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

[Stefan Priebe <s.priebe@profihost.ag>]

Am 14.02.2014 16:03, schrieb 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
>>> <s.priebe@profihost.ag> 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.

Yes that makes sense in the past it was just working due to a failing 
qemu process.

What is the right way to check for enough free memory and memory usage 
of a specific vm?

Stefan
>