linux-kernel - Re: [patch] mm: vmscan implement per-zone shrinkers

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Message-Id: <20101123162027.7BB0.A69D9226@jp.fujitsu.com>
Date:	Tue, 23 Nov 2010 16:21:54 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Nick Piggin <npiggin@...nel.dk>
Cc:	kosaki.motohiro@...fujitsu.com,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch] mm: vmscan implement per-zone shrinkers


Sorry for the delay. Recently I have no time at all ;)


> On Mon, Nov 15, 2010 at 09:50:36AM +0900, KOSAKI Motohiro wrote:
> > > > @@ -1835,8 +1978,6 @@ static void shrink_zone(int priority, st
> > > >  			break;
> > > >  	}
> > > >  
> > > > -	sc->nr_reclaimed = nr_reclaimed;
> > > > -
> > > >  	/*
> > > >  	 * Even if we did not try to evict anon pages at all, we want to
> > > >  	 * rebalance the anon lru active/inactive ratio.
> > > > @@ -1844,6 +1985,23 @@ static void shrink_zone(int priority, st
> > > >  	if (inactive_anon_is_low(zone, sc))
> > > >  		shrink_active_list(SWAP_CLUSTER_MAX, zone, sc, priority, 0);
> > > >  
> > > > +	/*
> > > > +	 * Don't shrink slabs when reclaiming memory from
> > > > +	 * over limit cgroups
> > > > +	 */
> > > > +	if (sc->may_reclaim_slab) {
> > > > +		struct reclaim_state *reclaim_state = current->reclaim_state;
> > > > +
> > > > +		shrink_slab(zone, sc->nr_scanned - nr_scanned,
> > > 
> > > Doubtful calculation. What mean "sc->nr_scanned - nr_scanned"?
> > > I think nr_scanned simply keep old slab balancing behavior.
> > 
> > And per-zone reclaim can lead to new issue. On 32bit highmem system,
> > theorically the system has following memory usage.
> > 
> > ZONE_HIGHMEM: 100% used for page cache
> > ZONE_NORMAL:  100% used for slab
> > 
> > So, traditional page-cache/slab balancing may not work. I think following
> 
> Yes, in theory you are right. I guess in theory the same hole exists
> if we have 0% page cache reclaimable globally, but this may be slightly
> more likely to hit.

I'm not worry about so much "0% page cache reclaimable globally" case
because I doubt it can be happen in real.


> > new calculation or somethinhg else is necessary.
> > 
> > 	if (zone_reclaimable_pages() > NR_SLAB_RECLAIMABLE) {
> > 		using current calculation
> > 	} else {
> > 		shrink number of "objects >> reclaim-priority" objects
> > 		(as page cache scanning calculation)
> > 	}
> > 
> > However, it can be separate this patch, perhaps.
> 
> I agree. In fact, perhaps the new calculation would work well in all
> cases anyway, so maybe we should move away from making slab reclaim a
> slave to pagecache reclaim.
> 
> Can we approach that in subsequent patches?

OK!



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