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

[Balbir Singh <balbir@linux.vnet.ibm.com>]

On Fri, Feb 19, 2010 at 8:42 PM, Christoph Lameter
<cl@linux-foundation.org> wrote:
> On Fri, 19 Feb 2010, Mel Gorman wrote:
>
>> > > The patch below sets a smaller value for RECLAIM_DISTANCE and thus e=
nables
>> > > zone reclaim.
>> >
>>
>> I've no problem with the patch anyway.
>
> Nor do I.
>
>> > - We seem to end up racing between zone_watermark_ok, zone_reclaim and
>> > =A0 buffered_rmqueue. Since everyone is in here the memory one thread =
reclaims
>> > =A0 may be stolen by another thread.
>> >
>>
>> You're pretty much on the button here. Only one thread at a time enters
>> 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.

Balbir Singh