virtio-console downgrade the virtio-pci-blk performance

[Feng Li]

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.


Thanks in advance.
-- 
Thanks and Best Regards,
Alex