[boost] Re: Boost.Threads: Do we need all those mutexes?

From: Matthew Vogt (mvogt_at_[hidden])
Date: 2004-07-05 22:29:15

Bruno Martínez Aguerre <br1 <at> internet.com.uy> writes:

> What about a latent_write_lock?
> 
> It's semantics would be to block on contruction until it only shares the  
> mutex with read_locks.  A write_lock could be contructed from a  
> latent_write_lock and this would always work, because another  
> latent_write_lock would still be blocked in it's constructor.  This kind  
> of lock would be useful for situations in which you have to read a  
> structure to know if you have to write on it.
> 
> Bruno Martinez

Interesting, this divides read locks into promotable and non-promotable 
variants?

If you have a design where a lot of threads take concurrent read locks, and the
case of promotion is rare, then having them all use latent_write_locks will 
unnecessarily serialise them.  Allowing promotion failure instead would only
affect rare cases of promotion contention.

If you have a case where promotion is common, then you probably won't gain much
over simply using write locks, will you?

If you can mix a number of read-only locks with a small number of 
latent_write_locks for which promotion is uncommon, then you have an optimum 
result.  

Matt