This is *NOT* a KVM issue, but may be worth adding into the FAQ...
We have a KVM host with 48Gb of RAM and run about 20 KVM clients on it.
After some time - different time depending on the kernel version - the
VM host kernel will start OOM-Killing the VM clients, even when there is
lots of free RAM (>10Gb) and free SWAP (>34Gb).
This seems to be caused by the kernel running out of LOWMEM (memory
below 1Gb) - because of the large amount of RAM a lot of LOWMEM (~400Mb)
is used by the memory map (32 bytes per 4Kb page), add in the kernel
itself and that leaves "only" about 460Mb of LOWMEM for kernel alloc.
This may not have been a problem, except Linux may also put cache blocks
and user processes in LOWMEM - it seems this can then lead to a LOWMEM
exhaust situation which triggers OOM-Killing even when there is LOADS of
SWAP and HIGHMEM free.
Sadly, killing userland processes is not a good way to try and free
LOWMEM, so what happens is a killing spree where by every process on the
VM host gets killed (inc all the VMs, sysklogd, klogd, sshd, udevd etc).
This is very bad in 2.6.32.6, quite bad in 2.6.32.9, better (but still
bad in) 2.6.31.12 - currently testing 2.6.33.3
See https://bugzilla.kernel.org/show_bug.cgi?id=15058
General advice seems to be, if you have more than 16Gb RAM then you
should run the VM host 64bit.
We didn't see this issue on a server with 32Gb running the same set of VMs.
James
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