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Re: [PATCH RFC] Simplify the Linux kernel by reducing its state space

To: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH RFC] Simplify the Linux kernel by reducing its state space
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date: Sat, 31 Mar 2012 14:21:49 -0700
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
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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 <paulmck@linux.vnet.ibm.com>
> > Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
> > ---
> 
> 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


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