On (Mon) 01 Sep 2014 [20:52:46], 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. > 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. > >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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
