Re: [patch] mm: default to node zonelist ordering when nodes have only lowmem

[David Rientjes <rientjes@google.com>]

On Fri, 26 Mar 2010, Mel Gorman wrote:

> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2582,7 +2582,7 @@ static int default_zonelist_order(void)
> >           * ZONE_DMA and ZONE_DMA32 can be very small area in the sytem.
> >  	 * If they are really small and used heavily, the system can fall
> >  	 * into OOM very easily.
> > -	 * This function detect ZONE_DMA/DMA32 size and confgigures zone order.
> > +	 * This function detect ZONE_DMA/DMA32 size and configures zone order.
> >  	 */
> 
> Spurious change here but it's not very important.
> 
> >  	/* Is there ZONE_NORMAL ? (ex. ppc has only DMA zone..) */
> >  	low_kmem_size = 0;
> > @@ -2594,6 +2594,15 @@ static int default_zonelist_order(void)
> >  				if (zone_type < ZONE_NORMAL)
> >  					low_kmem_size += z->present_pages;
> >  				total_size += z->present_pages;
> > +			} else if (zone_type == ZONE_NORMAL) {
> > +				/*
> 
> What if it was ZONE_DMA32?
> 

This is part of a zone iteration for each node, so if the node consists of 
only ZONE_DMA then it wouldn't have a populated ZONE_NORMAL either and 
will return ZONELIST_ORDER_NODE on the next iteration.

> > +				 * If any node has only lowmem, then node order
> > +				 * is preferred to allow kernel allocations
> > +				 * locally; otherwise, they can easily infringe
> > +				 * on other nodes when there is an abundance of
> > +				 * lowmem available to allocate from.
> > +				 */
> > +				return ZONELIST_ORDER_NODE;
> 
> It might be clearer if it was done as a similar check later
> 
> 		if (low_kmem_size &&
> 		    total_size > average_size && /* ignore small node */
> 		    low_kmem_size > total_size * 70/100)
> 			return ZONELIST_ORDER_NODE;
> 
> This is saying if low memory is > 70% of total, then use nodes. To take
> yours into account, it'd look something like;
> 
> if (low_kmwm_size && total_size > average_size) {
> 	if (lowmem_size == total_size)
> 		return ZONELIST_ORDER_ZONE;
> 
> 	if (lowmem_size > total_size * 70/100)
> 		return ZONELIST_ORDER_NODE;
> }

There's no guarantee that we'd ever detect the node consisiting of solely 
lowmem here since it may be asymmetrically smaller than the average node 
size.

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