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