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Re: CPU waiting for... what? (mistery)

From: Rick Denoire <100.17706_at_germanynet.de>
Date: Fri, 11 Apr 2003 22:32:34 +0200
Message-ID: <708e9vs1gv9ve1isp0fce2vd1d71dlpeuc@4ax.com>


Darren Dunham <ddunham_at_redwood.taos.com> wrote:

>CPU wait states say almost nothing about I/O. All it really says is
>that the CPU is somewhat idle. I/Owait + idle equals true CPU idle. So
>a box with 70% io wait has lots of CPU for processing.

Disagree. I/O wait: CPU would do something, just can't. CPU idle: there is really nothing to do.

Doing nothing has different causes.

>"wait queue"? What figures are you using that show you that? Are you
>talking about the "b" column from vmstat?

Sorry. I meant the secound column you get with vmstat. This is the number of blocked processes.

>How do you know you have an I/O bottleneck at all? You mention that
>you've seen 45MB/s transfer rates. That sounds good. As long as iostat
>is showing that the service times are small, then I think you've proved
>that the I/O is okay.

I am not that impressed by 45 MB/s if you consider that this is a Raid. Any top consumer type harddisk can achieve this sustained transfer rate ALONE.

Actually this is a poor performance. The host should be reading from cache due to the advanced prefetch mechanism of the storage subsystem. Reading from cache is much faster.

>Difficult. I think you'd have to write a utility that looked at the
>status of all the processes on the machine and recorded if any of them
>were in the "blocked" state. I have some information from someone else
>about the general aspects, but I've never attempted to code it up.

etruss was supposed to do that, it just does not work properly.

Bye
Rick Denoire Received on Fri Apr 11 2003 - 15:32:34 CDT

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