Re: [PATCH 01/10] mm: vmscan: Limit the number of pages kswapdreclaims at each priority

From: Johannes Weiner
Date: Thu Mar 21 2013 - 11:58:10 EST

On Sun, Mar 17, 2013 at 01:04:07PM +0000, Mel Gorman wrote:
> The number of pages kswapd can reclaim is bound by the number of pages it
> scans which is related to the size of the zone and the scanning priority. In
> many cases the priority remains low because it's reset every SWAP_CLUSTER_MAX
> reclaimed pages but in the event kswapd scans a large number of pages it
> cannot reclaim, it will raise the priority and potentially discard a large
> percentage of the zone as sc->nr_to_reclaim is ULONG_MAX. The user-visible
> effect is a reclaim "spike" where a large percentage of memory is suddenly
> freed. It would be bad enough if this was just unused memory but because
> of how anon/file pages are balanced it is possible that applications get
> pushed to swap unnecessarily.
> This patch limits the number of pages kswapd will reclaim to the high
> watermark. Reclaim will will overshoot due to it not being a hard limit as

will -> still?

> shrink_lruvec() will ignore the sc.nr_to_reclaim at DEF_PRIORITY but it
> prevents kswapd reclaiming the world at higher priorities. The number of
> pages it reclaims is not adjusted for high-order allocations as kswapd will
> reclaim excessively if it is to balance zones for high-order allocations.

I don't really understand this last sentence. Is the excessive
reclaim a result of the patch, a description of what's happening

> Signed-off-by: Mel Gorman <mgorman@xxxxxxx>

Nice, thank you. Using the high watermark for larger zones is more
reasonable than my hack that just always went with SWAP_CLUSTER_MAX,
what with inter-zone LRU cycle time balancing and all.

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
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