Subj : Re: Lock Free -- where to start
To   : comp.programming.threads
From : Joe Seigh
Date : Thu Sep 29 2005 06:00 pm

David Schwartz wrote:
> "Joe Seigh" <jseigh_01@xemaps.com> wrote in message 
> news:QradnVuGiqT2o6HeRVn-og@comcast.com...
> 
> 
>>So you're saying lock based solutions will scale better then lock-free 
>>ones?
> 
> 
>     It depends upon which of the dozens of possible meanings of "scale" you 
> intend. In general, if by "scale" you mean show improved real world 
> performance as load, in general, increases, then yes, lock-based solutions 
> will scale better than lock-free ones.
> 
>     Consider a case where you have four threads. Threads A and B want to 
> access the same collection and threads C and D want to access the same 
> collection. With only a single CPU, both a lock-free and lock-based solution 
> will run one thread at a time and it will run at full speed.
> 
>     Now, let's scale this to two CPUs. The lock-based solution will never 
> run thread A and B at the same time. It will always run one of A/B and one 
> of C/D. The scaling is perfect and the performance doubles since contending 
> threads never run.
> 
>     The lock-free solution will randomly match threads. Whenever it happens 
> to schedule A and B or C and D at the same time, each thread will run at 
> some small fraction of its maximum as the FSB bounces the shared/modified 
> data back and forth.
> 
>     Granted this is an extreme example, but the point is that contention is 
> bad and lock free algorithms tend to maximize contention.
> 

What happens when you scale to 4 cpu's?


-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.

.