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

[Feng Li]

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)