linux-kernel - Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather than rcu

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Message-ID: <20080822205339.GK6744@linux.vnet.ibm.com>
Date:	Fri, 22 Aug 2008 13:53:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Pekka Enberg <penberg@...helsinki.fi>, Ingo Molnar <mingo@...e.hu>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andi Kleen <andi@...stfloor.org>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] smp_call_function: use rwlocks on queues rather
	than rcu

On Fri, Aug 22, 2008 at 03:03:13PM -0500, Christoph Lameter wrote:
> Paul E. McKenney wrote:
> 
> > I was indeed thinking in terms of the free from RCU being specially marked.
> 
> Isnt there some way to shorten the rcu periods significantly? Critical
> sections do not take that long after all.

In theory, yes.  However, the shorter the grace period, the greater the
per-update overhead of grace-period detection -- the general approach
is to use a per-CPU high-resolution timer to force RCU grace period
processing every 100 microseconds or so.  Also, by definition, the RCU
grace period can be no shorter than the longest active RCU read-side
critical section.  Nevertheless, I have designed my current hierarchical
RCU patch with expedited grace periods in mind, though more for the
purpose of reducing latency of long strings of operations that involve
synchronize_rcu() than for cache locality.

> If the RCU periods are much shorter then the chance of cache hotness of the
> objects is increased.

How short does the grace period need to be to significantly increase the
chance of an RCU-protected data element remaining in cache across an RCU
grace period?  The last time I calculated this, the knee of the curve was
at a few tens of milliseconds, but to give you an idea of how long ago
that was, the workload I used was TPC/A.  Which might no longer be very
representative.  ;-)

							Thanx, Paul
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