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

[Daniel P. Berrange]

On Tue, Jan 26, 2016 at 10:41:12AM -0600, Chris Friesen wrote:
> Hi,
> 
> 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?

The qemuDomainMigratePrepareTunnel3Params() method is responsible for
starting the QEMU process on the target host. This should not normally
have any impact on host networking connectivity, since the CPUs on that
target QEMU wouldn't be running at that point. Perhaps the mere act of
starting QEMU and plugging the TAP dev into the network on the target
host causes some issue though ? eg are you using a bridge that is doing
STP or something like that.


Regards,
Daniel
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