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? 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,
