Re: [PATCH 6/9] mm: zone_reclaim: compaction: increase the high order pages in the watermarks

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Aug 06, 2013 at 12:08:38PM -0400, Johannes Weiner wrote:
> On Fri, Aug 02, 2013 at 06:06:33PM +0200, Andrea Arcangeli wrote:
> > Prevent the scaling down to reduce the watermarks too much.
> > 
> > Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> > ---
> >  mm/page_alloc.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 4401983..b32ecde 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1665,7 +1665,8 @@ static bool __zone_watermark_ok(struct zone *z, int order, unsigned long mark,
> >  		free_pages -= z->free_area[o].nr_free << o;
> >  
> >  		/* Require fewer higher order pages to be free */
> > -		min >>= 1;
> > +		if (o < (pageblock_order >> 2))
> > +			min >>= 1;
> 
> Okay, bear with me here:
>
> After an allocation, the watermark has to be met, all available pages
> considered.  That is reasonable because we don't want to deadlock and
> order-0 pages can be served from any page block.
> 
> Now say we have an order-2 allocation: in addition to the order-0 view
> on the watermark, after the allocation a quarter of the watermark has
> to be met with order-2 pages and up.  Okay, maybe we always want a few
> < PAGE_ALLOC_COSTLY_ORDER pages at our disposal, who knows.
>
> Then it kind of sizzles out towards higher order pages but it always
> requires the remainder to be positive, i.e. at least one page at the
> checked order available.  Only the actually requested order is not
> checked, so for an order-9 we only make sure that we could serve at
> least one order-8 page, maybe more depending on the zone size.

Argh, brainfart.  It subtracts free order-8 and what's left is order-9
and up.  So it makes sure there are watermark >> 9 worth of order-9
and up page blocks available after the allocation of an order-9 page.

> You're proposing to check at least for
> 
>   (watermark - min_watermark) >> (pageblock_order >> 2)
> 
> worth of order-8 pages instead.

Wouldn't it be enough to meet watermarks from an order-0 perspective,
maybe require *some* buffer up to PAGE_ALLOC_COSTLY_ORDER, and then be
okay with a single order-n page?  Why do we need more than one?

Anyway, I think the changelog should be a bit more verbose about the
motivation ;-)

Thanks!

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