Re: [PATCH 13/14] Do not compact within a preferred zone after acompaction failure

From: Andrew Morton
Date: Tue Apr 06 2010 - 20:08:21 EST

On Fri, 2 Apr 2010 17:02:47 +0100
Mel Gorman <mel@xxxxxxxxx> wrote:

> The fragmentation index may indicate that a failure is due to external
> fragmentation but after a compaction run completes, it is still possible
> for an allocation to 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 followed by an allocation failure, this patch
> defers further compaction in the zone for a period of time. The zone that
> is deferred is the first zone in the zonelist - i.e. the preferred zone.
> To defer compaction in the other zones, the information would need to be
> stored in the zonelist or implemented similar to the zonelist_cache.
> This would impact the fast-paths and is not justified at this time.

Your patch, it sucks!

> ---
> include/linux/compaction.h | 35 +++++++++++++++++++++++++++++++++++
> include/linux/mmzone.h | 7 +++++++
> mm/page_alloc.c | 5 ++++-
> 3 files changed, 46 insertions(+), 1 deletions(-)
> diff --git a/include/linux/compaction.h b/include/linux/compaction.h
> index ae98afc..2a02719 100644
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -18,6 +18,32 @@ extern int sysctl_extfrag_handler(struct ctl_table *table, int write,
> extern int fragmentation_index(struct zone *zone, unsigned int order);
> extern unsigned long try_to_compact_pages(struct zonelist *zonelist,
> int order, gfp_t gfp_mask, nodemask_t *mask);
> +
> +/* defer_compaction - Do not compact within a zone until a given time */
> +static inline void defer_compaction(struct zone *zone, unsigned long resume)
> +{
> + /*
> + * This function is called when compaction fails to result in a page
> + * allocation success. This is somewhat unsatisfactory as 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.
> + */

c'mon, let's not make this rod for our backs.

The "A suitable page may exist but watermarks are not met" case can be
addressed by testing the watermarks up-front, surely?

I bet the "Page migration cannot move all pages so fragmentation
remains" case can be addressed by setting some metric in the zone, and
suitably modifying that as a result on ongoing activity. To tell the
zone "hey, compaction migth be worth trying now". that sucks too, but not
so much.

Or something. Putting a wallclock-based throttle on it like this
really does reduce the usefulness of the whole feature.

Internet: "My application works OK on a hard disk but fails when I use an SSD!".

akpm: "Tell Mel!"

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