Re: [patch 3/3] vmscan: detect mapped file pages used only once

[Rik van Riel <riel@redhat.com>]

On 02/22/2010 02:49 PM, Johannes Weiner wrote:
> The VM currently assumes that an inactive, mapped and referenced file
> page is in use and promotes it to the active list.
>
> However, every mapped file page starts out like this and thus a problem
> arises when workloads create a stream of such pages that are used only
> for a short time.  By flooding the active list with those pages, the VM
> quickly gets into trouble finding eligible reclaim canditates.  The
> result is long allocation latencies and eviction of the wrong pages.
>
> This patch reuses the PG_referenced page flag (used for unmapped file
> pages) to implement a usage detection that scales with the speed of
> LRU list cycling (i.e. memory pressure).
>
> If the scanner encounters those pages, the flag is set and the page
> cycled again on the inactive list.  Only if it returns with another
> page table reference it is activated.  Otherwise it is reclaimed as
> 'not recently used cache'.
>
> This effectively changes the minimum lifetime of a used-once mapped
> file page from a full memory cycle to an inactive list cycle, which
> allows it to occur in linear streams without affecting the stable
> working set of the system.
>
> Signed-off-by: Johannes Weiner<hannes@cmpxchg.org>

Reviewed-by: Rik van Riel <riel@redhat.com>