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authorChao Fan <fanc.fnst@cn.fujitsu.com>2016-10-26 05:41:28 +0300
committerJonathan Corbet <corbet@lwn.net>2016-11-19 20:38:58 +0300
commit9c240d757658a3ae9968dd309e674c61f07c7f48 (patch)
treea50facdb5fabc9864528e103eb41bb76b05e38e2 /Documentation/filesystems
parentca9667fcc855676aa5c35978f201034d38790829 (diff)
downloadlinux-9c240d757658a3ae9968dd309e674c61f07c7f48.tar.xz
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>
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r--Documentation/filesystems/proc.txt11
1 files changed, 10 insertions, 1 deletions
diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 74329fd0add2..71f5096694b4 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