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

[David Rientjes <rientjes@google.com>]

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?

>  
>  	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().

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