From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759899Ab2IFUoh (ORCPT ); Thu, 6 Sep 2012 16:44:37 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:48090 "EHLO e33.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758131Ab2IFUof (ORCPT ); Thu, 6 Sep 2012 16:44:35 -0400 Date: Thu, 6 Sep 2012 13:37:42 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org, "Paul E. McKenney" Subject: Re: [PATCH tip/core/rcu 13/23] rcu: Control grace-period duration from sysfs Message-ID: <20120906203742.GV2448@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20120830181811.GA29154@linux.vnet.ibm.com> <1346350718-30937-1-git-send-email-paulmck@linux.vnet.ibm.com> <1346350718-30937-13-git-send-email-paulmck@linux.vnet.ibm.com> <1346940930.18408.14.camel@twins> <20120906175324.GN2448@linux.vnet.ibm.com> <1346956101.18408.61.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1346956101.18408.61.camel@twins> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12090620-2398-0000-0000-00000A5B7790 X-IBM-ISS-SpamDetectors: X-IBM-ISS-DetailInfo: BY=3.00000294; HX=3.00000196; KW=3.00000007; PH=3.00000001; SC=3.00000007; SDB=6.00171874; UDB=6.00038979; UTC=2012-09-06 20:44:33 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 06, 2012 at 08:28:21PM +0200, Peter Zijlstra wrote: > On Thu, 2012-09-06 at 10:53 -0700, Paul E. McKenney wrote: > > > > - how do I know if my workload wants a longer or shorter forced qs > > > period? > > > > Almost everyone can do just fine with the defaults. If you have more > > than about 1,000 CPUs, you might need a longer period. > > Because the cost of starting a grace period is on the same order (or > larger) in cost as this period? Because the overhead of rcu_gp_fqs() can then be multiple jiffies, so it doesn't make sense to run it so often. If nothing else, the rcu_gp_kthread() will start chewing up appreciable CPU time. > > Some embedded > > systems might need a shorter period -- the only specific example I know > > of is network diagnostic equipment running wireshark, which starts up > > slowly due to grace-period length. > > But but but 3 jiffies.. however is that too long? Because wireshark startup runs through a great many grace periods when starting up, and those 3-jiffy time periods add up. > > > Also, whatever made you want to provide this 'feature' in the first > > > place? > > > > Complaints from the two groups called out above. > > Does this really warrant a boot time knob for which even you cannot > quite explain what values to use when? If people look at me funny when I explain, I just tell them to leave it alone. One alternative at the low end would be to have a sysfs variable that converted normal grace periods to expedited grace periods. Would that be preferable? Thanx, Paul