Re: compaction: trying to understand the code

[Minchan Kim <minchan.kim@gmail.com>]

On Fri, Aug 20, 2010 at 07:22:16PM +0900, Minchan Kim wrote:
> On Fri, Aug 20, 2010 at 6:35 PM, Mel Gorman <mel@csn.ul.ie> wrote:
> > On Fri, Aug 20, 2010 at 01:34:47PM +0800, Wu Fengguang wrote:
> >> You do run lots of tasks: kernel_stack=1880kB.
> >>
> >> And you have lots of free memory, page reclaim has never run, so
> >> inactive_anon=0. This is where compaction is different from vmscan.
> >> In vmscan, inactive_anon is reasonably large, and will only be
> >> compared directly with isolated_anon.
> >>
> >
> > True, the key observation here was that compaction is being run via the
> > proc trigger. Normally it would be run as part of the direct reclaim
> > path when kswapd would already be awake. too_many_isolated() needs to be
> > different for compaction to take the whole system into account. What
> > would be the best alternative? Here is one possibility. A reasonable
> > alternative would be that when inactive < active that isolated can't be
> > more than num_online_cpus() * 2 (i.e. one compactor per online cpu).
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index 94cce51..1e000b7 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -215,14 +215,16 @@ static void acct_isolated(struct zone *zone, struct compact_control *cc)
> >  static bool too_many_isolated(struct zone *zone)
> >  {
> >
> > -       unsigned long inactive, isolated;
> > +       unsigned long active, inactive, isolated;
> >
> > +       active = zone_page_state(zone, NR_ACTIVE_FILE) +
> > +                                       zone_page_state(zone, NR_INACTIVE_ANON);
> >        inactive = zone_page_state(zone, NR_INACTIVE_FILE) +
> >                                        zone_page_state(zone, NR_INACTIVE_ANON);
> >        isolated = zone_page_state(zone, NR_ISOLATED_FILE) +
> >                                        zone_page_state(zone, NR_ISOLATED_ANON);
> >
> > -       return isolated > inactive;
> > +       return (inactive > active) ? isolated > inactive : false;
> >  }
> >
> >  /*
> >
> 
> 1. active : 1000 inactive : 1000
> 2. parallel reclaiming -> active : 1000 inactive : 500 isolated : 500
> 3. too_many_isolated return false.
> 
> But in this  case, there are already many isolated pages. So it should
> return true.
> 
> How about this?
> too_many_isolated()
> {
>       return (isolated > nr_zones * nr_nodes * nr_online_cpu *
> SWAP_CLUSTER_MAX);
> }

Above utterly not good. 
How about this?