Re: compaction: trying to understand the code

[Wu Fengguang <fengguang.wu@intel.com>]

On Fri, Aug 20, 2010 at 05:35:59PM +0800, Mel Gorman 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);

s/NR_INACTIVE_ANON/NR_ACTIVE_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;

Note that for anon LRU, inactive_ratio may be large numbers.
(inactive > active) is not easy, and not stable even when inactive_ratio=1.

Thanks,
Fengguang

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