linux-kernel - Re: [PATCH] v4 RCU: the bloatwatch edition

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Message-ID: <20090506202030.GI6771@linux.vnet.ibm.com>
Date:	Wed, 6 May 2009 13:20:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	mingo@...e.hu, dhowells@...hat.com, linux-kernel@...r.kernel.org,
	niv@...ibm.com, dvhltc@...ibm.com, lethal@...ux-sh.org,
	kernel@...tstofly.org, matthew@....cx
Subject: Re: [PATCH] v4 RCU: the bloatwatch edition

On Wed, May 06, 2009 at 12:19:08PM -0700, Andrew Morton wrote:
> On Wed, 6 May 2009 12:02:16 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > > > i'm wondering what Andrew thinks - he had objections, right?
> > > > 
> > > 
> > > More like "concerns".  It's unobvious to me that the modest .text
> > > savings justify the costs of an additional RCU implementation.  Where
> > > those costs include
> > > 
> > > - additional maintenance work and
> > > 
> > > - the reduced code reliability which comes from fragmenting the
> > >   tester base.  This will mostly affect users of the less popular RCU
> > >   implementations.
> > > 
> > > But hey, maybe I'm wrong.  And maybe I'm right, but we'll merge it anyway ;)
> > 
> > ;-)
> > 
> > How about if acceptance of Tiny RCU happens at the same time as Classic
> > RCU is dropped?  That would be a large net decrease in code size and
> > complexity.
> 
> It's a bit artificial to link the two actions.  Removing something:
> good.  Adding something: bad.  good+bad == less good ;)

Ah, but from a memory-footprint perspective, removing Classic RCU is
about 1.5K bad, given the larger memory footprint of Hierarchical RCU.
So, in this case, removing Classic RCU: good complexity, bad memory
footprint.  Adding Tiny RCU: slightly bad complexity, good memory
footprint.

So, replacing Classic RCU with Tiny RCU improves (reduces) both the
complexity and the memory footprint.

							Thanx, Paul
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