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

[Rik van Riel]

Wu Fengguang wrote:
> On Fri, Aug 07, 2009 at 11:17:22AM +0800, KOSAKI Motohiro wrote:
> > > Andrea Arcangeli wrote:
> > >
> > > > Likely we need a cut-off point, if we detect it takes more than X
> > > > seconds to scan the whole active list, we start ignoring young bits,
> > > We could just make this depend on the calculated inactive_ratio,
> > > which depends on the size of the list.
> > >
> > > For small systems, it may make sense to make every accessed bit
> > > count, because the working set will often approach the size of
> > > memory.
> > >
> > > On very large systems, the working set may also approach the
> > > size of memory, but the inactive list only contains a small
> > > percentage of the pages, so there is enough space for everything.
> > >
> > > Say, if the inactive_ratio is 3 or less, make the accessed bit
> > > on the active lists count.
> > Sound reasonable.
>
> Yes, such kind of global measurements would be much better.
>
> > How do we confirm the idea correctness?
>
> In general the active list tends to grow large on under-scanned LRU.
> I guess Rik is pretty familiar with typical inactive_ratio values of
> the large memory systems and may even have some real numbers :)
>
> > Wu, your X focus switching benchmark is sufficient test?
>
> It is a major test case for memory tight desktop.  Jeff presents
> another interesting one for KVM, hehe.
>
> Anyway I collected the active/inactive list sizes, and the numbers
> show that the inactive_ratio is roughly 1 when the LRU is scanned
> actively and may go very high when it is under-scanned.

inactive_ratio is based on the zone (or cgroup) size.

For zones it is a fixed value, which is available in
/proc/zoneinfo

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