[libvirt-users] Guests using more ram than specified

[Dennis Jacobfeuerborn]

On 16.01.2015 15:14, Michal Privoznik wrote:
> On 16.01.2015 13:33, Dennis Jacobfeuerborn wrote:
>> Hi,
>> today I noticed that one of my HVs started swapping aggressively and
>> noticed that the two guests running on it use quite a bit more ram than
>> I assigned to them. They respectively were assigned 124G and 60G with
>> the idea that the 192G system then has 8G for other purposes. In top I
>> see the VMs using about 128G and 64G which means there is nothing left
>> for the system. This is on a CentOS 7 system.
>> Any ideas what causes this or how I can calculate the actual maximum
>> amount of RAM I can assign to the guests on a HV without overcommitting RAM?
> 
> Well, this is an undecidable problem.
> One thing that may help is to use hugepages to back the memory for your
> guests. Because if you use the ordinary system pages, the translation
> table for ~200G is gonna be gigantic. Remember, that the table is
> counted in for memory usage.

According to the system information the qemu process uses transparent
hugepages and most of the Memory for a VM is reported under
AnonHugePages so that looks ok.

> Then, qemu itself consume some memory besides guest memory. How much?
> Nobody is able to tell.

Yes, and that worries me. The recommendation is not to over-commit
memory but if the System uses specified RAM + X and X is unknown and can
be tens of Gigabytes then I don't even know how to not over-commit the
system. I already reserved 8G for overhead but that doesn't seem to be
enough an now I don't know how to even calculate safe values for the guests.
One of the HVs actually crashed and rebooted itself. Not a pretty picture.

Regards,
  Dennis