Re: [PATCH 7/7] Do not compact within a preferred zone after a compaction failure

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

On Wed, Jan 13, 2010 at 03:28:16PM -0800, David Rientjes wrote:
> On Wed, 6 Jan 2010, Mel Gorman wrote:
> 
> > The fragmentation index may indicate that a failure it due to external
> > fragmentation, a compaction run complete and an allocation failure still
> > fail. There are two obvious reasons as to why
> > 
> >   o Page migration cannot move all pages so fragmentation remains
> >   o A suitable page may exist but watermarks are not met
> > 
> > In the event of compaction and allocation failure, this patch prevents
> > compaction happening for a short interval. It's only recorded on the
> > preferred zone but that should be enough coverage. This could have been
> > implemented similar to the zonelist_cache but the increased size of the
> > zonelist did not appear to be justified.
> > 
> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > ---
> >  include/linux/mmzone.h |    7 +++++++
> >  mm/page_alloc.c        |   15 ++++++++++++++-
> >  2 files changed, 21 insertions(+), 1 deletions(-)
> > 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index 30fe668..1d6ccbe 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -328,6 +328,13 @@ struct zone {
> >  	unsigned long		*pageblock_flags;
> >  #endif /* CONFIG_SPARSEMEM */
> >  
> > +#ifdef CONFIG_MIGRATION
> > +	/*
> > +	 * If a compaction fails, do not try compaction again until
> > +	 * jiffies is after the value of compact_resume
> > +	 */
> > +	unsigned long		compact_resume;
> > +#endif
> 
> CONFIG_COMPACTION?
> 

Yep

> >  
> >  	ZONE_PADDING(_pad1_)
> >  
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 7275afb..9c86606 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1729,7 +1729,7 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
> >  	cond_resched();
> >  
> >  	/* Try memory compaction for high-order allocations before reclaim */
> > -	if (order) {
> > +	if (order && time_after(jiffies, preferred_zone->compact_resume)) {
> >  		*did_some_progress = try_to_compact_pages(zonelist,
> >  						order, gfp_mask, nodemask);
> >  		if (*did_some_progress != COMPACT_INCOMPLETE) {
> > @@ -1748,6 +1748,19 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
> >  			 * but not enough to satisfy watermarks.
> >  			 */
> >  			count_vm_event(COMPACTFAIL);
> > +
> > +			/*
> > +			 * On failure, avoid compaction for a short time.
> > +			 * XXX: This is very unsatisfactory. The failure
> > +			 * 	to compact has nothing to do with time
> > +			 * 	and everything to do with the requested
> > +			 * 	order, the number of free pages and
> > +			 * 	watermarks. How to wait on that is more
> > +			 * 	unclear, but the answer would apply to
> > +			 * 	other areas where the VM waits based on
> > +			 * 	time.
> > +			 */
> > +			preferred_zone->compact_resume = jiffies + HZ/50;
> >  		}
> >  	}
> >  
> 
> This will need to be moved to (another) inline function dependent on 
> CONFIG_COMPACTION since we don't have zone->compact_resume without it; 
> it's probably better to seperate the function out rather than add #ifdef's 
> within __alloc_pages_direct_reclaim().
> 

Moved to an inline called defer_compaction()

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