[boost] Re: Lock unification (shared vs. upgradable)

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2004-08-10 12:24:44

On Aug 9, 2004, at 5:30 PM, Alexander Terekhov wrote:

> Howard Hinnant wrote:
> [...]
>> The alternative in this link looks very familiar.  I'm currently using
>> two condition variables, ...
>
> POSIX condition variables strive for "wait-morphing". So you'll end up
> with "serial wakes" (quite a waste on, say, 32-way box). The problem
> is that waiters must reacquire the mutex. tail_* stuff is supposed to
> NOT have this requirement (it's meant to use the associated mutex for
> waiters queue "protection"... a la Java monitors, in a way). Well,
> take also a look at:
>
> http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/rw.html

Ok, I think I understand now.  You are expediting the "thundering herd" 
of waiting readers when a writer unlocks on a multi-processor.

Thanks,
Howard