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

[Dr. David Alan Gilbert]

* Feng Li (address@hidden) 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.
  b) the host and the guest config (number of cpus etc)
  c) Why are you running it with iscsi back to the same host - why not
     just simplify the test back to a simple file?

Dave

> 
> Thanks in advance.
> -- 
> Thanks and Best Regards,
> Alex
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK