Messages in this thread | | | Date | Mon, 24 Sep 2012 14:26:44 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 8/9] mm: compaction: Cache if a pageblock was scanned and no pages were isolated |
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On Mon, 24 Sep 2012 10:39:38 +0100 Mel Gorman <mgorman@suse.de> wrote:
> On Fri, Sep 21, 2012 at 02:36:56PM -0700, Andrew Morton wrote: > > > Also, what has to be done to avoid the polling altogether? eg/ie, zap > > a pageblock's PB_migrate_skip synchronously, when something was done to > > that pageblock which justifies repolling it? > > > > The "something" event you are looking for is pages being freed or > allocated in the page allocator. A movable page being allocated in block > or a page being freed should clear the PB_migrate_skip bit if it's set. > Unfortunately this would impact the fast path of the alloc and free paths > of the page allocator. I felt that that was too high a price to pay.
We already do a similar thing in the page allocator: clearing of ->all_unreclaimable and ->pages_scanned. But that isn't on the "fast path" really - it happens once per pcp unload. Can we do something like that? Drop some hint into the zone without having to visit each page?
> > > > > > ... > > > > > > +static void reset_isolation_suitable(struct zone *zone) > > > +{ > > > + unsigned long start_pfn = zone->zone_start_pfn; > > > + unsigned long end_pfn = zone->zone_start_pfn + zone->spanned_pages; > > > + unsigned long pfn; > > > + > > > + /* > > > + * Do not reset more than once every five seconds. If allocations are > > > + * failing sufficiently quickly to allow this to happen then continually > > > + * scanning for compaction is not going to help. The choice of five > > > + * seconds is arbitrary but will mitigate excessive scanning. > > > + */ > > > + if (time_before(jiffies, zone->compact_blockskip_expire)) > > > + return; > > > + zone->compact_blockskip_expire = jiffies + (HZ * 5); > > > + > > > + /* Walk the zone and mark every pageblock as suitable for isolation */ > > > + for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) { > > > + struct page *page; > > > + if (!pfn_valid(pfn)) > > > + continue; > > > + > > > + page = pfn_to_page(pfn); > > > + if (zone != page_zone(page)) > > > + continue; > > > + > > > + clear_pageblock_skip(page); > > > + } > > > > What's the worst-case loop count here? > > > > zone->spanned_pages >> pageblock_order
What's the worst-case value of (zone->spanned_pages >> pageblock_order) :)
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