From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH 3/3] Convert the UDP hash lock to RCU Date: Tue, 7 Oct 2008 07:36:11 -0700 Message-ID: <20081007143611.GB6384@linux.vnet.ibm.com> References: <20081006185026.GA10383@minyard.local> <48EA8197.6080502@cosmosbay.com> <20081006.144002.56418911.davem@davemloft.net> <1223368290.26330.10.camel@lappy.programming.kicks-ass.net> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: David Miller , dada1@cosmosbay.com, minyard@acm.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, shemminger@vyatta.com To: Peter Zijlstra Return-path: Content-Disposition: inline In-Reply-To: <1223368290.26330.10.camel@lappy.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Tue, Oct 07, 2008 at 10:31:30AM +0200, Peter Zijlstra wrote: > On Mon, 2008-10-06 at 14:40 -0700, David Miller wrote: > > From: Eric Dumazet > > Date: Mon, 06 Oct 2008 23:22:31 +0200 > > > > > Me wondering what impact this synchronize_rcu() can have on mono-threaded > > > VOIP applications using lot of UDP sockets. What is the maximum delay of > > > this function ? > > > > The cost is enormous, we really can't use it here. > > > > I have a patch that did top-level socket destruction using RCU, > > and that didn't use synchronize_rcu(), and that killed connection > > rates by up to %20. > > Did you ever figure out why you lost those 20% ? > > > I can only imagine what the cost would be if I had to add such a call > > in there. > > Yeah, sync_rcu() is rediculously expensive, at best 3 jiffies IIRC. I could make it -much- faster, but at the expense of -serious- CPU overhead. Still, might be useful during boot time (when the system can't do anything useful anyway) to accelerate getting data structures initialized. Thanx, Paul