Re: [PATCH 05/10] mm: vmscan: Do not allow kswapd to scan at maximum priority

[Rik van Riel <riel@redhat.com>]

On 03/17/2013 09:04 AM, Mel Gorman wrote:
> Page reclaim at priority 0 will scan the entire LRU as priority 0 is
> considered to be a near OOM condition. Kswapd can reach priority 0 quite
> easily if it is encountering a large number of pages it cannot reclaim
> such as pages under writeback. When this happens, kswapd reclaims very
> aggressively even though there may be no real risk of allocation failure
> or OOM.
>
> This patch prevents kswapd reaching priority 0 and trying to reclaim
> the world. Direct reclaimers will still reach priority 0 in the event
> of an OOM situation.
>
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> ---
>   mm/vmscan.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 7513bd1..af3bb6f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -2891,7 +2891,7 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
>   		 */
>   		if (raise_priority || !this_reclaimed)
>   			sc.priority--;
> -	} while (sc.priority >= 0 &&
> +	} while (sc.priority >= 1 &&
>   		 !pgdat_balanced(pgdat, order, *classzone_idx));
>
>   out:
>

If priority 0 is way way way way way too aggressive, what makes
priority 1 safe?

This makes me wonder, are the priorities useful at all to kswapd?

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