Re: [patch 1/5] mm: exclude reserved pages from dirtyable memory

[Mel Gorman <mgorman@suse.de>]

On Fri, Sep 30, 2011 at 09:17:20AM +0200, Johannes Weiner wrote:
> The amount of dirtyable pages should not include the full number of
> free pages: there is a number of reserved pages that the page
> allocator and kswapd always try to keep free.
> 
> The closer (reclaimable pages - dirty pages) is to the number of
> reserved pages, the more likely it becomes for reclaim to run into
> dirty pages:
> 
>        +----------+ ---
>        |   anon   |  |
>        +----------+  |
>        |          |  |
>        |          |  -- dirty limit new    -- flusher new
>        |   file   |  |                     |
>        |          |  |                     |
>        |          |  -- dirty limit old    -- flusher old
>        |          |                        |
>        +----------+                       --- reclaim
>        | reserved |
>        +----------+
>        |  kernel  |
>        +----------+
> 
> This patch introduces a per-zone dirty reserve that takes both the
> lowmem reserve as well as the high watermark of the zone into account,
> and a global sum of those per-zone values that is subtracted from the
> global amount of dirtyable pages.  The lowmem reserve is unavailable
> to page cache allocations and kswapd tries to keep the high watermark
> free.  We don't want to end up in a situation where reclaim has to
> clean pages in order to balance zones.
> 
> Not treating reserved pages as dirtyable on a global level is only a
> conceptual fix.  In reality, dirty pages are not distributed equally
> across zones and reclaim runs into dirty pages on a regular basis.
> 
> But it is important to get this right before tackling the problem on a
> per-zone level, where the distance between reclaim and the dirty pages
> is mostly much smaller in absolute numbers.
> 
> Signed-off-by: Johannes Weiner <jweiner@redhat.com>
> Reviewed-by: Rik van Riel <riel@redhat.com>

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs