Re: [PATCH 8/8] vmscan: Kick flusher threads to clean pages when reclaim is encountering dirty pages

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Jul 19, 2010 at 03:37:37PM +0100, Mel Gorman wrote:
> On Mon, Jul 19, 2010 at 10:23:49AM -0400, Christoph Hellwig wrote:
> > On Mon, Jul 19, 2010 at 02:11:30PM +0100, Mel Gorman wrote:
> > > +	/*
> > > +	 * If reclaim is encountering dirty pages, it may be because
> > > +	 * dirty pages are reaching the end of the LRU even though
> > > +	 * the dirty_ratio may be satisified. In this case, wake
> > > +	 * flusher threads to pro-actively clean some pages
> > > +	 */
> > > +	wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty + nr_dirty / 2);
> > > +
> > 
> > Where is the laptop-mode magic coming from?
> > 
> 
> It comes from other parts of page reclaim where writing pages is avoided
> by page reclaim where possible. Things like this
> 
> 	wakeup_flusher_threads(laptop_mode ? 0 : total_scanned);

Actually, it's not avoiding writing pages in laptop mode, instead it
is lumping writeouts aggressively (as I wrote in my other mail,
.nr_pages=0 means 'write everything') to keep disk spinups rare and
make maximum use of them.

> although the latter can get disabled too. Deleting the magic is an
> option which would trade IO efficiency for power efficiency but my
> current thinking is laptop mode preferred reduced power.

Maybe couple your wakeup with sc->may_writepage?  It is usually false
for laptop_mode but direct reclaimers enable it at one point in
do_try_to_free_pages() when it scanned more than 150% of the reclaim
target, so you could use existing disk spin-up points instead of
introducing new ones or disabling the heuristics in laptop mode.