linux-kernel - Re: [RCU] adds a prefetch() in rcu_do

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Date:	Thu, 30 Nov 2006 09:55:52 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Andrew Morton <akpm@...l.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RCU] adds a prefetch() in rcu_do_batch()

On Thursday 30 November 2006 02:25, Paul E. McKenney wrote:
> On Wed, Nov 22, 2006 at 04:02:29PM +0100, Eric Dumazet wrote:
> > On some workloads, (for example when lot of close() syscalls are done),
> > RCU qlen can be quite large, and RCU heads are no longer in cpu cache
> > when rcu_do_batch() is called.
> >
> > This patches adds a prefetch() in rcu_do_batch() to give CPU a hint to
> > bring back cache lines containing 'struct rcu_head's.
> >
> > Most list manipulations macros include prefetch(), but not open coded
> > ones (at least with current C compilers :) )
> >
> > I got a nice speedup on a trivial benchmark  (3.48 us per iteration
> > instead of 3.95 us on a 1.6 GHz Pentium-M)
> > while (1) { pipe(p); close(fd[0]); close(fd[1]);}
>
> Interesting!  How much of the speedup was due to the prefetch() and how
> much to removing the extra store to rdp->donelist?

I only benchmarked the prefetch() case.
 
Then, when cooking the patch I found I could do the rdp->donelist affectation 
after the loop. I am not sure it's worth to do another benchmark for this 
trivial optimization (Please dont tell me its not a valid one :) )

Eric
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