Re: [PATCH] kvm: don't call mmu_shrinker w/o used_mmu_pages

[Takuya Yoshikawa <takuya.yoshikawa@gmail.com>]

On Fri, 20 Apr 2012 19:15:24 -0700
Mike Waychison <mikew@google.com> wrote:

> In our situation, we simple disable the shrinker altogether.  My
> understanding is that we EPT or NPT, the amount of memory used by
> these tables is bounded by the size of guest physical memory, whereas
> with software shadowed tables, it is bounded by the addresses spaces
> in the guest.  This bound makes it reasonable to not do any reclaim
> and charge it as a "system overhead tax".

IIRC, KVM's mmu_shrink is mainly for protecting the host from pathological
guest without EPT or NPT.

You can see Avi's summary: -- http://www.spinics.net/lists/kvm/msg65671.html
===
We should aim for the following:
- normal operation causes very little shrinks (some are okay)
- high pressure mostly due to kvm results in kvm being shrunk (this is a
pathological case caused by a starting a guest with a huge amount of
memory, and mapping it all to /dev/zero (or ksm), and getting the guest
the create shadow mappings for all of it)
- general high pressure is shared among other caches like dcache and icache

The cost of reestablishing an mmu page can be as high as half a
millisecond of cpu time, which is the reason I want to be conservative.
===

Thanks,
	Takuya