>> >>> 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. > I use "-device virtio-serial,vectors=4" instead of "-device virtio-serial", but the degradation still happened, nothing changed. with virtio-serial enabled: 64k-write-sequence: 4200 IOPS with virtio-serial disabled: 64k-write-sequence: 5300 IOPS How to confirm whether it's MSI in windows? Thanks, Zhang Haoyu >> >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? -- 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
