High sleep_on_page latency

[Prateek Sharma]

On Mon, Jan 16, 2012 at 10:41 AM, Mulyadi Santosa
<mulyadi.santosa at gmail.com> wrote:
> 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?
>

I am using qemu-kvm 0.15, with KVM.

> 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
>

   My initial understanding was that sleep_on_page is 'called' by
__lock_page, which is usually called by file_read .
    So, i assumed that sleep_on_page is for the page-lock contention.
My primary confusion is with the interpretation of latencytop output.
2 seconds even for I/O to complete seems awfully long.


> Hope it helps....
>
> --
> regards,
>
> Mulyadi Santosa
> Freelance Linux trainer and consultant
>
> blog: the-hydra.blogspot.com
> training: mulyaditraining.blogspot.com