[libvirt-users] start a domain, minimal memory request.

[Michal Privoznik]

On 04/09/2018 09:00 AM, llilulu wrote:
> Hi:
>    I use libvirt manage virtual guest, When I start many virtual domain(my host 128G physical memory）, example, I start 100 guest, 1G per guest。sometime I will encounter "cannot allocate memory", "cannot set up guest memory \'pc.ram\'： cannot allocate memory". sometime encounter qemu process killed by system(oom),
>   I want to know what minimal memory need when start a domain ? and how many host free memory reserve can ensure vm running steady.

There's no answer to this question I'm afraid. Not only it's undecidable
problem but we also have history with it. I mean, at some point in the
past somebody came up with 'great' idea to have libvirt computing some
max value and setting it in cgroups to prevent qemu with a memleak
eating all the resources. It's better to be safe than sorry, right?
We've came up with several equations trying to figure that out. But then
we had to revert the patches because no matter how hard we tried some
users were still hitting the limit with valid use cases.

IOW, qemu memory consumption is not static through domain lifetime. It
changes. IIRC, hotplug operation was consuming surprisingly lot of
memory (on the other hand, for a fraction of a second). Also, memory
consumption is going to be dependant on domain configuration. Then, qemu
is linking with some libraries too - they will add their own overhead.

Long story short, I don't think there's an answer to your question.

Michal