On Tue, Dec 14, 2010 at 02:43:41PM -0800, Andrew Morton wrote:
> On Fri, 10 Dec 2010 15:46:21 +0000
> Mel Gorman <mel@xxxxxxxxx> wrote:
>
> > When reclaiming for high-orders, kswapd is responsible for balancing a
> > node but it should not reclaim excessively. It avoids excessive reclaim by
> > considering if any zone in a node is balanced then the node is balanced.
>
> Here you're referring to your [patch 1/6] yes? Not to current upstream.
>
Yes.
> > In
> > the cases where there are imbalanced zone sizes (e.g. ZONE_DMA with both
> > ZONE_DMA32 and ZONE_NORMAL), kswapd can go to sleep prematurely as just
> > one small zone was balanced.
>
> Since [1/6]?
>
Yes.
> > This alters the sleep logic of kswapd slightly. It counts the number of pages
> > that make up the balanced zones. If the total number of balanced pages is
>
> Define "balanced page"? Seems to be the sum of the total sizes of all
> zones which have reached their desired free-pages threshold?
>
Correct.
> But this includes all page orders, whereas here we're targetting a
> particular order. Although things should work out OK due to the
> scaling/sizing proportionality.
>
It's the size of the whole zone that is being accounted for and as it's
a watermark check, the order is being taken into account.
> > more than a quarter of the zone, kswapd will go back to sleep. This should
> > keep a node balanced without reclaiming an excessive number of pages.
>
> ick.
>
> > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> > Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
> > ---
> > mm/vmscan.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++++---------
> > 1 files changed, 49 insertions(+), 9 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 625dfba..6723101 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2191,10 +2191,40 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
> > }
> > #endif
> >
> > +/*
> > + * pgdat_balanced is used when checking if a node is balanced for high-order
> > + * allocations.
>
> Is this the correct use of the term "balanced"? I think "balanced" is
> something that happens *between* zones: They've all achieved the same
> (perhaps weighted) ratio of free pages.
>
What would be a better term? pgdat_sufficiently_but_not_fully_balanced()? If
it returns true, it can mean the node is either fully "balanced" as you
define it or that enough zones have enough free suitably-ordered pages for
allocations to succeed.
> > Only zones that meet watermarks and are in a zone allowed
> > + * by the callers classzone_idx are added to balanced_pages. The total of
>
> caller's
>
Right.
> > + * balanced pages must be at least 25% of the zones allowed by classzone_idx
> > + * for the node to be considered balanced. Forcing all zones to be balanced
> > + * for high orders can cause excessive reclaim when there are imbalanced zones.
>
> Excessive reclaim of what?
>
slab, list rotations and pages within the imbalanced zones that may never
become balanced. Minimally, kswapd just stays awake consuming CPU.
> If one particular zone is having trouble achieving its desired level of
> free pages of a partocular order, are you saying that kswapd sits there
> madly scanning other zones, which have already reached their desired
> level? If so, that would be bad.
>
As far as I can gather, yes, this is what is happening. I don't have a local
reproduction case so I'm basing this on a bug report. He has two problems -
kswapd stays awake constantly and way too many pages are free.
> I think you're saying that we just keep on scanning away at this one
> zone. But what was wrong with doing that?
>
It wastes CPU.
> > + * The choice of 25% is due to
> > + * o a 16M DMA zone that is balanced will not balance a zone on any
> > + * reasonable sized machine
>
> How does a zone balance another zone?
>
That should have been "will not balance a node".
> > + * o On all other machines, the top zone must be at least a reasonable
> > + * precentage of the middle zones. For example, on 32-bit x86, highmem
> > + * would need to be at least 256M for it to be balance a whole node.
> > + * Similarly, on x86-64 the Normal zone would need to be at least 1G
> > + * to balance a node on its own. These seemed like reasonable ratios.
> > + */
> > +static bool pgdat_balanced(pg_data_t *pgdat, unsigned long balanced_pages,
> > + int classzone_idx)
> > +{
> > + unsigned long present_pages = 0;
> > + int i;
> > +
> > + for (i = 0; i <= classzone_idx; i++)
> > + present_pages += pgdat->node_zones[i].present_pages;
> > +
> > + return balanced_pages > (present_pages >> 2);
> > +}
> > +
> >
> > ...
> >
>
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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