On Sat, Jan 17, 2015 at 3:00 AM, Dennis Jacobfeuerborn <denni...@conversis.de> wrote: > 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 >
It`s may be interesting to check how various compaction technologies (KSM/PKSM) will behave on such large VMs. If you are running those VMs on a host where a single NUMA node has smaller amount of memory than a single VM allocation, you also may face mm-related issues and NUMA-related allocation faults. _______________________________________________ libvirt-users mailing list libvirt-users@redhat.com https://www.redhat.com/mailman/listinfo/libvirt-users
