On Thu, Jul 31, 2014 at 3:37 PM, Benoît Canet <benoit.ca...@irqsave.net> 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 <pbonz...@redhat.com> wrote: >> > Il 30/07/2014 19:15, Ming Lei ha scritto: >> >> On Wed, Jul 30, 2014 at 9:45 PM, Paolo Bonzini <pbonz...@redhat.com> >> >> 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,
