Re: compaction: trying to understand the code

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

On Mon, Aug 23, 2010 at 07:23:16AM +0800, Wu Fengguang wrote:
> > From: Minchan Kim <minchan.kim@gmail.com>
> > Date: Mon, 23 Aug 2010 00:20:44 +0900
> > Subject: [PATCH] compaction: handle active and inactive fairly in too_many_isolated
> > 
> > Iram reported compaction's too_many_isolated loops forever.
> > (http://www.spinics.net/lists/linux-mm/msg08123.html)
> > 
> > The meminfo of situation happened was inactive anon is zero.
> > That's because the system has no memory pressure until then.
> > While all anon pages was in active lru, compaction could select
> > active lru as well as inactive lru. That's different things
> > with vmscan's isolated. So we has been two too_many_isolated.
> > 
> > While compaction can isolated pages in both active and inactive,
> > current implementation of too_many_isolated only considers inactive.
> > It made Iram's problem.
> > 
> > This patch handles active and inactie with fair.
> > That's because we can't expect where from and how many compaction would
> > isolated pages.
> > 
> > This patch changes (nr_isolated > nr_inactive) with
> > nr_isolated > (nr_active + nr_inactive) / 2.
> 
> The change looks good, thanks. However I'm not sure if it's enough.
> 
> I wonder where the >40MB isolated pages come about.  inactive_anon
> remains 0 and free remains high over a long time, so it seems there
> are no concurrent direct reclaims at all. Are the pages isolated by
> the compaction process itself?

I think it can't happen without kswapd or direct reclaim.
But I think direct reclaim doesn't happen becuase Iram has no activity on system 
at that time. So just geussing following scenario.

1. trigger compaction by proc
2. isolate some pages and then migrate_pages
3. migrate_pages calls cond_resched
4. someone need big page(I am not sure this part)
4. kswapd: shrink anon active list due to inactive_anon_is_low
5. kswapd: isolate_lru_pages for order > 0 (ex, 0.5M page) so 0.5 M * 32 = 16M are isolated
6. kswapd: shrink_zone : shrink anon active list due to inactive_anon_is_low 
7. kswapd: isolate_lru_pages for order > 0 (ex, 0.5M page) so 0.5 M * 32  are isolated again.

Does it make sense?

-- 
Kind regards,
Minchan Kim

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