Re: Interpreting iowait value from output of iostat

On 10 January 2011 11:40, Ask Questions <[email protected]> wrote:
> Hi,
>
> Need some clarifications on my understanding of =A0iostat command.
>
> Pasting the first line of output from this command .
>
> $iostat
> Linux 2.6.32-21-generic (desktop) =A0 =A0 =A0 Monday 10 January 2011 =
=A0_i686_ =A0(2
> CPU)
>
> avg-cpu: =A0%user =A0 %nice =A0%system =A0%iowait =A0 =A0%steal =A0 %=
idle
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 0.09 =A0 =A0 =A0 0.03 =A0 =A0 =A0 0.7=
4 =A0 =A0 =A0 =A0 0.07
> 0.00 =A0 =A0 99.08
>
> My confusion is with the %iowait column. The man page says : =A0 iost=
at
> "show the percentage of time that the CPU or CPUs were idle during
> which the system had an outstanding disk I/O request ".
>
> So , what I understood is that when the CPU is mostly idle and have
> free cycles, and =A0there is an IO request , the CPU can immediately
> handle it since it has free cycles. Now if the CPU is 100% busy and
> has no free cycles to handle an IO request , and there is a IO reques=
t
> during that time , %iowait value is expected to increase based on my
> understanding as the the request is waiting because CPU is busy and
> has no free cycles left.
>
> So , if there is an increase in IO wait time , =A0we need to check th=
e
> CPU and memory utilization . =A0There might me a possibility of bad
> blocks in the disk also .
>
> To simulate this , I tried the following .
>
> I executed dd if=3D/dev/zero of=3D/home/test =A0till the disk is satu=
rated.
> disk saturation I identified by running the iostat command on another
> terminal and looking at the %util column after every 5 secs. %iowait
> was mostly fluctuating within 35.00 when the disk was 100% saturated.
>
> After waiting for around 3 minutes , I fired another IO intensive
> command using dd . =A0The disk was already 100% busy as per the %util
> column. Now since the disk is fully saturated and has no free cycles
> for the next dd command , I can see the %iowait went to around 70.00
> and was fluctuating between and 50 and 70.
>
> I terminated the second dd command and can see value of %iostat comin=
g
> down .
>
> Is my understanding clear . Please clarify
>
> Thanks in Advance
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It is exactly what the man page says: =A0it's the time the CPU was idle
and has to wait on the results of an I/O request before it can
continue its operation.
More concretely, if you'd give it something else to do in the
meantime, it'd be happy to work on that.

Linux is a time-sharing OS. =A0It means that it tries to give a fair
chunk of cpu time (known as a time slice) to all the tasks that are
ready to run on the cpu.

=46or the sake of simplicity, let's assume we have a computer with only
one cpu and two tasks waiting on the cpu.

The scheduler starts task 1 on the cpu, until its time slice is
consumed. =A0At that time, the scheduler will interrupt it, and give
task 2 its fair chunk.

Now, suppose that task 2 requires data to be read from the disk. =A0Dis=
k
access is still the slowest operation on the system. =A0So, task 2 will
be put on hold (uninterruptible sleep, i.e. the "D" status in e.g. "ps
auxw", i.e. the "b" column in "vmstat") until that I/O is ready to be
delivered. =A0It will also increase the priority of that task.

If there is no other task ready to run on the cpu, the cpu is idle,
waiting on I/O, and your iowait counter will go up.

If task 1 has unfinished business, that task will get the cpu, until
the I/O is ready to be delivered. =A0At that time, task 2 will change
state from sleeping to runable, and as it has a higher priority than
task 1, the scheduler will interrupt task 1 and place it back on the
runable queue, and give the cpu back to task 2 where it can now finish
its time slice, and lower its priority back to normal. =A0The cpu hasn'=
t
been idle, so your counter won't go up.

Hope this helps.

Kind regards,

Herta

--
"Life on Earth may be expensive,
=A0but it comes with a free ride around the Sun."
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