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

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Message-ID: <20090506225115.GM6771@linux.vnet.ibm.com>
Date:	Wed, 6 May 2009 15:51:15 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, 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 11:22:54PM +0100, David Howells wrote:
> Andrew Morton <akpm@...ux-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
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