Messages in this thread | | | Date | Thu, 29 May 2014 21:35:50 -0600 | From | Jens Axboe <> | Subject | Re: [PATCH] block: virtio_blk: don't hold spin lock during world switch |
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On 2014-05-29 21:34, Ming Lei wrote: > On Fri, May 30, 2014 at 11:19 AM, Jens Axboe <axboe@kernel.dk> wrote: >> On 2014-05-29 20:49, Ming Lei wrote: >>> >>> Firstly, it isn't necessary to hold lock of vblk->vq_lock >>> when notifying hypervisor about queued I/O. >>> >>> Secondly, virtqueue_notify() will cause world switch and >>> it may take long time on some hypervisors(such as, qemu-arm), >>> so it isn't good to hold the lock and block other vCPUs. >>> >>> On arm64 quad core VM(qemu-kvm), the patch can increase I/O >>> performance a lot with VIRTIO_RING_F_EVENT_IDX enabled: >>> - without the patch: 14K IOPS >>> - with the patch: 34K IOPS >> >> >> Patch looks good to me. I don't see a hit on my qemu-kvm testing, but it >> definitely makes sense and I can see it hurting in other places. > > It isn't easy to observe the improvement on x86 VM, especially > with few vCPUs, because qemu-system-x86_64 only takes > several microseconds to handle the notification, but on arm64, it > may take hundreds of microseconds, so the improvement is > obvious on arm VM. > > I hope this patch can be merged, at least arm VM can benefit > from it.
If Rusty agrees, I'd like to add it for 3.16 with a stable marker.
-- Jens Axboe
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