +0800 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@cn.fujitsu.com> Signed-off-by: Chao Fan <fanc.fnst@cn.fujitsu.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net> --- 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 74329fd0add22..71f5096694b40 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 -- GitLab