On Thu, Nov 24, 2011 at 10:07:55AM +0900, KAMEZAWA Hiroyuki wrote:
>
>
> Can I make a question ?
>
> On Wed, 23 Nov 2011 14:34:16 +0100
> Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
>
> > + /*
> > + * When allocating a page cache page for writing, we
> > + * want to get it from a zone that is within its dirty
> > + * limit, such that no single zone holds more than its
> > + * proportional share of globally allowed dirty pages.
> > + * The dirty limits take into account the zone's
> > + * lowmem reserves and high watermark so that kswapd
> > + * should be able to balance it without having to
> > + * write pages from its LRU list.
> > + *
> > + * This may look like it could increase pressure on
> > + * lower zones by failing allocations in higher zones
> > + * before they are full. But the pages that do spill
> > + * over are limited as the lower zones are protected
> > + * by this very same mechanism. It should not become
> > + * a practical burden to them.
> > + *
> > + * XXX: For now, allow allocations to potentially
> > + * exceed the per-zone dirty limit in the slowpath
> > + * (ALLOC_WMARK_LOW unset) before going into reclaim,
> > + * which is important when on a NUMA setup the allowed
> > + * zones are together not big enough to reach the
> > + * global limit. The proper fix for these situations
> > + * will require awareness of zones in the
> > + * dirty-throttling and the flusher threads.
> > + */
> > + if ((alloc_flags & ALLOC_WMARK_LOW) &&
> > + (gfp_mask & __GFP_WRITE) && !zone_dirty_ok(zone))
> > + goto this_zone_full;
> >
> > BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
> > if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
>
> This wil call
>
> if (NUMA_BUILD)
> zlc_mark_zone_full(zonelist, z);
>
> And this zone will be marked as full.
>
> IIUC, zlc_clear_zones_full() is called only when direct reclaim ends.
> So, if no one calls direct-reclaim, 'full' mark may never be cleared
> even when number of dirty pages goes down to safe level ?
> I'm sorry if this is alread discussed.
It does not remember which zones are marked full for longer than a
second - see zlc_setup() - and also ignores this information when an
iteration over the zonelist with the cache enabled came up
empty-handed.
I thought it would make sense to take advantage of the cache and save
the zone_dirty_ok() checks against ineligible zones too on subsequent
iterations.
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