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

[Amit Shah]

On (Mon) 01 Sep 2014 [20:52:46], 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.
> >>
> >I don't think so,
> >I use iometer to test 64k-read(or write)-sequence case, if I disable the 
> >virtio-serial dynamically via device manager->virtio-serial => disable,
> >then the performance get promotion about 25% immediately, then I re-enable 
> >the virtio-serial via device manager->virtio-serial => enable,
> >the performance got back again, very obvious.
> add comments:
> Although the virtio-serial is enabled, I don't use it at all, the degradation 
> still happened.

Using the vectors= option as mentioned below, you can restrict the
number of MSI vectors the virtio-serial device gets.  You can then
confirm whether it's MSI that's related to these issues.

> >So, I think it has no business with legacy interrupt mode, right?
> >
> >I am going to observe the difference of perf top data on qemu and perf kvm 
> >stat data when disable/enable virtio-serial in guest,
> >and the difference of perf top data on guest when disable/enable 
> >virtio-serial in guest,
> >any ideas?
> >
> >Thanks,
> >Zhang Haoyu
> >>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