Re: [Qemu-devel] [PATCH v3 0/7] virtio-blk: multiqueue support

[Stefan Hajnoczi]

On Tue, Jun 07, 2016 at 05:28:24PM +0100, Stefan Hajnoczi wrote:
> 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.
> 
> Results show that there is no regression anymore, thanks to dropping the
> batch notify BH patch.
> 
> 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
> unpatched-d6550e9ed2             19269820.2 ± 1.36%
> unpatched-dataplane-d6550e9ed2   22351400.4 ± 1.07%
> v3-dataplane                     22318511.2 ± 0.77%
> v3-no-dataplane                  18936103.8 ± 1.12%
> v3-queues-4-no-dataplane         19177021.8 ± 1.45%
> v3-smp-4-no-dataplane            25509585.2 ± 29.50%
> v3-smp-4-no-dataplane-no-mq      12466177.2 ± 7.88%
> 
> Configuration:
> Name                             Patched? Dataplane? SMP? MQ?
> unpatched-d6550e9ed2                    N          N    N   N
> unpatched-dataplane-d6550e9ed2          N          Y    N   N
> v3-dataplane                            Y          Y    N   N
> v3-no-dataplane                         Y          N    N   N
> v3-queues-4-no-dataplane                Y          N    N   Y
> v3-smp-4-no-dataplane                   Y          N    Y   Y
> v3-smp-4-no-dataplane-no-mq             Y          N    Y   N
> 
> SMP means -smp 4.
> MQ means virtio-blk-pci,num-queues=4.
> 
> 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(-)

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