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

From: Mel Gorman
Date: Tue Mar 19 2013 - 05:55:24 EST

On Tue, Mar 19, 2013 at 07:53:16AM +0800, Simon Jeons wrote:
> Hi Mel,
> On 03/17/2013 09:04 PM, 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
> Since there is nr_reclaimed >= nr_to_reclaim check if priority is
> large than DEF_PRIORITY in shrink_lruvec, how can a large percentage
> of memory is suddenly freed happen?

Because of the priority checks made in get_scan_count(). Patch 5 has
more detail on why this happens.

Mel Gorman
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