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SubjectRe: [RCU] adds a prefetch() in rcu_do_batch()
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?

Thanx, Paul

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

> --- linux-2.6.19-rc6/kernel/rcupdate.c 2006-11-16 05:03:40.000000000 +0100
> +++ linux-2.6.19-rc6-ed/kernel/rcupdate.c 2006-11-22 15:12:09.000000000 +0100
> @@ -235,12 +235,14 @@ static void rcu_do_batch(struct rcu_data
>
> list = rdp->donelist;
> while (list) {
> - next = rdp->donelist = list->next;
> + next = list->next;
> + prefetch(next);
> list->func(list);
> list = next;
> if (++count >= rdp->blimit)
> break;
> }
> + rdp->donelist = list;
>
> local_irq_disable();
> rdp->qlen -= count;

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