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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