Re: make swappiness safer to use

Andrea Arcangeli
2007-07-31T23:02:51Z

On Tue, Jul 31, 2007 at 03:51:09PM -0700, Andrew Morton wrote:
> Yeah, I misread the paranthesisation. sorry.

never mind!

> I nice way of coding this would be:
>
> /*
> * comment goes here
> */
> adjust = zone_page_state(zone, NR_ACTIVE) /
> (zone_page_state(zone, NR_INACTIVE) + 1);
>
> /*
> * comment goes here
> */
> adjust *= (vm_swappiness + 1) / 100;
>
> /*
> * comment goes here
> */
> adjust *= mapped_ratio / 100;
>
> /*
> * comment goes here
> */
> swap_tendency += adjust;
>
> so there's no confusion over parenthesisation or associativity, and the
> reader can see the logic as it unfolds. The compiler should do exactly the
> same thing.
>
> It is worth expending the extra effort and screen space for clarity in that
> part of the kernel, given the amount of trouble it causes, and the amount
> of time people spend sweating over it. Those would want to be good
> comments, too.

Ok.

Signed-off-by: Kurt Garloff <garloff@suse.de>
Signed-off-by: Andrea Arcangeli <andrea@suse.de>

diff --git a/mm/vmscan.c b/mm/vmscan.c
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -879,6 +879,7 @@ static void shrink_active_list(unsigned
long mapped_ratio;
long distress;
long swap_tendency;
+ long imbalance;

if (zone_is_near_oom(zone))
goto force_reclaim_mapped;
@@ -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;
+
+ /*
+ * 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;
+
+ /* apply imbalance feedback to swap_tendency */
+ swap_tendency += imbalance;

/*
* Now use this metric to decide whether to start moving mapped