Re: [PATCH] v4 RCU: the bloatwatch edition

Paul E. McKenney
2009-05-06T19:02:32Z

On Wed, May 06, 2009 at 11:24:59AM -0700, Andrew Morton wrote:
> On Wed, 6 May 2009 14:09:09 +0200
> Ingo Molnar <mingo@elte.hu> wrote:
>
> >
> > * David Howells <dhowells@redhat.com> wrote:
> >
> > > Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > >
> > > > This patch is a version of RCU designed for (!SMP && EMBEDDED)
> > > > provided as a proof of concept of a small-footprint RCU implementation.
> > > > In particular, the implementation of synchronize_rcu() is extremely
> > > > lightweight and high performance. It passes rcutorture testing in each
> > > > of the four relevant configurations (combinations of NO_HZ and PREEMPT)
> > > > on x86. This saves about 900 bytes compared to Classic RCU, and a
> > > > couple kilobytes compared to Hierarchical RCU (updated to 2.6.29):
> > > > ...
> > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > >
> > > Acked-by: David Howells <dhowells@redhat.com>
> >
> > 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.

Thanx, Paul