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

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Nov 17, 2016 at 07:38:45PM +0200, Michael S. Tsirkin wrote:
> On Thu, Nov 17, 2016 at 01:27:49PM +0000, Stefan Hajnoczi wrote:
> > 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
> 
> Sure. Pls remember to ping or re-post after the release.

The block pull request I just sent with AioContext polling includes
these patches.  Just noticed this and remembered this email thread.
Feel free to NACK the pull request if you wish to discuss this series
again: <20170104133414.6524-1-stefanha@redhat.com>

Stefan

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