linux-kernel - Re: [PATCH] mm/filemap.c: unconditionally call mark_page

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Message-Id: <200703160351.l2G3p3GJ020217@turing-police.cc.vt.edu>
Date:	Thu, 15 Mar 2007 23:51:03 -0400
From:	Valdis.Kletnieks@...edu
To:	Rik van Riel <riel@...hat.com>
Cc:	Andreas Mohr <andi@...x01.fht-esslingen.de>,
	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	Ashif Harji <asharji@...uwaterloo.ca>, linux-mm@...ck.org,
	Nick Piggin <npiggin@...e.de>, Jan Kara <jack@...e.cz>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/filemap.c: unconditionally call mark_page_accessed

On Thu, 15 Mar 2007 14:35:17 EDT, Rik van Riel said:
> Valdis.Kletnieks@...edu wrote:
> > On Wed, 14 Mar 2007 22:33:17 BST, Andreas Mohr said:
> > 
> >> it'd seem we need some kind of state management here to figure out good
> >> intervals of when to call mark_page_accessed() *again* for this page. E.g.
> >> despite non-changing access patterns you could still call mark_page_accessed()
> >> every 32 calls or so to avoid expiry, but this would need extra helper
> >> variables.
> > 
> > What if you did something like
> > 
> > 	if (jiffies%32) {...
> > 
> > (Possibly scaling it so the low-order bits change).  No need to lock it, as
> > "right most of the time" is close enough.
> 
> Bad idea.  That way you would only count page accesses if the
> phase of the moon^Wjiffie is just right.

On the other hand, Andreas suggested only marking it once every 32 calls,
but that required a helper variable.  Statistically, jiffies%32 should
end up about the same as a helper variable %32.

This of course, if just calling mark_page_accessed() is actually expensive
enough that we don't want to do it unconditionally.


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