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Date: Wed, 12 Mar 2014 20:15:42 +0100
From: Stefan Priebe <s.priebe@profihost.ag>
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Subject: Re: [Qemu-devel] memory allocation of migration changed?
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To: Eric Blake <eblake@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>
Cc: Orit Wasserman <owasserm@redhat.com>, Juan Quintela <quintela@redhat.com>, Peter Lieven <pl@kamp.de>, qemu-devel <qemu-devel@nongnu.org>, Dave Gilbert <dgilbert@redhat.com>


Am 24.02.2014 17:13, schrieb Eric Blake:
> On 02/24/2014 08:00 AM, Stefan Hajnoczi wrote:
>
>>> What is the right way to check for enough free memory and memory
>>> usage of a specific vm?
>>
>> I would approach it in terms of guest RAM allocation plus QEMU overhead:
>>
>>    host_ram >= num_guests * guest_ram_size + num_guests * qemu_overhead
>>
>> The qemu_overhead is the question mark.  It depends on the QEMU features
>> the guests have enabled and number of devices.
>>
>> QEMU also does not have a strict policy on predictable memory
>> consumption, which makes it hard to give a formula for it.
>
> In fact, at one point libvirt tried to put an automatic cap on the
> memory usable by qemu by multiplying RAM size and accounting for a
> margin of overhead, but no matter what heuristics we tried, we still got
> complaints from users that their guests were killed when they ran out of
> memory, and so we ended up reverting the automatic limits from libvirt.
>   (You can still enforce a limit as an end user, although the libvirt
> documentation no longer recommends attempting that, for as long as the
> qemu allocation remains unpredictable.)
>

Might something like:
vm.overcommit_ratio=100
vm.overcommit_memory=2

help? So that the migration might not happen but that's better than 
killing a random process?

Stefan