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

Magnus Damm
2006-03-13T03:05:39Z

On 3/12/06, Peter Zijlstra <peter@programming.kicks-ass.net> wrote:
> 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.

In shrink_zone(), there is +1 logic that adds at least one to
nr_scan_active/nr_scan_inactive, and resets them to zero when they
have reached sc->swap_cluster_max (32 or higher in some cases).

So nr_scan_active/nr_scan_inactive will in most cases be 16
(SWAP_CLUSTER_MAX / 2), regardless of the size of the zone. So, a
total of 256 calls to shrink_zone() on a zone with 4096 pages will
likely scan through 100% of the pages on both LRU lists, while 256
calls to shrink_zone() on a zone with say 8096 pages will result in
around 50% of the pages on the lists are scanned through.

Maybe not entirely true, but the bottom line is that the +1 logic will
scan though smaller zones faster than large ones.

/ magnus