Messages in this thread | | | Date | Fri, 29 Feb 2008 16:18:28 +0100 | From | Tomasz Chmielewski <> | Subject | Re: Very high IOWait during all disk activity |
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> 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
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