Messages in this thread | | | Date | Fri, 23 Dec 2005 09:24:17 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [patch 0/9] mutex subsystem, -V4 |
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On Thu, 22 Dec 2005, Andrew Morton wrote:
> Christoph Hellwig <hch@infradead.org> wrote: > > I really don't get why you hate mutex primitives so much. > > I've just spelled out in considerable detail why this work is premature. > How can you not "get" it? Why do I have to keep saying the same thing in > different ways? It's really quite simple. > > > So here is permutation #4: > > If we can optimise semaphores for speed and space, the mutexes are > *unneeded*.
How can't you get the fact that semaphores could _never_ be as simple as mutexes? This is a theoritical impossibility, which maybe turns out not to be so true on x86, but which is damn true on ARM where the fast path (the common case of a mutex) is significantly more efficient.
Semaphores _require_ an atomic decrement, mutexes do not. On some architectures that makes a huge difference.
> And I think we _should_ optimise semaphores for speed and space. Don't you?
No one disagrees with that.
> If we can do that, there is no point at all in adding a new lock type which > has no speed advantage and no space advantage and which has less > functionality than semaphores.
The very point is that mutexes will always have a speed advantage by nature.
> And, repeating myself yet again: if we can demonstrate that it is not > feasible to optimise semaphores to the same performance and space efficiency > of mutexes then (and only then) we have a reason for adding mutexes.
I spent the whole week making that demonstration repeatedly. Why are you ignoring me?
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