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

[Juan Quintela]

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).

Later, Juan.