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    SubjectRe: [patch 1/4] mm: exclude reserved pages from dirtyable memory
    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:
    > > 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.

    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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