* Christoph Lameter [off-list ref] [2010-02-19 09:51:12]:

On Fri, 19 Feb 2010, Balbir Singh wrote:

quoted

quoted

quoted

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.

Thanks for the explanation!

-- 
	Three Cheers,
	Balbir