On Tue, 23 Mar 2010 12:25:46 +0000
Mel Gorman <mel@xxxxxxxxx> 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.
>
>
> ...
>
> +/* 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.
um. "Two wrongs don't make a right". We should fix the other sites,
not use them as excuses ;)
What _is_ a good measure of "time" in this code? "number of pages
scanned" is a pretty good one in reclaim. We want something which will
adapt itself to amount-of-memory, number-of-cpus, speed-of-cpus,
nature-of-workload, etc, etc.
Is it possible to come up with some simple metric which approximately
reflects how busy this code is, then pace ourselves via that?
> + */
> + zone->compact_resume = resume;
> +}
> +
> +static inline int compaction_deferred(struct zone *zone)
> +{
> + /* init once if necessary */
> + if (unlikely(!zone->compact_resume)) {
> + zone->compact_resume = jiffies;
> + return 0;
> + }
> +
> + return time_before(jiffies, zone->compact_resume);
> +}
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