Re: [PATCH] powerpc: Set a smaller value for RECLAIM_DISTANCE to enable zone reclaim

[Christoph Lameter <cl@linux-foundation.org>]

On Fri, 19 Feb 2010, Balbir Singh wrote:

> >> zone_reclaim. The others back off and try the next zone in the zonelist
> >> instead. I'm not sure what the original intention was but most likely it
> >> was to prevent too many parallel reclaimers in the same zone potentially
> >> dumping out way more data than necessary.
> >
> > Yes it was to prevent concurrency slowing down reclaim. At that time the
> > number of processors per NUMA node was 2 or so. The number of pages that
> > are reclaimed is limited to avoid tossing too many page cache pages.
> >
>
> That is interesting, I always thought it was to try and free page
> cache first. For example with zone->min_unmapped_pages, if
> zone_pagecache_reclaimable is greater than unmapped pages, we start
> reclaim the cached pages first. The min_unmapped_pages almost sounds
> like the higher level watermark - or am I misreading the code.

Indeed the purpose is to free *old* page cache pages.

The min_unmapped_pages is to protect a mininum of the page cache pages /
fs metadata from zone reclaim so that ongoing file I/O is not impacted.