From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from nlpi129.prodigy.net (nlpi129.sbcis.sbc.com [207.115.36.143]) by ozlabs.org (Postfix) with ESMTP id 9B634B7D00 for ; Sat, 20 Feb 2010 02:51:46 +1100 (EST) Date: Fri, 19 Feb 2010 09:51:12 -0600 (CST) From: Christoph Lameter To: Balbir Singh Subject: Re: [PATCH] powerpc: Set a smaller value for RECLAIM_DISTANCE to enable zone reclaim In-Reply-To: <661de9471002190741k34ddb1acidf64d089bf9ff284@mail.gmail.com> Message-ID: References: <20100218222923.GC31681@kryten> <20100219000730.GD31681@kryten> <20100219145523.GN30258@csn.ul.ie> <661de9471002190741k34ddb1acidf64d089bf9ff284@mail.gmail.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: Mel Gorman , linuxppc-dev@lists.ozlabs.org, Anton Blanchard List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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.