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.
add comments:
Although the virtio-serial is enabled, I don't use it at all, the degradation 
still happened.

>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