Re: [PATCH 02/12] mm: supporting variables and functions for balanced zone aging

Marcelo Tosatti <marcelo.tosatti <at> cyclades.com> wrote:
>
> Hi Andrew,
> 
> On Thu, Dec 01, 2005 at 02:37:14AM -0800, Andrew Morton wrote:
> > Wu Fengguang <wfg <at> mail.ustc.edu.cn> wrote:
> > >
> > >  The zone aging rates are currently imbalanced,
> > 
> > ZONE_DMA is out of whack.  It shouldn't be, and I'm not aware of anyone
> > getting in and working out why.  I certainly wouldn't want to go and add
> > all this stuff without having a good understanding of _why_ it's out of
> > whack.  Perhaps it's just some silly bug, like the thing I pointed at in
> > the previous email.
> 
> I think that the problem is caused by the interaction between 
> the way reclaiming is quantified and parallel allocators.

Could be.  But what about the bug which I think is there?  That'll cause
overscanning of the DMA zone.

> The zones have different sizes, and each zone reclaim iteration
> scans the same number of pages. It is unfair.

Nope.  See how shrink_zone() bases nr_active and nr_inactive on
zone->nr_active and zone_nr_inactive.  These calculations are intended to
cause the number of scanned pages in each zone to be

	(zone->nr-active + zone->nr_inactive) >> sc->priority.

> On top of that, kswapd is likely to block while doing its job, 
> which means that allocators have a chance to run.

kswapd should only block under rare circumstances - huge amounts of dirty
pages coming off the tail of the LRU.

> --- mm/vmscan.c.orig	2006-01-01 12:44:39.000000000 -0200
> +++ mm/vmscan.c	2006-01-01 16:43:54.000000000 -0200
> @@ -616,8 +616,12 @@
>  {

Please use `diff -p'.