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 <oliver.hookins@xxxxxxxxx> wrote: > > > On Tue, Nov 15, 2011 at 11:47:58AM +0100, ext Juan Quintela wrote: > > >> Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx> 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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
