Re: [Qemu-devel] virtio-console downgrade the virtio-pci-blk performance

[Feng Li]

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='virtio-serial' index='0' ports='2'/>), the performance downgrade
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@xxxxxxxxx> ä2018å10æ1æåä äå10:58åéï
>
> Hi Dave,
> My comments are in-line.
>
> Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx> ä2018å10æ1æåä äå7:41åéï
> >
> > * Feng Li (lifeng1519@xxxxxxxxx) 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=iscsi://127.0.0.1:3260/iqn.2016-02.com.test:system:fl-iscsi/1,format=raw,if=none,id=drive-virtio-disk0,cache=none,aio=native
> > > -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x6,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1,write-cache=on
> > >
> > > If I add "-device
> > > virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x5  ", 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=libaio
> iodepth=128
> runtime=120
> time_based
> direct=1
>
> [randread]
> stonewall
> bs=4k
> filename=/dev/vdb
> rw=randread
>
> >   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@xxxxxxxxxx / Manchester, UK
>
>
>
> --
> Thanks and Best Regards,
> Feng Li(Alex)



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