Re: [Qemu-block] [PATCH] virtio-blk: dataplane: Don't batch notifications if EVENT_IDX is present

[Stefan Hajnoczi]

On Wed, Mar 07, 2018 at 12:44:59PM +0100, Sergio Lopez wrote:
> Commit 5b2ffbe4d99843fd8305c573a100047a8c962327 ("virtio-blk: dataplane:
> notify guest as a batch") deferred guest notification to a BH in order
> batch notifications, with purpose of avoiding flooding the guest with
> interruptions.
> 
> This optimization came with a cost. The average latency perceived in the
> guest is increased by a few microseconds, but also when multiple IO
> operations finish at the same time, the guest won't be notified until
> all completions from each operation has been run. On the contrary,
> virtio-scsi issues the notification at the end of each completion.
> 
> On the other hand, nowadays we have the EVENT_IDX feature that allows a
> better coordination between QEMU and the Guest OS to avoid sending
> unnecessary interruptions.
> 
> With this change, virtio-blk/dataplane only batches notifications if the
> EVENT_IDX feature is not present.
> 
> Some numbers obtained with fio (ioengine=sync, iodepth=1, direct=1):
>  - Test specs:
>    * fio-3.4 (ioengine=sync, iodepth=1, direct=1)
>    * qemu master
>    * virtio-blk with a dedicated iothread (default poll-max-ns)
>    * backend: null_blk nr_devices=1 irqmode=2 completion_nsec=280000
>    * 8 vCPUs pinned to isolated physical cores
>    * Emulator and iothread also pinned to separate isolated cores
>    * variance between runs < 1%
> 
>  - Not patched
>    * numjobs=1:  lat_avg=327.32  irqs=29998
>    * numjobs=4:  lat_avg=337.89  irqs=29073
>    * numjobs=8:  lat_avg=342.98  irqs=28643
> 
>  - Patched:
>    * numjobs=1:  lat_avg=323.92  irqs=30262
>    * numjobs=4:  lat_avg=332.65  irqs=29520
>    * numjobs=8:  lat_avg=335.54  irqs=29323
> 
> Signed-off-by: Sergio Lopez <address@hidden>
> ---
>  hw/block/dataplane/virtio-blk.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)

Thanks, applied to my block tree!

I merged this quickly because I don't want to forget this patch for the
upcoming QEMU 2.12 release.  If anyone has questions or wants to
discuss, please go ahead and I can hold back the patch.

https://github.com/stefanha/qemu/commits/block

Stefan

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