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. Paolo
For comparison, we have a DPDK-based fastpath NIC type that we added (sort of like vhost-net), and it continues to process packets while the dirty page scanning is going on. Only the actual cutover affects it.