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Subject: Re: [PATCH 3/7] CART - an advanced page replacement policy
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Marcelo <marcelo.tosatti@cyclades.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       linux-mm@kvack.org
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Date: Fri, 30 Sep 2005 21:16:20 +0200
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On Fri, 2005-09-30 at 15:44 -0300, Marcelo wrote:
> Hi Peter,
> 
> On Thu, Sep 29, 2005 at 08:08:48PM +0200, Peter Zijlstra wrote:
> > The flesh of the CART implementation. Again comments in the file should be
> > clear.
> 
> Having per-zone "B1" target accounted at fault-time instead of a global target
> strikes me.
> 
> The ARC algorithm adjusts the B1 target based on the fact that being-faulted-pages
> were removed from the same memory region where such pages will reside.
> 
> The per-zone "B1" target as you implement it means that the B1 target accounting
> happens for the zone in which the page for the faulting data has been allocated,
> _not_ on the zone from which the data has been evicted. Seems quite unfair.
> 
> So for example, if a page gets removed from the HighMem zone while in the 
> B1 list, and the same data gets faulted in later on a page from the normal
> zone, Normal will have its "B1" target erroneously increased.
> 
> A global inactive target scaled to the zone size would get rid of that problem.

Good, good, I'll think this though, this probably means the other
targets: 'p' and 'r' would similarly benefit.

> Another issue is testing: You had some very interesting numbers before, 
> how are things now?

I managed to reproduce that 10% gain on the kernel build time once more,
however it seems very unstable, I generally get only 2-3%.

> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > 
> >  mm/cart.c                  |  631 +++++++++++++++++++++++++++++++++++++++++++++
> >  7 files changed, 682 insertions(+), 35 deletions(-)
> > 
> > Index: linux-2.6-git/mm/cart.c
> > ===================================================================
> > --- /dev/null
> > +++ linux-2.6-git/mm/cart.c
> > @@ -0,0 +1,639 @@
> 
> <snip>
> 
> > +#define cart_cT ((zone)->nr_active + (zone)->nr_inactive + (zone)->free_pages)
> > +#define cart_cB ((zone)->present_pages)
> > +
> > +#define T2B(x) (((x) * cart_cB) / cart_cT)
> > +#define B2T(x) (((x) * cart_cT) / cart_cB)
> > +
> > +#define size_T1 ((zone)->nr_active)
> > +#define size_T2 ((zone)->nr_inactive)
> > +
> > +#define list_T1 (&(zone)->active_list)
> > +#define list_T2 (&(zone)->inactive_list)
> > +
> > +#define cart_p ((zone)->nr_p)
> > +#define cart_q ((zone)->nr_q)
> > +
> > +#define size_B1 ((zone)->nr_evicted_active)
> > +#define size_B2 ((zone)->nr_evicted_inactive)
> > +
> > +#define nr_Ns ((zone)->nr_shortterm)
> > +#define nr_Nl (size_T1 + size_T2 - nr_Ns)
> 
> These defines are not not easy to read inside the code which
> uses them, I personally think that "zone->nr_.." explicitly is 
> much clearer.

Yes, I plan to replace them by explicit referenced, however they were
handy while playing with the definitions. And since nobody outside of
the cart code still refers to them I plan to rename the struct zone
members to match these names; zone->nr_active to zone->size_T1 and
zone->active_list to zone->list_T1.

-- 
Peter Zijlstra <a.p.zijlstra@chello.nl>

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