Re: Free memory never fully used, swapping

[Shaohua Li]

On Fri, 2010-11-26 at 17:18 +0800, KOSAKI Motohiro wrote:
> > On Fri, 2010-11-26 at 10:31 +0800, KOSAKI Motohiro wrote:
> > > > record the order seems not sufficient. in balance_pgdat(), the for look
> > > > exit only when:
> > > > priority <0 or sc.nr_reclaimed >= SWAP_CLUSTER_MAX.
> > > > but we do if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
> > > >                         order = sc.order = 0;
> > > > this means before we set order to 0, we already reclaimed a lot of
> > > > pages, so I thought we need set order to 0 earlier before there are
> > > > enough free pages. below is a debug patch.
> > > > 
> > > > 
> > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > index d31d7ce..ee5d2ed 100644
> > > > --- a/mm/vmscan.c
> > > > +++ b/mm/vmscan.c
> > > > @@ -2117,6 +2117,26 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
> > > >  }
> > > >  #endif
> > > >  
> > > > +static int all_zone_enough_free_pages(pg_data_t *pgdat)
> > > > +{
> > > > +	int i;
> > > > +
> > > > +	for (i = 0; i < pgdat->nr_zones; i++) {
> > > > +		struct zone *zone = pgdat->node_zones + i;
> > > > +
> > > > +		if (!populated_zone(zone))
> > > > +			continue;
> > > > +
> > > > +		if (zone->all_unreclaimable)
> > > > +			continue;
> > > > +
> > > > +		if (!zone_watermark_ok(zone, 0, high_wmark_pages(zone) * 8,
> > > > +								0, 0))
> > > > +			return 0;
> > > > +	}
> > > > +	return 1;
> > > > +}
> > > > +
> > > >  /* is kswapd sleeping prematurely? */
> > > >  static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
> > > >  {
> > > > @@ -2355,7 +2375,8 @@ out:
> > > >  		 * back to sleep. High-order users can still perform direct
> > > >  		 * reclaim if they wish.
> > > >  		 */
> > > > -		if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
> > > > +		if (sc.nr_reclaimed < SWAP_CLUSTER_MAX ||
> > > > +		    (order > 0 && all_zone_enough_free_pages(pgdat)))
> > > >  			order = sc.order = 0;
> > > 
> > > Ummm. this doesn't work. this place is processed every 32 pages reclaimed.
> > > (see below code and comment). Theresore your patch break high order reclaim
> > > logic.
> > Yes, this will break high order reclaim, but we need a compromise.
> > wrongly reclaim pages is more worse. could increase the watermark in
> > all_zone_enough_free_pages() better?
> > 
> 
> Hmm..
> I guess I haven't catch your mention. you wrote 
> 
> > > > but we do if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
> > > >                         order = sc.order = 0;
> > > > this means before we set order to 0, we already reclaimed a lot of
> > > > pages
> 
> and I wrote it's not a lot. So, I don't understand why you are talking
> about watermark increasing now. Personally you seems to talk unrelated
> topic. Can you please elablate your point more
ok let me clarify, in the for-loop of balance_pgdat() we reclaim 32
pages one time. but we have
if (!all_zones_ok) {
...
		if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
			order = sc.order = 0;

		goto loop_again;
	}
only when sc.nr_reclaimed < SWAP_CLUSTER_MAX or priority < 0, we set
order to 0. before this, we still use high order for zone_watermark_ok()
and it will fail and we keep doing page reclaim. So in the proposed
patch by you or Mel, checking the freed pages or order in kswapd() is
later. so I suggest we check if there is enough free pages in
balance_pgdat() and break high order allocation if yes.

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