Re: Real-time rw-locks (Re: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.32-15)

[Ingo Molnar <mingo@elte.hu>]

* William Lee Irwin III <wli@holomorphy.com> wrote:

> On Fri, Jan 28, 2005 at 08:38:56AM +0100, Ingo Molnar wrote:
> > no, it's not a big scalability problem. rwlocks are really a mistake -
> > if you want scalability and spinlocks/semaphores are not enough then one
> > should either use per-CPU locks or lockless structures. rwlocks/rwsems
> > will very unlikely help much.
> 
> I wouldn't be so sure about that. SGI is already implicitly relying on
> the parallel holding of rwsems for the lockless pagefaulting, and
> Oracle has been pushing on mapping->tree_lock becoming an rwlock for a
> while, both for large performance gains.

i dont really buy it. Any rwlock-type of locking causes global cacheline
bounces. It can make a positive scalability difference only if the
read-lock hold time is large, at which point RCU could likely have
significantly higher performance. There _may_ be an intermediate locking
pattern that is both long-held but has a higher mix of write-locking
where rwlocks/rwsems may have a performance advantage over RCU or
spinlocks.

Also this is about PREEMPT_RT, mainly aimed towards embedded systems,
and at most aimed towards small (dual-CPU) SMP systems, not the really
big systems.

But, the main argument wrt. PREEMPT_RT stands and is independent of any
scalability properties: rwlocks/rwsems have so bad deterministic
behavior that they are almost impossible to implement in a sane way.

	Ingo