On Wed, Sep 21, 2011 at 04:03:28PM +0100, Mel Gorman wrote:

On Wed, Sep 21, 2011 at 03:04:23PM +0100, Mel Gorman wrote:

quoted

On Tue, Sep 20, 2011 at 03:45:12PM +0200, Johannes Weiner wrote:

quoted

The amount of dirtyable pages should not include the total 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  |
       +----------+

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 <redacted>
---
 include/linux/mmzone.h |    1 +
 mm/page-writeback.c    |    8 +++++---
 mm/page_alloc.c        |    1 +
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 1ed4116..e28f8e0 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h

@@ -316,6 +316,7 @@ struct zone {
 	 * sysctl_lowmem_reserve_ratio sysctl changes.
 	 */
 	unsigned long		lowmem_reserve[MAX_NR_ZONES];
+	unsigned long		totalreserve_pages;
 

This is nit-picking but totalreserve_pages is a poor name because it's
a per-zone value that is one of the lowmem_reserve[] fields instead
of a total. After this patch, we have zone->totalreserve_pages and
totalreserve_pages but are not related to the same thing.
but they are not the same.

As you correctly pointed out to be on IRC, zone->totalreserve_pages
is not the lowmem_reserve because it takes the high_wmark into
account. Sorry about that, I should have kept thinking.  The name is
still poor though because it does not explain what the value is or
what it means.

zone->FOO value needs to be related to lowmem_reserve because this
	is related to balancing zone usage.

zone->FOO value should also be related to the high_wmark because
	this is avoiding writeback from page reclaim

err....... umm... this?

	/*
	 * When allocating a new page that is expected to be
	 * dirtied soon, the number of free pages and the
	 * dirty_balance reserve are taken into account. The
	 * objective is that the globally allowed number of dirty
	 * pages should be distributed throughout the zones such
	 * that it is very unlikely that page reclaim will call
	 * ->writepage.
	 *
	 * dirty_balance_reserve takes both lowmem_reserve and
	 * the high watermark into account. The lowmem_reserve
	 * is taken into account because we don't want the
	 * distribution of dirty pages to unnecessarily increase
	 * lowmem pressure. The watermark is taken into account
	 * because it's correlated with when kswapd wakes up
	 * and how long it stays awake.
	 */
	unsigned long		dirty_balance_reserve.

Yes, that's much better, thanks.

I assume this is meant the same for both the zone and the global level
and we should not mess with totalreserve_pages in either case?

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