[RFC PATCH 0/2] block: virtio-blk: support multi vq per virtio-blk

Hi,

This patches try to support multi virtual queues(multi-vq) in one
virtio-blk device, and maps each virtual queue(vq) to blk-mq's
hardware queue.

With this approach, both scalability and performance problems on
virtio-blk device get improved.

For verifying the improvement, I implements virtio-blk multi-vq over
qemu's dataplane feature, and both handling host notification
from each vq and processing host I/O are still kept in the per-device
iothread context, the changes are based on qemu v2.0.0 release, and
can be accessed from below tree:

	git://kernel.ubuntu.com/ming/qemu.git #v2.0.0-virtblk-dataplane-mq

For enabling the multi-vq feature, 'num_queues=N' need to be added into
'-device virtio-blk-pci ...' of qemu command line, and suggest to pass
'vectors=N+1' to keep one MSI irq vector per each vq, and the feature
depends on x-data-plane.

Fio(libaio, randread, iodepth=64, bs=4K, jobs=N) is run inside VM to
verify the improvement.

I just create a small quadcore VM and run fio inside the VM, and
num_queues of the virtio-blk device is set as 2, but looks the
improvement is still obvious.

1), about scalability
- without mutli-vq feature
	-- jobs=2, thoughput: 145K iops
	-- jobs=4, thoughput: 100K iops
- without mutli-vq feature
	-- jobs=2, thoughput: 186K iops
	-- jobs=4, thoughput: 199K iops

2), about thoughput
- without mutli-vq feature
	-- top thoughput: 145K iops
- with mutli-vq feature
    -- top thoughput: 199K iops

So even for one quadcore VM, if the virtqueue number is increased
from 1 to 2, both scalability and performance can get improved a
lot.

Thanks,
--
Ming Lei