Re: make swappiness safer to use

[Fengguang Wu <fengguang.wu@gmail.com>]

On Wed, Aug 01, 2007 at 01:02:51AM +0200, Andrea Arcangeli wrote:
> diff --git a/mm/vmscan.c b/mm/vmscan.c
[...]
> @@ -912,6 +913,44 @@ static void shrink_active_list(unsigned 
>  		 * altogether.
>  		 */
>  		swap_tendency = mapped_ratio / 2 + distress + sc->swappiness;
> +
> +		/*
> +		 * If there's huge imbalance between active and inactive
> +		 * (think active 100 times larger than inactive) we should
> +		 * become more permissive, or the system will take too much
> +		 * cpu before it start swapping during memory pressure.
> +		 * Distress is about avoiding early-oom, this is about
> +		 * making swappiness graceful despite setting it to low
> +		 * values.
> +		 *
> +		 * Avoid div by zero with nr_inactive+1, and max resulting
> +		 * value is vm_total_pages.
> +		 */
> +		imbalance = zone_page_state(zone, NR_ACTIVE) /
> +                        (zone_page_state(zone, NR_INACTIVE) + 1);
> +
> +		/*
> +		 * Reduce the effect of imbalance if swappiness is low,
> +		 * this means for a swappiness very low, the imbalance
> +		 * must be much higher than 100 for this logic to make
> +		 * the difference.
> +		 *
> +		 * Max temporary value is vm_total_pages*100.
> +		 */
> +		imbalance *= (vm_swappiness + 1) / 100;
                             ~~~~~~~~~~~~~~~~~~~~~~~~~ It will be zero!

Better to scale it up before the division:
		imbalance *= (vm_swappiness + 1) * 1024 / 100;

> +
> +		/*
> +		 * If not much of the ram is mapped, makes the imbalance
> +		 * less relevant, it's high priority we refill the inactive
> +		 * list with mapped pages only in presence of high ratio of
> +		 * mapped pages.
> +		 *
> +		 * Max temporary value is vm_total_pages*100.
> +		 */
> +		imbalance *= mapped_ratio / 100;

		imbalance *= mapped_ratio * 1024 / 100;

> +		/* apply imbalance feedback to swap_tendency */
> +		swap_tendency += imbalance;

		swap_tendency += imbalance / 1024 / 1024;

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>