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.
>> 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?
>
>So it's a windows guest; it could be something windows driver
>specific, then?  Do you see the same on Linux guests too?
>
I suspect windows driver specific, too.
I have not test linux guest, I'll test it later.

Thanks,
Zhang Haoyu
>               Amit