From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH RFC] Simplify the Linux kernel by reducing its state space Date: Sat, 31 Mar 2012 14:21:49 -0700 Message-ID: <20120331212149.GI2450@linux.vnet.ibm.com> References: <20120331163321.GA15809@linux.vnet.ibm.com> <1333227608.2325.4054.camel@edumazet-glaptop> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <1333227608.2325.4054.camel@edumazet-glaptop> Sender: linux-alpha-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Eric Dumazet Cc: linux-kernel@vger.kernel.org, linux-alpha@vger.kernel.org, linux-arm-kernel@lists.infradead.org, uclinux-dist-devel@blackfin.uclinux.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linux-m32r@ml.linux-m32r.org, linux-m32r-ja@ml.linux-m32r.org, linux-mips@linux-mips.org, linux-am33-list@redhat.com, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, tglx@linutronix.de, linux@arm.linux.org.uk, dhowells@redhat.com, jejb@parisc-linux.org, linux390@de.ibm.com, x86@kernel.org, cmetcalf@tilera.com On Sat, Mar 31, 2012 at 11:00:08PM +0200, Eric Dumazet wrote: > On Sun, 2012-04-01 at 00:33 +0800, Paul E. McKenney wrote: > > Although there have been numerous complaints about the complexity of > > parallel programming (especially over the past 5-10 years), the plain > > truth is that the incremental complexity of parallel programming over > > that of sequential programming is not as large as is commonly believed. > > Despite that you might have heard, the mind-numbing complexity of modern > > computer systems is not due so much to there being multiple CPUs, but > > rather to there being any CPUs at all. In short, for the ultimate in > > computer-system simplicity, the optimal choice is NR_CPUS=0. > > > > This commit therefore limits kernel builds to zero CPUs. This change > > has the beneficial side effect of rendering all kernel bugs harmless. > > Furthermore, this commit enables additional beneficial changes, for > > example, the removal of those parts of the kernel that are not needed > > when there are zero CPUs. > > > > Signed-off-by: Paul E. McKenney > > Reviewed-by: Thomas Gleixner > > --- > > Hmm... I believe you could go one step forward and allow negative values > as well. Antimatter was proven to exist after all. > > Hint : nr_cpu_ids is an "int", not an "unsigned int" > > Bonus: Existing bugs become "must have" features. ;-) ;-) ;-) > Of course there is no hurry and this can wait 365 days. James Bottomley suggested imaginary numbers of CPUs some time back, and I suppose there is no reason you cannot have fractional numbers of CPUs, and perhaps irrational numbers as well. Of course, these last two would require use of floating-point arithmetic (or something similar) in the kernel. So I guess we have at several years worth. Over to you for the negative numbers. ;-) Thanx, Paul