Re: [PATCH 3/5] mm improvements

[Andrew Morton <akpm@osdl.org>]

Nikita Danilov <Nikita@Namesys.COM> wrote:
>
>  Rik van Riel writes:
>   > On Wed, 4 Feb 2004, Nick Piggin wrote:
>   > > Nick Piggin wrote:
>   > > 
>   > > > 3/5: vm-lru-info.patch
>   > > >     Keep more referenced info in the active list. Should also improve
>   > > >     system time in some cases. Helps swapping loads significantly.
>   > 
>   > I suspect this is one of the more important ones in this
>   > batch of patches...
> 
>  I don't understand how this works. This patch just parks mapped pages on
>  the "ignored" segment of the active list, where they rest until
>  reclaim_mapped mode is entered.
> 
>  This only makes a difference for the pages that were page_referenced():
> 
>  1. they are moved to the ignored segment rather than to the head of the
>  active list.
> 
>  2. their referenced bit is not cleared
> 
>  Now, as "ignored" segment is not scanned in !reclaim_mode, (2) would
>  only make a difference when VM rapidly oscillates between reclaim_mapped
>  and !reclaim_mapped, because after a long period of !reclaim_mapped
>  operation preserved referenced bit on a page only means "this page has
>  been referenced in the past, but not necessary recently".

Yes, reclaim_mapped shouldn't change at all frequently, unless the
zone->prev_priority thing is broken.  prev_priority is supposed to remember
the reclaim_mapped state between successive scan attempts so we go straight
into doing the right thing.

It _was_ working - I instrumented and tested that when it went in.  It was
a bit tricky to get right.

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