Messages in this thread | | | Date | Fri, 18 Mar 2005 17:55:44 +0100 (MET) | From | Esben Nielsen <> | Subject | Re: Real-Time Preemption and RCU |
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On Fri, 18 Mar 2005, Ingo Molnar wrote:
> > * Bill Huey <bhuey@lnxw.com> wrote: > > > I'd like to note another problem. Mingo's current implementation of > > rt_mutex (super mutex for all blocking synchronization) is still > > missing reader counts and something like that would have to be > > implemented if you want to do priority inheritance over blocks. > > i really have no intention to allow multiple readers for rt-mutexes. We > got away with that so far, and i'd like to keep it so. Imagine 100 > threads all blocked in the same critical section (holding the read-lock) > when a highprio writer thread comes around: instant 100x latency to let > all of them roll forward. The only sane solution is to not allow > excessive concurrency. (That limits SMP scalability, but there's no > other choice i can see.) >
Unless a design change is made: One could argue for a semantics where write-locking _isn't_ deterministic and thus do not have to boost all the readers. Readers boost the writers but not the other way around. Readers will be deterministic, but not writers. Such a semantics would probably work for a lot of RT applications happening not to take any write-locks - these will in fact perform better. But it will give the rest a lot of problems.
> Ingo
Esben
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