Re: [patch 1/4] mm: exclude reserved pages from dirtyable memory

Mel Gorman
2011-09-21T15:03:35Z

On Wed, Sep 21, 2011 at 03:04:23PM +0100, Mel Gorman wrote:
> On Tue, Sep 20, 2011 at 03:45:12PM +0200, Johannes Weiner wrote:
> > 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 <jweiner@redhat.com>
> > ---
> > 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.

--
Mel Gorman
SUSE Labs