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List:       linux-kernel
Subject:    Re: Real-Time Preemption and RCU
From:       Ingo Molnar <mingo () elte ! hu>
Date:       2005-03-19 16:31:28
Message-ID: 20050319163128.GB28958 () elte ! hu
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* Herbert Xu <herbert@gondor.apana.org.au> wrote:

> Ingo Molnar <mingo@elte.hu> wrote:
> >
> > 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.)
> 
> What about allowing only as many concurrent readers as there are CPUs?

since a reader may be preempted by a higher prio task, there is no
linear relationship between CPU utilization and the number of readers
allowed. You could easily end up having all the nr_cpus readers
preempted on one CPU. It gets pretty messy.

	Ingo
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