[Haskell-cafe] Automatic Reference Counting

[Thomas Davie]

On 2 Jul 2011, at 17:52, Chris Smith wrote:

> On Sat, 2011-07-02 at 17:35 +0100, Thomas Davie wrote:
>> It's interesting that you cite that GC is both faster and lower memory
>> overhead – Apple's stated reasons for implementing this were that GC
>> was both too slow and too memory intensive to use sensibly on iDevices
>> and that ARC was both faster and less memory intensive.
> 
> This is a little more complex that just "better" or "worse".  The speed
> and memory overhead of reference counting depend on what percentage of
> your data structures are pointers, and of your program is performing
> pointer updates.  Presumably, iOS developers would try to avoid a lot of
> small heap allocations, and instead use packed data structures with
> in-place updates to larger contiguous blocks of memory.  In that case,
> it's possible that reference counting is much faster and reduces memory
> usage compared to garbage collection.  This is certainly not the case in
> a typical functional language, though.
> 
> When asking about memory usage, you also want to distinguish between
> "hot" memory usage (how much memory is actively used and so we want it
> to fit in cache) and overall memory usage (total heap size, even though
> a lot of it may be swapped out to disk on a desktop).  Garbage
> collection typically increases the overall heap size, but is better than
> reference counting when it comes to reducing the size of the "hot" area
> of memory.
> 
> So basically, I wouldn't call Apple's claims unusual when they are made
> for iOS and Objective C (though "garbage collection is too slow to use
> sensibly on iDevices" is definitely pushing the ridiculous side of
> things), but I also wouldn't expect their rather specific environment to
> carry over to general purpose computing, or especially to Haskell.

Thanks, that's a great explanation :)