Re: [Qemu-devel] [PATCH 01/15] qemu coroutine: support bypass mode

[Ming Lei]

On Fri, Aug 1, 2014 at 9:13 PM, Stefan Hajnoczi <address@hidden> wrote:
> On Fri, Aug 01, 2014 at 10:54:02AM +0800, Ming Lei wrote:
>> On Fri, Aug 1, 2014 at 12:30 AM, Paolo Bonzini <address@hidden> wrote:
>> > Il 31/07/2014 18:13, Ming Lei ha scritto:
>> >> Follows 'perf report' result on cycles event for with/without bypass
>> >> coroutine:
>> >>
>> >>     http://pastebin.com/ae0vnQ6V
>> >>
>> >> From the profiling result, looks bdrv_co_do_preadv() is a bit slow
>> >> without bypass coroutine.
>> >
>> > Yeah, I can count at least 3.3% time spent here:
>> >
>> > 0.87%          bdrv_co_do_preadv
>> > 0.79%          bdrv_aligned_preadv
>> > 0.71%          qemu_coroutine_switch
>> > 0.52%          tracked_request_begin
>> > 0.45%          coroutine_swap
>> >
>> > Another ~3% wasted in malloc, etc.
>>
>> That should be related with coroutine and the BH in bdrv_co_do_rw().
>> In this post I didn't apply Stephan's coroutine resize patch which might
>> decrease usage of malloc() for coroutine.
>
> Please rerun with "[PATCH v3 0/2] coroutine: dynamically scale pool
> size".

No problem, will do that. Actually in my last post with rfc, this patchset
was against your coroutine resize patches.

I will provide the profile data tomorrow.

>
>> At least, coroutine isn't cheap from the profile result.
>
> Instead of bypassing coroutines we should first understand the overhead
> that they impose.  Is it due to the coroutine implementation (switching
> stacks) or due to the bdrv_co_*() code that happens to use coroutines
> but slow for other reasons.

>From the 3th patch(block: support to bypass qemu coroutinue)
and the 5th patch(dataplane: enable selective bypassing coroutine),
the change is to bypass coroutine and BH, and the other bdrv code
path is same, so it is due to the coroutine implementation, IMO.

Thanks,