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

[Zhang Haoyu]

>> >>> 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.
>
I use "-device virtio-serial,vectors=4" instead of "-device virtio-serial", but 
the degradation still happened, nothing changed.
with virtio-serial enabled:
64k-write-sequence: 4200 IOPS
with virtio-serial disabled:
64k-write-sequence: 5300 IOPS

How to confirm whether it's MSI in windows?

Thanks,
Zhang Haoyu

>> >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?