Re: Very high IOWait during all disk activity

[Tomasz Chmielewski <mangoo@wpkg.org>]

> I recently noticed on a number of my Linux boxes that during disk
> activity, CPU usage was consistently hitting 100%. A little digging
> showed that the CPU was spending up to around 65% of its time in an
> IOWait state. Checked this with kernels 2.6.22 and 2.6.25-rc3, and
> also across SATA and PATA drives on three different machines, all with
> the same results. I also checked back with an old Ubuntu 6.06 Live CD
> and that also exhibits the problem.
> 
> Having done some digging on the net, I can't get a definitive answer
> as to whether this is considered "normal". Some people suggest that
> IOWait is informational and doesn't indicate a problem, but based on
> my admittedly limited understanding of such things, the CPU shouldn't
> need to spend much time on disk I/O these days due to the use of DMA.
> 
> Is it expected behaviour for the CPU to spend such a large amount of
> time in the IOWait state during disk I/O?

Unless you can write to the disk faster than fetch data from /dev/zero - 
yes, it is normal.

BTW, it doesn't mean that your CPU's cycles are wasted. You will see big 
"wa" numbers when there are no other tasks to schedule at the same time.

Try running:

   cat /dev/zero | bzip2 -c >/dev/null

when your IOwait is big (because you write a big file), and then watch 
the numbers.


-- 
Tomasz Chmielewski
http://wpkg.org