On 05/13/2010 03:20 PM, James Stevens wrote:
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.
I'd go with 64-bit at 2GB and above. It's both faster and safer.
The lowmem load is about 0.5% of guest memory, so 48GB means 240MB
lowmem allocated. Thin ice.
Since you can run a 64-bit kernel with your existing userspace, at least
you have a simple upgrade path.
--
error compiling committee.c: too many arguments to function
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