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

[Nick Piggin <npiggin@kernel.dk>]

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.


> 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?

Thanks,
Nick