Re: [PATCH] 2.4.0-prerelease: preemptive kernel.

[Andi Kleen]

On Thu, Jan 04, 2001 at 01:39:57PM -0800, Nigel Gamble wrote:
> Experience has shown that adaptive spinlocks are not worth the extra
> overhead (if you mean the type that spin for a short time
> and then decide to sleep).  It is better to use spin_lock_irqsave()
> (which, by definition, disables kernel preemption without the need
> to set a no-preempt flag) to protect regions where the lock is held
> for a maximum of around 100us, and to use a sleeping mutex lock for
> longer regions.  This is what I'm working towards.

What experience ?  Only real-time latency testing or SMP scalability 
testing? 

The case I was thinking about is a heavily contended lock like the
inode semaphore of a file that is used by several threads on several
CPUs in parallel or the mm semaphore of a often faulted shared mm. 

It's not an option to convert them to a spinlock, but often the delays
are short enough that a short spin could make sense. 

-Andi