linux-kernel - Re: [PATCH] mm: remove zone->prev

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Message-Id: <20090211031201.cace1c68.akpm@linux-foundation.org>
Date:	Wed, 11 Feb 2009 03:12:01 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	MinChan Kim <minchan.kim@...il.com>,
	kamezawa.hiroyu@...fujitsu.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, riel@...hat.com
Subject: Re: [PATCH] mm: remove zone->prev_prioriy

On Wed, 11 Feb 2009 20:06:46 +0900 (JST) KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:

> > On Tue, 10 Feb 2009 19:57:01 +0900
> > MinChan Kim <minchan.kim@...il.com> wrote:
> > 
> > > As you know, prev_priority is used as a measure of how much stress page reclaim.
> > > But now we doesn't need it due to split-lru's way.
> > > 
> > > I think it would be better to remain why prev_priority isn't needed any more
> > > and how split-lru can replace prev_priority's role in changelog.
> > > 
> > > In future, it help mm newbies understand change history, I think.
> > 
> > Yes, I'd be fascinated to see that explanation.
> > 
> > In http://groups.google.pn/group/linux.kernel/browse_thread/thread/fea9c9a0b43162a1
> > it was asserted that we intend to use prev_priority again in the future.
> > 
> > We discussed this back in November:
> > http://lkml.indiana.edu/hypermail/linux/kernel/0811.2/index.html#00001
> > 
> > And I think that I still think that the VM got worse due to its (new)
> > failure to track previous state.  IIRC, the response to that concern
> > was quite similar to handwavy waffling.
> 
> Yes.
> I still think it's valuable code.
> I think, In theory, VM sould take parallel reclaim bonus.

prev_priority had nothing to do with concurrent reclaim?

It was there so that when a task enters direct reclaim against a zone,
it will immediately adopt the state which the task which most recently
ran direct reclaim had.

Without this feature, each time a task enters direct reclaim it will need
to "relearn" that state - ramping up, making probably-incorrect
decisions as it does so.


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