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:

> The performance relative to mutual exclusion is quantifiable and very
> reproducible. [...]

yes, i dont doubt the results - my point is that it's not proven that
the other, more read-friendly types of locking underperform rwlocks. 
Obviously spinlocks and rwlocks have the same cache-bounce properties,
so rwlocks can outperform spinlocks if the read path overhead is higher
than that of a bounce, and reads are dominant. But it's still a poor
form of scalability. In fact, when the read path is really expensive
(larger than say 10-20 usecs) an rwlock can produce the appearance of
linear scalability, when compared to spinlocks.

> As far as performance relative to RCU goes, I suspect cases where
> write-side latency is important will arise for these. Other lockless
> methods are probably more appropriate, and are more likely to dominate
> rwlocks as expected. For instance, a reimplementation of the radix
> trees for lockless insertion and traversal (c.f. lockless pagetable
> patches for examples of how that's carried out) is plausible, where
> RCU memory overhead in struct page is not.

yeah.

	Ingo