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

[Dr. David Alan Gilbert]

* Stefan Hajnoczi (address@hidden) wrote:
> 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.

When you said 'in the past (Qemu 1.5)' is that actual 1.5 release?
I ask because a bit of bisecting leads me to 7dda5dc82a776a39a799 'migration: 
initialize RAM to zero'
(16th April 2013 slightly before 1.5.0 time) - Although I think it's effect 
maybe just to make these
previous changes have the effect they were intended to.
So if the behaviour you're seeing is between 1.5/1.7 then it's something else

I think one of the ways to think about it is that previously you could start
a guest on a host relying on overcommit (although it might OOM) but you were 
unlikely
to be able to migrate it in to a host with overcommit because it would write 
all it's
0 pages.

However, if you're seeing the difference between a 1.5 release and 1.7 then 
maybe
it's something more subtle.

Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK