Re: [patch 1/1] vmscan: give referenced, active and unmapped pages a second trip around the LRU

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2007-05-25 at 00:28 -0700, Andrew Morton wrote:
> On Fri, 25 May 2007 09:23:30 +0200 Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > > >  		}
> > > >  		list_add(&page->lru, &l_inactive);
> > > >  	}
> > > 
> > > That does a bit of extra work in the !PageReferenced && !page_mapped case,
> > > but whatever.
> > > 
> > > The question is: what effect does the change have on page reclaim
> > > effectiveness?   And how much more swappy does it become?  And
> > > how much more oom-killery?
> > 
> > All very good questions, of which I'd like to know the answers too :-(
> 
> hm.  We've always had this problem.
> 
> > I'm sitting on a huge pile of reclaim code, and have no real way of
> > answering these questions; I did start writing some synthetic benchmark
> > suite, but never really finished it - perhaps I ought to dive into that
> > again after OLS.
> 
> hm.
> 
> > The trouble I had with the previous patch is that it somehow looks to
> > PG_referenced but not the PTE state, that seems wrong to me.
> 
> 		if (page_mapped(page)) {
> 			if (!reclaim_mapped ||
> 			    (total_swap_pages == 0 && PageAnon(page)) ||
> 			    page_referenced(page, 0)) {
> 				list_add(&page->lru, &l_active);
> 				continue;
> 			}
> 		} else if (TestClearPageReferenced(page)) {
> 			list_add(&page->lru, &l_active);
> 			continue;
> 		}
> 
> When we run TestClearPageReferenced() we know that the page isn't
> page_mapped(): there aren't any pte's which refer to it.

D'0h, I guess I need my morning juice...

OK, that was my biggest beef - another small nit: I think it should do
the page_referenced() first, and then the other checks (in the
page_mapped() branch). Otherwise we might 'leak' the referenced state
and give it yet another cycle on the active list - even though it was
not used since last we were here.

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