Re: [Qemu-devel] [PATCH 0/3] virtio: disable notifications in blk and scsi

[Christian Borntraeger]

On 11/16/2016 10:53 PM, Stefan Hajnoczi wrote:
> Disabling notifications during virtqueue processing reduces the number of
> exits.  The virtio-net device already uses virtio_queue_set_notifications() 
> but
> virtio-blk and virtio-scsi do not.
> 
> The following benchmark shows a 15% reduction in virtio-blk-pci MMIO exits:
> 
>   (host)$ qemu-system-x86_64 \
>               -enable-kvm -m 1024 -cpu host \
>               -drive if=virtio,id=drive0,file=f24.img,format=raw,\
>                      cache=none,aio=native
>   (guest)$ fio # jobs=4, iodepth=8, direct=1, randread
>   (host)$ sudo perf record -a -e kvm:kvm_fast_mmio
> 
> Number of kvm_fast_mmio events:
> Unpatched: 685k
> Patched: 592k (-15%, lower is better)
> 
> Note that a workload with iodepth=1 and a single thread will not benefit - 
> this
> is a batching optimization.  The effect should be strongest with large iodepth
> and multiple threads submitting I/O.  The guest I/O scheduler also affects the
> optimization.

I have trouble seeing any difference in terms of performances or CPU load 
(other than 
a reduced number of kicks).
I was expecting some benefit by reducing the spinlock hold times in virtio-blk,
but this needs some more setups to actually find the sweet spot.

Maybe it will show its benefit with the polling thing?
> 
> Stefan Hajnoczi (3):
>   virtio: add missing vdev->broken check
>   virtio-blk: suppress virtqueue kick during processing
>   virtio-scsi: suppress virtqueue kick during processing
> 
>  hw/block/virtio-blk.c | 18 ++++++++++++------
>  hw/scsi/virtio-scsi.c | 36 +++++++++++++++++++++---------------
>  hw/virtio/virtio.c    |  4 ++++
>  3 files changed, 37 insertions(+), 21 deletions(-)
>