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 14:25:22 -0800 Message-ID: <20121113222521.GI2489@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> 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: Content-Disposition: inline In-Reply-To: <20121113214720.GC13472@elliptictech.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org 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. Thanx, Paul