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

[Paolo Bonzini]

On 26/01/2016 17:41, Chris Friesen wrote:
> I'm using libvirt (1.2.12) with qemu (2.2.0) in the context of OpenStack.
> 
> If I live-migrate a guest with virtio network interfaces, I see a
> ~1200msec delay in processing the network packets, and several hundred
> of them get dropped.  I get the dropped packets, but I'm not sure why
> the delay is there.
> 
> I instrumented qemu and libvirt, and the strange thing is that this
> delay seems to happen before qemu actually starts doing any
> migration-related work.  (i.e. before qmp_migrate() is called)
> 
> Looking at my timestamps, the start of the glitch seems to coincide with
> libvirtd calling qemuDomainMigratePrepareTunnel3Params(), and the end of
> the glitch occurs when the migration is complete and we're up and
> running on the destination.
> 
> 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?

Paolo