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Re: [Qemu-devel] [PATCH 0/3] virtio: disable notifications in blk and sc
From: |
Christian Borntraeger |
Subject: |
Re: [Qemu-devel] [PATCH 0/3] virtio: disable notifications in blk and scsi |
Date: |
Thu, 17 Nov 2016 12:01:30 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 |
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(-)
>
- Re: [Qemu-devel] [PATCH 1/3] virtio: add missing vdev->broken check, (continued)
[Qemu-devel] [PATCH 3/3] virtio-scsi: suppress virtqueue kick during processing, Stefan Hajnoczi, 2016/11/16
Re: [Qemu-devel] [PATCH 0/3] virtio: disable notifications in blk and scsi, Michael S. Tsirkin, 2016/11/16
Re: [Qemu-devel] [PATCH 0/3] virtio: disable notifications in blk and scsi,
Christian Borntraeger <=