On 01/09/14 14:52, 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.
This is just wild guessing: If virtio-blk and virtio-serial share an IRQ, the guest operating system has to check each virtqueue for activity. Maybe there is some inefficiency doing that. AFAIK virtio-serial registers 64 virtqueues (on 31 ports + console) even if everything is unused. Christian -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
