Re: [PATCH 06/27] mm, vmscan: Make kswapd reclaim in terms of nodes

Vlastimil Babka
2016-04-28T08:36:40Z

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...