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

[Andrey Korolyov]

On Tue, Sep 2, 2014 at 10:36 AM, Amit Shah <address@hidden> wrote:
> 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
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Can confirm serious degradation comparing to the 1.1 with regular
serial output  - I am able to hang VM forever after some tens of
seconds after continuously printing dmest to the ttyS0. VM just ate
all available CPU quota during test and hanged over some tens of
seconds, not even responding to regular pings and progressively
raising CPU consumption up to the limit.