I understand, You could potentially encapsulate all else - except your VM's in a small cgroup and frequently reclaim from there using the memory cgroup. If drop caches works for you, that is good too. I am surprised that cache allocations are causing lowmem exhaustion.
I'm suirprised that LOWMEM exhaustion causes oom-killer (when HIGH & SWAP are half empty) but there you go, life is full of surprises ;)
Of course, it could be something else, but its definitely disk i/o that triggers the oom-killer and clearing the cache stops it from happening (2.6.31.12) - any alternative explain really really welcome.
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