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

[Ming Lei]

On Thu, Jul 31, 2014 at 7:37 AM, Paolo Bonzini <address@hidden> wrote:
> Il 30/07/2014 19:15, Ming Lei ha scritto:
>> On Wed, Jul 30, 2014 at 9:45 PM, Paolo Bonzini <address@hidden> wrote:
>>> Il 30/07/2014 13:39, Ming Lei ha scritto:
>>>> This patch introduces several APIs for supporting bypass qemu coroutine
>>>> in case of being not necessary and for performance's sake.
>>>
>>> No, this is wrong.  Dataplane *must* use the same code as non-dataplane,
>>> anything else is a step backwards.
>>
>> As we saw, coroutine has brought up performance regression
>> on dataplane, and it isn't necessary to use co in some cases, is it?
>
> Yes, and it's not necessary on non-dataplane either.  It's not necessary
> on virtio-scsi, and it will not be necessary on virtio-scsi dataplane
> either.

It is good to know these cases, so they might benefit from this patch
in future too, :-)

>>> If you want to bypass coroutines, bdrv_aio_readv/writev must detect the
>>> conditions that allow doing that and call the bdrv_aio_readv/writev
>>> directly.
>>
>> That is easy to detect, please see the 5th patch.
>
> No, that's not enough.  Dataplane right now prevents block jobs, but
> that's going to change and it could require coroutines even for raw devices.

Could you explain it a bit why co is required for raw devices in future?

If some cases for not requiring co can be detected beforehand, these
patches are useful, IMO.

>>> To begin with, have you benchmarked QEMU and can you provide a trace of
>>> *where* the coroutine overhead lies?
>>
>> I guess it may be caused by the stack switch, at least in one of
>> my box, bypassing co can improve throughput by ~7%, and by
>> ~15% in another box.
>
> No guesses please.  Actually that's also my guess, but since you are
> submitting the patch you must do better and show profiles where stack
> switching disappears after the patches.

OK, no problem, I will provide some profile result.

Thanks,