Re: [Qemu-devel] memory allocation of migration changed?

[Stefan Hajnoczi]

On Tue, Feb 11, 2014 at 07:30:54PM +0100, Stefan Priebe wrote:
> Am 11.02.2014 16:44, schrieb Stefan Hajnoczi:
> >On Tue, Feb 11, 2014 at 3:54 PM, Stefan Priebe - Profihost AG
> ><address@hidden> wrote:
> >>in the past (Qemu 1.5) a migration failed if there was not enogh memory
> >>on the target host available directly at the beginning.
> >>
> >>Now with Qemu 1.7 i've seen succeeded migrations but the kernel OOM
> >>memory killer killing qemu processes. So the migration seems to takes
> >>place without having anough memory on the target machine?
> >
> >How much memory is the guest configured with?  How much memory does
> >the host have?
> 
> Guest: 48GB
> Host: 192GB
> 
> >I wonder if there are zero pages that can be migrated almost "for
> >free" and the destination host doesn't touch.  When they are touched
> >for the first time after migration handover, they need to be allocated
> >on the destination host.  This can lead to OOM if you overcommitted
> >memory.
> 
> In the past the migration failed immediatly with exit code 255.
> 
> >Can you reproduce the OOM reliably?  It should be possible to debug it
> >and figure out whether it's just bad luck or a true regression.
> 
> So there is no known patch changing this behaviour?
> 
> What is about those?
> fc1c4a5d32e15a4c40c47945da85ef9c1e0c1b54
> 211ea74022f51164a7729030b28eec90b6c99a08
> f1c72795af573b24a7da5eb52375c9aba8a37972

Yes, that's what I was referring to when I mentioned zero pages.

The problem might just be that the destination host didn't have enough
free memory.  Migration succeeded due to memory overcommit on the host,
but quickly ran out of memory after handover.  The quick answer there is
to reconsider your overcommitting memory and also checking memory
availability before live migrating.

Stefan