Re: compaction: trying to understand the code

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

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);
}
-- 
Kind regards,
Minchan Kim

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