Re: [PATCH 3/5] mm improvements

Nikita Danilov [off-list ref] 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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