Re: [PATCH 3/3] Do not unconditionally treat zones that fail zone_reclaim() as full

[Mel Gorman <mel@csn.ul.ie>]

On Tue, Jun 09, 2009 at 11:11:19AM +0800, Wu Fengguang wrote:
> On Mon, Jun 08, 2009 at 09:01:30PM +0800, Mel Gorman wrote:
> > On NUMA machines, the administrator can configure zone_reclaim_mode that
> > is a more targetted form of direct reclaim. On machines with large NUMA
> > distances for example, a zone_reclaim_mode defaults to 1 meaning that clean
> > unmapped pages will be reclaimed if the zone watermarks are not being
> > met. The problem is that zone_reclaim() failing at all means the zone
> > gets marked full.
> > 
> > This can cause situations where a zone is usable, but is being skipped
> > because it has been considered full. Take a situation where a large tmpfs
> > mount is occuping a large percentage of memory overall. The pages do not
> > get cleaned or reclaimed by zone_reclaim(), but the zone gets marked full
> > and the zonelist cache considers them not worth trying in the future.
> > 
> > This patch makes zone_reclaim() return more fine-grained information about
> > what occured when zone_reclaim() failued. The zone only gets marked full if
> > it really is unreclaimable. If it's a case that the scan did not occur or
> > if enough pages were not reclaimed with the limited reclaim_mode, then the
> > zone is simply skipped.
> > 
> > There is a side-effect to this patch. Currently, if zone_reclaim()
> > successfully reclaimed SWAP_CLUSTER_MAX, an allocation attempt would
> > go ahead. With this patch applied, zone watermarks are rechecked after
> > zone_reclaim() does some work.
> >
> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> 
> Thanks for making the code a lot more readable :)
> 
> Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
> 

Thanks.

> >  	/*
> >  	 * Do not scan if the allocation should not be delayed.
> >  	 */
> >  	if (!(gfp_mask & __GFP_WAIT) || (current->flags & PF_MEMALLOC))
> > -			return 0;
> > +			return ZONE_RECLAIM_NOSCAN;
> 
> Why not kill the extra tab?
> 

Why not indeed. Tab is now killed.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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