On Tue, 23 Mar 2010, Mel Gorman wrote:

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> The fragmentation index may indicate that a failure it due to external

s/it/is/

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> fragmentation, a compaction run complete and an allocation failure still

???

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> 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

compaction is "recorded"? deferred?

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> 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.

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> @@ -1787,6 +1787,9 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
>  			 */
>  			count_vm_event(COMPACTFAIL);
>
> +			/* On failure, avoid compaction for a short time. */
> +			defer_compaction(preferred_zone, jiffies + HZ/50);
> +

20ms? How was that interval determined?

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