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

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Message-Id: <200611221602.29597.dada1@cosmosbay.com>
Date:	Wed, 22 Nov 2006 16:02:29 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Dipankar Sarma <dipankar@...ibm.com>, linux-kernel@...r.kernel.org
Subject: [RCU] adds a prefetch() in rcu_do_batch()

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]);}

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

View attachment "rcu.prefetch.patch" of type "text/plain" (493 bytes)