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

[Amit Shah]

On (Mon) 01 Sep 2014 [20:38:20], 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.
> 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?

                Amit