On 04/15/2016 11:13 AM, Mel Gorman wrote:
/*
- * If a zone reaches its high watermark, consider it to be no longer
- * congested. It's possible there are dirty pages backed by congested
- * BDIs but as pressure is relieved, speculatively avoid congestion
- * waits.
+ * Fragmentation may mean that the system cannot be rebalanced for
+ * high-order allocations. If twice the allocation size has been
+ * reclaimed then recheck watermarks only at order-0 to prevent
+ * excessive reclaim. Assume that a process requested a high-order
+ * can direct reclaim/compact.
Also kcompactd is woken up in this case...
*/
- if (pgdat_reclaimable(zone->zone_pgdat) &&
- zone_balanced(zone, sc->order, false, 0, classzone_idx)) {
- clear_bit(PGDAT_CONGESTED, &pgdat->flags);
- clear_bit(PGDAT_DIRTY, &pgdat->flags);
- }
+ if (sc->order && sc->nr_reclaimed >= 2UL << sc->order)
+ sc->order = 0;
return sc->nr_scanned >= sc->nr_to_reclaim;
This looks indeed simpler than my earlier zone_balanced() modification
you removed. However I think there's still potential of overreclaim due
to a stream of kswapd_wakeups where each will have to reclaim 2UL <<
sc->order pages, regardless of watermarks. Could be some high-order
wakeups from GFP_ATOMIC context that have order-0 fallbacks but will
cause kswapd to keep reclaiming when kcompactd can't keep up due to
fragmentation...
