Re: [PATCH 3/7] mm: vmscan: Reclaim order-0 and use compaction instead of lumpy reclaim

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Dec 01, 2010 at 10:56:49AM +0000, Mel Gorman wrote:
> On Wed, Dec 01, 2010 at 11:27:45AM +0100, Johannes Weiner wrote:
> > On Mon, Nov 22, 2010 at 03:43:51PM +0000, Mel Gorman wrote:
> > > Lumpy reclaim is disruptive. It reclaims a large number of pages and ignores
> > > the age of the pages it reclaims. This can incur significant stalls and
> > > potentially increase the number of major faults.
> > > 
> > > Compaction has reached the point where it is considered reasonably stable
> > > (meaning it has passed a lot of testing) and is a potential candidate for
> > > displacing lumpy reclaim. This patch introduces an alternative to lumpy
> > > reclaim whe compaction is available called reclaim/compaction. The basic
> > > operation is very simple - instead of selecting a contiguous range of pages
> > > to reclaim, a number of order-0 pages are reclaimed and then compaction is
> > > later by either kswapd (compact_zone_order()) or direct compaction
> > > (__alloc_pages_direct_compact()).
> > > 
> > > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > 
> > > @@ -286,18 +290,20 @@ static void set_lumpy_reclaim_mode(int priority, struct scan_control *sc,
> > >  	lumpy_mode syncmode = sync ? LUMPY_MODE_SYNC : LUMPY_MODE_ASYNC;
> > >  
> > >  	/*
> > > -	 * Some reclaim have alredy been failed. No worth to try synchronous
> > > -	 * lumpy reclaim.
> > > +	 * Initially assume we are entering either lumpy reclaim or
> > > +	 * reclaim/compaction.Depending on the order, we will either set the
> > > +	 * sync mode or just reclaim order-0 pages later.
> > >  	 */
> > > -	if (sync && sc->lumpy_reclaim_mode & LUMPY_MODE_SINGLE)
> > > -		return;
> > > +	if (COMPACTION_BUILD)
> > > +		sc->lumpy_reclaim_mode = LUMPY_MODE_COMPACTION;
> > > +	else
> > > +		sc->lumpy_reclaim_mode = LUMPY_MODE_CONTIGRECLAIM;
> > 
> > Isn't this a regression for !COMPACTION_BUILD in that earlier kernels
> > would not do sync lumpy reclaim when somebody disabled it during the
> > async run?
> > 
> 
> You'll need to clarify your question I'm afraid. In 2.6.36 for example,
> if lumpy reclaim gets disabled then sync reclaim does not happen at all.
> This was due to large stalls being observed when copying large amounts
> of data to slow storage such as a USB external drive.

Sorry for the noise, I just verified that it really was dead code.  We
have

	if (should_reclaim_stall())
		set_lumpy_reclaim_mode(.sync=true)

but because the branch is never taken if lumpy is disabled, the
conditional in set_lumpy_reclaim_mode() is dead.

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