Re: [Qemu-devel] [PATCH v4 0/7] virtio-blk: multiqueue support

[Christian Borntraeger]

On 06/21/2016 02:13 PM, Stefan Hajnoczi wrote:
> v4:
>  * Rebased onto qemu.git/master
>  * Included latest performance results
> 
> v3:
>  * Drop Patch 1 to batch guest notify for non-dataplane
> 
>    The Linux AIO completion BH and the virtio-blk batch notify BH changed 
> order
>    in the AioContext->first_bh list as a side-effect of moving the BH from
>    hw/block/dataplane/virtio-blk.c to hw/block/virtio-blk.c.  This caused a
>    serious performance regression for both dataplane and non-dataplane.
> 
>    I've decided not to move the BH in this series and work on a separate
>    solution for making batch notify generic.
> 
>    The remaining patches have been reordered and cleaned up.
> 
>  * See performance data below.
> 
> v2:
>  * Simplify s->rq live migration [Paolo]
>  * Use more efficient bitmap ops for batch notification [Paolo]
>  * Fix perf regression due to batch notify BH in wrong AioContext [Christian]
> 
> The virtio_blk guest driver has supported multiple virtqueues since Linux 
> 3.17.
> This patch series adds multiple virtqueues to QEMU's virtio-blk emulated
> device.
> 
> Ming Lei sent patches previously but these were not merged.  This series
> implements virtio-blk multiqueue for QEMU from scratch since the codebase has
> changed.  Live migration support for s->rq was also missing from the previous
> series and has been added.
> 
> It's important to note that QEMU's block layer does not support multiqueue 
> yet.
> Therefore virtio-blk device processes all virtqueues in the same AioContext
> (IOThread).  Further work is necessary to take advantage of multiqueue support
> in QEMU's block layer once it becomes available.
> 
> Performance results:
> 
> Using virtio-blk-pci,num-queues=4 can produce a speed-up but -smp 4
> introduces a lot of variance across runs.  No pinning was performed.
> 
> RHEL 7.2 guest on RHEL 7.2 host with 1 vcpu and 1 GB RAM unless otherwise
> noted.  The default configuration of the Linux null_blk driver is used as
> /dev/vdb.
> 
> $ cat files/fio.job
> [global]
> filename=/dev/vdb
> ioengine=libaio
> direct=1
> runtime=60
> ramp_time=5
> gtod_reduce=1
> 
> [job1]
> numjobs=4
> iodepth=16
> rw=randread
> bs=4K
> 
> $ ./analyze.py runs/
> Name                                   IOPS   Error
> v4-smp-4-dataplane               13326598.0 ± 6.31%
> v4-smp-4-dataplane-no-mq         11483568.0 ± 3.42%
> v4-smp-4-no-dataplane            18108611.6 ± 1.53%
> v4-smp-4-no-dataplane-no-mq      13951225.6 ± 7.81%

This differs from the previous numbers. What is with
and what is without patch? I am surprised to see dataplane
to be slower than no-dataplane - this contradicts everything
that I have seen in the past.

> 
> Stefan Hajnoczi (7):
>   virtio-blk: add VirtIOBlockConf->num_queues
>   virtio-blk: multiqueue batch notify
>   virtio-blk: tell dataplane which vq to notify
>   virtio-blk: associate request with a virtqueue
>   virtio-blk: live migrate s->rq with multiqueue
>   virtio-blk: dataplane multiqueue support
>   virtio-blk: add num-queues device property
> 
>  hw/block/dataplane/virtio-blk.c | 81 
> +++++++++++++++++++++++++++++------------
>  hw/block/dataplane/virtio-blk.h |  2 +-
>  hw/block/virtio-blk.c           | 52 +++++++++++++++++++++-----
>  include/hw/virtio/virtio-blk.h  |  6 ++-
>  4 files changed, 105 insertions(+), 36 deletions(-)
>