Re: [PATCH 5/7] mm: vmscan: Do not writeback filesystem pages inkswapd except in high priority

From: Mel Gorman
Date: Thu Aug 11 2011 - 16:38:14 EST

On Thu, Aug 11, 2011 at 02:18:54PM -0400, Rik van Riel wrote:
> On 08/10/2011 06:47 AM, Mel Gorman wrote:
> >It is preferable that no dirty pages are dispatched for cleaning from
> >the page reclaim path. At normal priorities, this patch prevents kswapd
> >writing pages.
> >
> >However, page reclaim does have a requirement that pages be freed
> >in a particular zone. If it is failing to make sufficient progress
> >(reclaiming< SWAP_CLUSTER_MAX at any priority priority), the priority
> >is raised to scan more pages. A priority of DEF_PRIORITY - 3 is
> >considered to be the point where kswapd is getting into trouble
> >reclaiming pages. If this priority is reached, kswapd will dispatch
> >pages for writing.
> >
> >Signed-off-by: Mel Gorman<mgorman@xxxxxxx>
> >Reviewed-by: Minchan Kim<>
> My only worry with this patch is that maybe we'll burn too
> much CPU time freeing pages from a zone.

The throttling patch prevents too much CPU being used if pages under
writeback are being encountered during scanning. That said, I shared
your concern and recorded kswapd CPU usage over time.

> However, chances
> are we'll have freed pages from other zones when scanning
> one zone multiple times (the page cache dirty limit is global,
> the clean pages have to be _somewhere_).
> Since the bulk of the allocators are not too picky about
> which zone they get their pages from, I suspect this patch
> will be an overall improvement pretty much all the time.

This is roughly similar to my own reasoning.

I uploaded all the kswapd CPU usage charts to

These are smoothened as the raw figures are barely readable. If you
go through them, you'll see that kswapd CPU usage is sometimes higher
but generally within 2-3%.

> Acked-by: Rik van Riel <riel@xxxxxxxxxx>


Mel Gorman
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