Re: [Qemu-devel] [question] virtio-blk performance degradation happened with virito-serial

Amit Shah
2014-08-29T14:39:05Z

On (Fri) 29 Aug 2014 [15:45:30], Zhang Haoyu wrote:
> Hi, all
>
> I start a VM with virtio-serial (default ports number: 31), and found that virtio-blk performance degradation happened, about 25%, this problem can be reproduced 100%.
> without virtio-serial:
> 4k-read-random 1186 IOPS
> with virtio-serial:
> 4k-read-random 871 IOPS
>
> but if use max_ports=2 option to limit the max number of virio-serial ports, then the IO performance degradation is not so serious, about 5%.
>
> And, ide performance degradation does not happen with virtio-serial.

Pretty sure it's related to MSI vectors in use. It's possible that
the virtio-serial device takes up all the avl vectors in the guests,
leaving old-style irqs for the virtio-blk device.

If you restrict the number of vectors the virtio-serial device gets
(using the -device virtio-serial-pci,vectors= param), does that make
things better for you?

Amit