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From: Michael Glassford (glassfordm_at_[hidden])
Date: 2005-04-25 08:32:43

Peter Dimov wrote:
> What are the design goals of read_write_mutex? (Lock proliferation aside
> for a moment.)
> My understanding has always been that read/write locks are an
> optimization, that is, a read/write lock-based algorithm should deliver
> better performance than the same algorithm with "ordinary" locks. Right?
> Wrong.
> The program at the end of this message demonstrates that in this
> specific (contrived?) case ordinary mutexes outperform a read/write
> mutex by a factor of 2.5 or more in almost every scenario, even when no
> writers are active!
> In some scenarios (16 readers, 4 writers, 10,000,000 iterations) the
> ordinary mutex case completes in under 8 seconds on my machine, but the
> read/write case exceeded my patience threshold.
> Am I missing something?

It doesn't help that the Boost read/write mutex, as I've mentioned
before, is pretty inefficient; I'm not very happy with it. There are at
least three reasons for the inefficiency: 1) Supporting four scheduling
policies in one code base makes the code too complex; 2) Trying to
adhere to the scheduling policies too rigidly is also adding code
complexity and inefficiency; 3) The complexity makes it harder to
optimize, and not much time has been spent optimizing it in the first place.

I mentioned elsewhere that I have some thoughts about ways to improve
Boost.Threads; some of these ideas deal with addressing these three
points and getting a much better read/write mutex in place.


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