Re: make swappiness safer to use

Fengguang Wu
2007-08-01T01:19:25Z

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;