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Date: Thu, 17 Nov 2016 19:38:45 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 0/3] virtio: disable notifications in blk
 and scsi
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>, Kevin Wolf <kwolf@redhat.com>, zhunxun@gmail.com, Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org, Christian Borntraeger <borntraeger@de.ibm.com>, Paolo Bonzini <pbonzini@redhat.com>

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.