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

From: Glen Knowles (gknowles_at_[hidden])
Date: 2004-07-09 22:00:12

> From: Howard Hinnant [mailto:hinnant_at_[hidden]] 
> On Jul 9, 2004, at 7:10 PM, Glen Knowles wrote:
> >> Without try locks, lock_both can't be made to work without risking 
> >> deadlock.
> >
> > Lock_both can be made to work with try locks? Either you 
> have a way to 
> > deterministicly set the order you lock the mutexes or you have 
> > problems. You can avoid deadlocking with try lock, but you 
> will fail 
> > to get the pair of locks in cases were you would have gotten it by 
> > waiting. I suppose you could just keep try-locking both 
> ways until you 
> > get it, but that's not good for performance. :)
> 
> Well actually keep try-locking both ways is what I've been 
> doing.  I'd agree with you that in a heavily contested 
> context, performance could theoretically suffer arbitrarily badly.
> 
> > One solution to this problem would be mutex::operator<
> 
> <nod>  I just implemented lock_both::lock two ways:  Once 
> just keep try-locking both ways, and once locking in the 
> order of the addresses of the locks (with only two locks in 
> the test, this was a safe operator<).  My test looks like:

The problem case for spinning is when one thread gets the locks and the
holds them for a non-trivial length of time. During that time the other,
spinning thread, just wastes cpu as fast as it can. This just gets magnified
as more threads are involved.

Glen