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

[Will Huck <will.huckk@gmail.com>]

Hi Johannes,
On 03/21/2013 11:57 PM, Johannes Weiner wrote:
> 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
> now...?
>
>> Signed-off-by: Mel Gorman <mgorman@suse.de>
> 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@cmpxchg.org>

One offline question, how to understand this in function balance_pgdat:
/*
  * Do some background aging of the anon list, to give
  * pages a chance to be referenced before reclaiming.
  */
age_acitve_anon(zone, &sc);
>
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