Re: [PATCH 2/5] mm: vmscan: Scale number of pages reclaimed byreclaim/compaction based on failures

[Minchan Kim]

On Thu, Aug 09, 2012 at 02:49:22PM +0100, Mel Gorman wrote:
> If allocation fails after compaction then compaction may be deferred for
> a number of allocation attempts. If there are subsequent failures,
> compact_defer_shift is increased to defer for longer periods. This patch
> uses that information to scale the number of pages reclaimed with
> compact_defer_shift until allocations succeed again. The rationale is
> that reclaiming the normal number of pages still allowed compaction to
> fail and its success depends on the number of pages. If it's failing,
> reclaim more pages until it succeeds again.
> 
> Note that this is not implying that VM reclaim is not reclaiming enough
> pages or that its logic is broken. try_to_free_pages() always asks for
> SWAP_CLUSTER_MAX pages to be reclaimed regardless of order and that is
> what it does. Direct reclaim stops normally with this check.
> 
> 	if (sc->nr_reclaimed >= sc->nr_to_reclaim)
> 		goto out;
> 
> should_continue_reclaim delays when that check is made until a minimum number
> of pages for reclaim/compaction are reclaimed. It is possible that this patch
> could instead set nr_to_reclaim in try_to_free_pages() and drive it from
> there but that's behaves differently and not necessarily for the better. If
> driven from do_try_to_free_pages(), it is also possible that priorities
> will rise. When they reach DEF_PRIORITY-2, it will also start stalling
> and setting pages for immediate reclaim which is more disruptive than not
> desirable in this case. That is a more wide-reaching change that could
> cause another regression related to THP requests causing interactive jitter.
> 
> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Reviewed-by: Minchan Kim <minchan@xxxxxxxxxx>

-- 
Kind regards,
Minchan Kim
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