Re: [RFC] respect the referenced bit of KVM guest pages?

[Avi Kivity <avi@redhat.com>]

On 08/05/2009 05:15 PM, Rik van Riel wrote:
>> If that's indeed the case, we can have the EPT ageing mechanism give 
>> pages a bit more time around by using an available bit in the EPT 
>> PTEs to return accessed on the first pass and not-accessed on the 
>> second.
>
> Can we find out which pages are EPT pages?
>

No need to (see below).

> If so, we could unmap them when they get moved from the
> active to the inactive list, and soft fault them back in
> on access, emulating the referenced bit for EPT pages and
> making page replacement on them work like it should.

It should be easy to implement via the mmu notifier callback: when the 
mm calls clear_flush_young(), mark it as young, and unmap it from the 
EPT pagetable.

> Your approximation of pretending the page is accessed the
> first time and pretending it's not the second time sounds
> like it will just lead to less efficient FIFO replacement,
> not to anything even vaguely approximating LRU.

Right, it's just a hack that gives EPT pages higher priority, like the 
original patch suggested.  Note that LRU for VMs is not a good 
algorithm, since the VM will also reference the least recently used 
page, leading to thrashing.

-- 
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