Re: [PATCH 10/25] mm, compaction: Ignore the fragmentation avoidance boost for isolation and compaction

[Vlastimil Babka]

On 1/4/19 1:49 PM, Mel Gorman wrote:
> When pageblocks get fragmented, watermarks are artifically boosted to
> reclaim pages to avoid further fragmentation events. However, compaction
> is often either fragmentation-neutral or moving movable pages away from
> unmovable/reclaimable pages. As the true watermarks are preserved, allow
> compaction to ignore the boost factor.
> 
> The expected impact is very slight as the main benefit is that compaction
> is slightly more likely to succeed when the system has been fragmented
> very recently. On both 1-socket and 2-socket machines for THP-intensive
> allocation during fragmentation the success rate was increased by less
> than 1% which is marginal. However, detailed tracing indicated that
> failure of migration due to a premature ENOMEM triggered by watermark
> checks were eliminated.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 57ba9d1da519..05c9a81d54ed 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2958,7 +2958,7 @@ int __isolate_free_page(struct page *page, unsigned int order)
>  		 * watermark, because we already know our high-order page
>  		 * exists.
>  		 */
> -		watermark = min_wmark_pages(zone) + (1UL << order);
> +		watermark = zone->_watermark[WMARK_MIN] + (1UL << order);
>  		if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA))
>  			return 0;
>  
>