High sleep_on_page latency

[Mulyadi Santosa]

Hi Prateek...

On Mon, Jan 16, 2012 at 11:48, Prateek Sharma <prateek3.14 at gmail.com> wrote:
> The latencytop output for the process doing the IO (qemu) is below:
>
> Process qemu-system-x86 (6239)             Total: 31363.7 msec
> [sleep_on_page]                                   1966.5 msec         81.4 %
> Waiting for event (select)                          5.0 msec          7.8 %
> [kvm_vcpu_block]                                    5.0 msec          8.5 %
> synchronous write                                   1.7 msec          0.0 %
> Userspace lock contention                           1.5 msec          2.3 %

Which qemu version do you use now? AFAIK certain new qemu versions
already uses iothreads by default. That should reduce I/O latency...
AFAIK too iothreads is not enabled by default...but for qemu 1.0 above
it's enabled by default. Are you compiling from source?

Regarding the function sleep_on_page(). It's in turn io_schedule().
And here's the comment above the function declaration of
io_schedule():
/*
* This task is about to go to sleep on IO. Increment rq->nr_iowait so
* that process accounting knows that this is a task in IO wait state.
*/

You can confirm it by yourself in:
http://lxr.linux.no/#linux+v3.2.1/kernel/sched.c#L5872

Hope it helps....

-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com