Re: [libvirt-users] Guests using more ram than specified

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