From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: Does anyone use CONFIG_TINY_PREEMPT_RCU? Date: Tue, 13 Nov 2012 16:05:34 -0800 Message-ID: <20121114000534.GM2489@linux.vnet.ibm.com> References: <20121113004906.GA10557@linux.vnet.ibm.com> <20121113144620.GA27426@elliptictech.com> <20121113170804.GA2489@linux.vnet.ibm.com> <20121113175654.GA30119@elliptictech.com> <20121113211952.GC2489@linux.vnet.ibm.com> <20121113214720.GC13472@elliptictech.com> <20121113222521.GI2489@linux.vnet.ibm.com> <20121113224040.GA14594@elliptictech.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org To: Nick Bowler Return-path: Received: from e7.ny.us.ibm.com ([32.97.182.137]:56479 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756018Ab2KNAZN (ORCPT ); Tue, 13 Nov 2012 19:25:13 -0500 Received: from /spool/local by e7.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 13 Nov 2012 19:25:10 -0500 Content-Disposition: inline In-Reply-To: <20121113224040.GA14594@elliptictech.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Tue, Nov 13, 2012 at 05:40:40PM -0500, Nick Bowler wrote: > On 2012-11-13 14:25 -0800, Paul E. McKenney wrote: > > On Tue, Nov 13, 2012 at 04:47:20PM -0500, Nick Bowler wrote: > > > On 2012-11-13 13:19 -0800, Paul E. McKenney wrote: > > > > On Tue, Nov 13, 2012 at 12:56:54PM -0500, Nick Bowler wrote: > > > > > On 2012-11-13 09:08 -0800, Paul E. McKenney wrote: > > > > > > Suppose that TREE_PREEMPT_RCU was available for !SMP && PREEMPT builds. > > > > > > Would that work for you? > > > > > > > > > > To be honest I don't really know what the difference is, other than what > > > > > the help text says, which is: > > > > > > > > > > [TINY_PREEMPT_RCU] greatly reduces the memory footprint of RCU. > > > > > > > > > > "Greatly reduced memory footprint" sounds pretty useful... > > > > > > > > OK, so from your viewpoint, the only possible benefit is smaller > > > > memory? > > > > > > Well, I have no idea. If I was given the choice between TREE_PREEMPT_RCU > > > and TINY_PREEMPT_RCU, absent any information not in the description of > > > these options, I would choose TINY. The description suggests that the > > > memory savings come at the expense of SMP support, which sounds like a > > > great tradeoff to make for a UP system. > > > > > > > How much memory does your device have, if I may ask? > > > > > > It's a (pretty old!) desktop. I recently had to upgrade it to two > > > gigabytes due to unbearable thrashing with only one... > > > > If you have two gigabytes (or even one gigabyte), you won't notice the > > few kilobytes of difference between TINY_PREEMPT_RCU and TREE_PREEMPT_RCU. > > Well then TINY_PREEMPT_RCU doesn't sound all that useful for me! > Perhaps the help text could be improved... such as changing the words > "greatly reduced" to "marginally reduced" as a first step? > > Is there no significant cache impact due to the larger implementation? > I don't really have the time or expertise to do measurements in this > regard, but if TREE_PREEMPT_RCU was actually a selectable option I could > at least choose it to see if anything explodes horribly... Indeed, if I do this, the first step would be to re-introduce the choice between CONFIG_TREE_PREEMPT_RCU and CONFIG_TINY_PREEMPT_RCU so that you could check for horrible explosions. If the past is any guide, there would be at least a few. ;-) Thanx, Paul