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