On Tue, 09/02 12:06, Amit Shah wrote: > 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. Amit, It's related to the big number of ioeventfds used in virtio-serial-pci. With virtio-serial-pci's ioeventfd=off, the performance is not affected no matter if guest initializes it or not. In my test, there are 12 fds to poll in qemu_poll_ns before loading guest virtio_console.ko, whereas 76 once modprobe virtio_console. Looks like the ppoll takes more time to poll more fds. Some trace data with systemtap: 12 fds: time rel_time symbol 15 (+1) qemu_poll_ns [enter] 18 (+3) qemu_poll_ns [return] 76 fd: 12 (+2) qemu_poll_ns [enter] 18 (+6) qemu_poll_ns [return] I haven't looked at virtio-serial code, I'm not sure if we should reduce the number of ioeventfds in virtio-serial-pci or focus on lower level efficiency. Haven't compared with g_poll but I think the underlying syscall should be the same. Any ideas? Fam > > > >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 -- 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
