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Subject: Re: [PATCH 00/14] Concurrent Page Cache
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Christoph Lameter <clameter@sgi.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
       Andrew Morton <akpm@osdl.org>, Nick Piggin <nickpiggin@yahoo.com.au>,
       Ingo Molnar <mingo@elte.hu>, Rik van Riel <riel@redhat.com>
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Date: Mon, 29 Jan 2007 19:56:17 +0100
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On Mon, 2007-01-29 at 10:15 -0800, Christoph Lameter wrote:
> On Mon, 29 Jan 2007, Peter Zijlstra wrote:
> 
> > Ladder locking would end up:
> > 
> > lock A0
> > lock B1
> > unlock A0 -> a new operation can start
> > lock C2
> > unlock B1
> > lock D5
> > unlock C2
> > ** we do stuff to D5
> > unlock D5
> > 
> 
> Instead of taking one lock we would need to take 4?

Yep.

> Wont doing so cause significant locking overhead?
> We probably would want to run some benchmarks. 

Right, I was hoping the extra locking overhead would be more than
compensated by the reduction in lock contention time. But testing is
indeed in order.

> Maybe disable the scheme for systems with a small number of 
> processors?

CONFIG_RADIX_TREE_CONCURRENT does exactly this.