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

[Stefan Hajnoczi]

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.

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(-)

-- 
2.7.4