Re: [libvirt] high outage times for qemu virtio network links during live migration, trying to debug

On 26/01/2016 18:21, Chris Friesen wrote: >>> >>> My question is, why doesn't qemu continue processing virtio packets >>> while the dirty page scanning and memory transfer over the network is >>> proceeding? >> >> QEMU (or vhost) _are_ processing virtio traffic, because otherwise you'd >> have no delay---only dropped packets. Or am I missing something? > > I have separate timestamps embedded in the packet for when it was sent > and when it was echoed back by the target (which is the one being > migrated). What I'm seeing is that packets to the guest are being sent > every msec, but they get delayed somewhere for over a second on the way > to the destination VM while the migration is in progress. Once the > migration is over, a bunch of packets get delivered to the app in the > guest and are then processed all at once and echoed back to the sender > in a big burst (and a bunch of packets are dropped, presumably due to a > buffer overflowing somewhere). That doesn't exclude a bug somewhere in net/ code. It doesn't pinpoint it to QEMU or vhost-net. In any case, what I would do is to use tracing at all levels (guest kernel, QEMU, host kernel) for packet rx and tx, and find out at which layer the hiccup appears.