Messages in this thread | | | From | Prateek Sharma <> | Date | Mon, 16 Jan 2012 11:24:51 +0530 | Subject | Re: High sleep_on_page latency |
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On Mon, Jan 16, 2012 at 10:41 AM, Mulyadi Santosa <mulyadi.santosa@gmail.com> wrote: > Hi Prateek... > > On Mon, Jan 16, 2012 at 11:48, Prateek Sharma <prateek3.14@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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