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Subject[PATCH] Change the document about iowait
Date
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>
---
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


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