Re: [Qemu-devel] Memory sync algorithm during migration

[Oliver Hookins]

On Tue, Nov 22, 2011 at 10:31:58AM +0100, ext Juan Quintela wrote:
> Oliver Hookins <address@hidden> wrote:
> > On Tue, Nov 15, 2011 at 11:47:58AM +0100, ext Juan Quintela wrote:
> >> Takuya Yoshikawa <address@hidden> wrote:
> >> > Adding qemu-devel ML to CC.
> >> >
> >> > Your question should have been sent to qemu-devel ML because the logic
> >> > is implemented in QEMU, not KVM.
> >> >
> >> > (2011/11/11 1:35), Oliver Hookins wrote:
> >> >> Hi,
> >> >>
> >> >> I am performing some benchmarks on KVM migration on two different types 
> >> >> of VM.
> >> >> One has 4GB RAM and the other 32GB. More or less idle, the 4GB VM takes 
> >> >> about 20
> >> >> seconds to migrate on our hardware while the 32GB VM takes about a 
> >> >> minute.
> >> >>
> >> >> With a reasonable amount of memory activity going on (in the hundreds 
> >> >> of MB per
> >> >> second) the 32GB VM takes 3.5 minutes to migrate, but the 4GB VM never
> >> >> completes. Intuitively this tells me there is some watermarking of 
> >> >> dirty pages
> >> >> going on that is not particularly efficient when the dirty pages ratio 
> >> >> is high
> >> >> compared to total memory, but I may be completely incorrect.
> >> 
> >> > You can change the ratio IIRC.
> >> > Hopefully, someone who knows well about QEMU will tell you better ways.
> >> >
> >> >  Takuya
> >> >
> >> >>
> >> >> Could anybody fill me in on what might be going on here? We're using 
> >> >> libvirt
> >> >> 0.8.2 and kvm-83-224.el5.centos.1
> >> 
> >> This is pretty old qemu/kvm code base.
> >> In principle, it makes no sense that with 32GB RAM migration finishes,
> >> and with 4GB RAM it is unable (intuitively it should be, if ever, the
> >> other way around).
> >> 
> >> Do you have an easy test that makes the problem easily reproducible?
> >> Have you tried ustream qemu.git? (some improvements on that department).
> >
> > I've just tried the qemu-kvm 0.14.1 tag which seems to be the latest that 
> > builds
> > on my platform. For some strange reason migrations always seem to fail in 
> > one
> > direction with "Unknown savevm section or instance 'hpet' 0" messages.
> 
> What is your platform?  This seems like you are running with hpet in one
> side, but without it in the other.  What command line are you using?

Yes, my mistake. We were also testing later kernels and my test machines managed
to get out of sync. One had support for hpet clocksource but the other one
didn't.

> 
> > This seems to point to different migration protocols on either end but they 
> > are
> > both running the same version of qemu-kvm I built. Does this ring any bells 
> > for
> > anyone?
> 
> Command line mismatch.  But, what is your platform?

CentOS5.6. Now running the VMs through qemu-kvm 0.14.1, unloaded migrations take
about half the time but with memory I/O load now both VMs never complete the
migration. In practical terms I'm writing about 50MB/s into memory and we have a
10Gbps network (and I've seen real speeds up to 8-9Gbps on the wire) so there
should be enough capacity to sync up the dirty pages.

So now the 32GB and 4GB VMs have matching behaviour (which makes more sense) but
I'm not any closer to figuring out what is going on.