On 06/15/2010 04:55 PM, Dave Hansen wrote:
This is a big RFC for the moment. These need a bunch more runtime testing. -- We've seen contention in the mmu_shrink() function.
First of all, that's surprising. I tried to configure the shrinker so it would stay away from kvm unless memory was really tight. The reason is that kvm mmu pages can cost as much as 1-2 ms of cpu time to build, perhaps even more, so we shouldn't drop them lightly.
It's certainly a neglected area that needs attention, though.
This patch set reworks it to hopefully be more scalable to large numbers of CPUs, as well as large numbers of running VMs. The patches are ordered with increasing invasiveness. These seem to boot and run fine. I'm running about 40 VMs at once, while doing "echo 3> /proc/sys/vm/drop_caches", and killing/restarting VMs constantly.
Will drop_caches actually shrink the kvm caches too? If so we probably need to add that to autotest since it's a really good stress test for the mmu.
Seems to be relatively stable, and seems to keep the numbers of kvm_mmu_page_header objects down.
That's no necessarily a good thing, those things are expensive to recreate. Of course, when we do need to reclaim them, that should be efficient.
We also do a very bad job of selecting which page to reclaim. We need to start using the accessed bit on sptes that point to shadow page tables, and then look those up and reclaim unreferenced pages sooner. With shadow paging there can be tons of unsync pages that are basically unused and can be reclaimed at no cost to future runtime.
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