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From:   Feng Li <lifeng1519@gmail.com>
Date:   Thu, 11 Oct 2018 18:15:41 +0800
Message-ID: <CAEK8JBC-HKGb43UynWx69Lj-y48Em038eNMpOb7G72XfLF_mDg@mail.gmail.com>
Subject: Re: [Qemu-devel] virtio-console downgrade the virtio-pci-blk performance
To:     dgilbert@redhat.com, amit@kernel.org,
        virtualization@lists.linux-foundation.org
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        qemu-discuss@nongnu.org, qemu-devel@nongnu.org,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>
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Add Amit Shah.

After some tests, we found:
- the virtio serial port number is inversely proportional to the iSCSI
virtio-blk-pci performance.
If we set the virio-serial ports to 2("<controller
type=3D'virtio-serial' index=3D'0' ports=3D'2'/>), the performance downgrad=
e
is minimal.

- use local disk/ram disk as virtio-blk-pci disk, the performance
downgrade is still obvious.


Could anyone give some help about this issue?

Feng Li <lifeng1519@gmail.com> =E4=BA=8E2018=E5=B9=B410=E6=9C=881=E6=97=A5=
=E5=91=A8=E4=B8=80 =E4=B8=8B=E5=8D=8810:58=E5=86=99=E9=81=93=EF=BC=9A
>
> Hi Dave,
> My comments are in-line.
>
> Dr. David Alan Gilbert <dgilbert@redhat.com> =E4=BA=8E2018=E5=B9=B410=E6=
=9C=881=E6=97=A5=E5=91=A8=E4=B8=80 =E4=B8=8B=E5=8D=887:41=E5=86=99=E9=81=93=
=EF=BC=9A
> >
> > * Feng Li (lifeng1519@gmail.com) wrote:
> > > Hi,
> > > I found an obvious performance downgrade when virtio-console combined
> > > with virtio-pci-blk.
> > >
> > > This phenomenon exists in nearly all Qemu versions and all Linux
> > > (CentOS7, Fedora 28, Ubuntu 18.04) distros.
> > >
> > > This is a disk cmd:
> > > -drive file=3Discsi://127.0.0.1:3260/iqn.2016-02.com.test:system:fl-i=
scsi/1,format=3Draw,if=3Dnone,id=3Ddrive-virtio-disk0,cache=3Dnone,aio=3Dna=
tive
> > > -device virtio-blk-pci,scsi=3Doff,bus=3Dpci.0,addr=3D0x6,drive=3Ddriv=
e-virtio-disk0,id=3Dvirtio-disk0,bootindex=3D1,write-cache=3Don
> > >
> > > If I add "-device
> > > virtio-serial-pci,id=3Dvirtio-serial0,bus=3Dpci.0,addr=3D0x5  ", the =
virtio
> > > disk 4k iops (randread/randwrite) would downgrade from 60k to 40k.
> > >
> > > In VM, if I rmmod virtio-console, the performance will back to normal=
.
> > >
> > > Any idea about this issue?
> > >
> > > I don't know this is a qemu issue or kernel issue.
> >
> > It sounds odd;  can you provide more details on:
> >   a) The benchmark you're using.
> I'm using fio, the config is:
> [global]
> ioengine=3Dlibaio
> iodepth=3D128
> runtime=3D120
> time_based
> direct=3D1
>
> [randread]
> stonewall
> bs=3D4k
> filename=3D/dev/vdb
> rw=3Drandread
>
> >   b) the host and the guest config (number of cpus etc)
> The qemu cmd is : /usr/libexec/qemu-kvm --device virtio-balloon -m 16G
> --enable-kvm -cpu host -smp 8
> or qemu-system-x86_64 --device virtio-balloon -m 16G --enable-kvm -cpu
> host -smp 8
>
> The result is the same.
>
> >   c) Why are you running it with iscsi back to the same host - why not
> >      just simplify the test back to a simple file?
> >
>
> Because my ISCSI target could supply a high IOPS performance.
> If using a slow disk, the performance downgrade would be not so obvious.
> It's easy to be seen, you could try it.
>
>
> > Dave
> >
> > >
> > > Thanks in advance.
> > > --
> > > Thanks and Best Regards,
> > > Alex
> > >
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>
>
>
> --
> Thanks and Best Regards,
> Feng Li(Alex)


--
Thanks and Best Regards,
Feng Li(Alex)