Messages in this thread | | | Date | Thu, 11 Aug 2011 21:38:05 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH 5/7] mm: vmscan: Do not writeback filesystem pages in kswapd except in high priority |
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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@suse.de> > >Reviewed-by: Minchan Kim<minchan.kim@gmail.com> > > 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 http://www.csn.ul.ie/~mel/postings/riel-20110811
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@redhat.com>
Thanks.
-- Mel Gorman SUSE Labs
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