On 2014-05-29 21:34, Ming Lei wrote:
On Fri, May 30, 2014 at 11:19 AM, Jens Axboe <axboe@xxxxxxxxx> 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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