[PATCH v1 0/2] block: virtio-blk: support multi vq per virtio-blk

[Ming Lei <ming.lei@canonical.com>]

Hi,

These 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 on virtio-blk
device can 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 change is 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-mq.1

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
- with 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 in my test, even for a quad-core VM, if the virtqueue number
is increased from 1 to 2, both scalability and performance can
get improved a lot.

V1:
	- remove RFC since no one objects
	- add '__u8 unused' for pending as suggested by Rusty
	- use virtio_cread_feature() directly, suggested by Rusty


Thanks,
--
Ming Lei