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

[Stefan Hajnoczi]

On Thu, Nov 17, 2016 at 12:17:57AM +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 16, 2016 at 09:53:06PM +0000, 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)
> 
> Any chance to see a gain in actual benchmark numbers?
> This is important to make sure we are not just
> shifting overhead around.

Good idea.  I reran this morning without any tracing and compared
against bare metal.

Total reads for a 30-second 4 KB random read benchmark with 4 processes
x iodepth=8:

Bare metal: 26440 MB
Unpatched:  19799 MB
Patched:    21252 MB

Patched vs Unpatched: +7% improvement
Patched vs Bare metal: 20% virtualization overhead

The disk image is a 8 GB raw file on XFS on LVM on dm-crypt on a Samsung
MZNLN256HCHP 256 GB SATA SSD.  This is just my laptop.

Seems like a worthwhile improvement to me.

Stefan

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