Re: [PATCH] v4 RCU: the bloatwatch edition

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, May 06, 2009 at 11:22:54PM +0100, David Howells wrote:
> Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> > 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
> 
> It may be more than just modest .text savings.  Being optimised to be as
> simple as possible, and to only support one CPU, it may be quicker too.  I'm
> not sure how best to benchmark it though.

The read side is unchanged, but the update side is another story, given
that synchronize_rcu()'s latency decreases from multiple milliseconds
to the sub-microsecond range:

	void synchronize_rcu(void)
	{
		unsigned long flags;

		local_irq_save(flags);
		rcu_ctrlblk.completed++;
		local_irq_restore(flags);
	}

So boot speed is one possible metric, depending on how many synchronize_rcu()
invocations are in your arch's boot path.  It appears that x86 has a
fair number.  ;-)

							Thanx, Paul