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 -- 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
