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

[Oliver Hookins]

On Tue, Nov 22, 2011 at 02:04:17PM +0100, Oliver Hookins wrote:
> 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.

Resending as it got rejected the first time:

I have not set the maximum permissible migration time but that default
certainly points to a possibility of a solution. As for the writing semantics
it was a straight dd from disk to /dev/shm so I can't speak for the kernel but
naively I would think it may be contiguous space.