Re: [Qemu-devel] [question] virtio-blk performance degradationhappened 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?

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