Subj : Re: Memory reordering problem (or is it?)
To   : comp.programming.threads
From : Joe Seigh
Date : Wed Jun 15 2005 08:47 am

Giancarlo Niccolai wrote:
> Maciej Sobczak wrote:
....
>>I'm not an expert in today's GC technology, but considering the fact
>>that it has a lot to do with threads, I would like to ask you whether
>>this is considered to be a problem and what GC (or the language and its
>>runtime) can do to avoid this.
>>
>>Regards,
>>
> 
> 
> The most common solution, and not one of the worst, is to stop all threads
> in a safe "stop point" and start collection, allowing the threads to resume
> again after the collection is done. This is called "sequential mark-sweep".
> Another approach, which may or may not be better, depending on which
> situations and requirements you face, is that of never stopping the threads
> and using a "competitive" garbage collector. The most common is the "three
> color" approach, in which the GC thread scan endlessly the memory in search
> of unreachable nodes (the description is imprecise, but it gives the idea).
> The algorithm is less efficient than a mark-sweep, but it never stop
> threads, which may be good if you need high responsiveness, and on a
> massive parallel machine being finetuned with your program (i.e. a SMP with
> the a number of CPU nearing the number of your threads), it may end in
> higher overall performances.
> 
> More can be found at: http://www.memorymanagement.org/
> 

I don't think any of them will be as efficient as RCU or SMR.  Some of those
GC algorithms seem to have horrific overhead that make SMR even with its full
memory barriers look efficient.   They seem to have this attitude that it is
important to GC *everything* no matter what the cost.  The only one that comes
nearest is distributed reference counting where they just scan the thread
stacks and refcount the heap.  The problem with that is the only place it's
safe to examine the stack is from the thread itself which has some problems.

-- 
Joe Seigh

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

.