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

[Ming Lei]

On Thu, Jul 31, 2014 at 3:37 PM, Benoît Canet <address@hidden> wrote:
> The Thursday 31 Jul 2014 à 11:55:28 (+0800), Ming Lei wrote :
>> 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?
>
> Block mirroring of a device for example is done using coroutines.
> As block mirroring can be done on a raw device you need coroutines.

If block layer knows the mirroring is in-progress, it still can enable
coroutine by ignoring bypass coroutine, or just let device disable
bypass coroutine in case of mirroring, and the current APIs are very
flexible.

Thanks,