Re: [PATCH 00/03] Unmapped: Separate unmapped and mapped pages

Peter Zijlstra
2006-03-11T21:08:15Z

On Fri, 2006-03-10 at 14:38 +0100, Magnus Damm wrote:
> On 3/10/06, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> > Breaking the LRU in two like this breaks the page ordering, which makes
> > it possible for pages to stay resident even though they have much less
> > activity than pages that do get reclaimed.
>
> Yes, true. But this happens already with a per-zone LRU. LRU pages
> that happen to end up in the DMA zone will probably stay there a
> longer time than pages in the normal zone. That does not mean it is
> right to break the page ordering though, I'm just saying it happens
> already and the oldest piece of data in the global system will not be
> reclaimed first - instead there are priorities such as unmapped pages
> will be reclaimed over mapped and so on. (I strongly feel that there
> should be per-node LRU:s, but that's another story)

If reclaim works right* there is equal pressure on each zone
(proportional to their size) and hence each page will have an equal life
time expectancy.

(*) this is of course not possible for all workloads, however
balance_pgdat and the page allocator take pains to make it as true as
possible.

Peter