Disadvantages of ARC

ponce contact at gam3sfrommars.fr
Thu Feb 6 07:11:15 PST 2014


On Thursday, 6 February 2014 at 11:37:59 UTC, Max Klyga wrote:
> Anti-GC crowd tries to promote ARC as an deterministic 
> alternative for memory management.
> I noticed that people promoting ARC do not provide any 
> disadvantages for proposed approach.
>
> The thing is in gamedev and other soft-realitime software 
> background only a handfull types of resources are really 
> managed by RC and memory usage patterns are VERY specific to 
> their domain (mostly linear allocation/deallocation and objects 
> with non deterministic lifetime are preallocated in pools).
>
> Trying to use RC as a general method of memory management leads 
> to some problems.
> A pretty detailed view by John Harrop (He is somewhat known for 
> trolling in PL community, but nonetheless knows what he is 
> talking about) - 
> http://www.quora.com/Computer-Programming/How-do-reference-counting-and-garbage-collection-compare/answer/Jon-Harrop-1?srid=3Gvg&share=1#
>
>
> So RC could also introduce unpredictable pause times at 
> undesired places.
>
> This is also confirmed by research from HP - 
> http://www.hpl.hp.com/personal/Hans_Boehm/popl04/refcnt.pdf
>
> My point is that we should not ruin the language ease of use. 
> We do need to deal with Phobos internal allocations, but we 
> should not switch to ARC as a default memory management scheme. 
> In practice people promoting ARC will probably not use phobos 
> anyway. Currently its just an excuse to not use D.
>
> Look at c++ and STL, etc. People will roll their own solutions 
> no matter what you try.

I think of RC as a greater evil that GC. From what I've seen it 
does creates leaking cycles in tree structures AND pauses. From a 
low-level point of view, RC pointers are ugly (separate counter 
that will trash your cache) and do atomics all over the place 
(ie. memory barriers). That they don't have to because of shared 
is TBD.

It is comforting to me to know that a GC pointer is still just a 
pointer. If we go the RC route, we will have to constantly think 
about the higher cost of RC pointer vs weak-ref, instead of 
thinking about other things instead
Right now a GC pointer is the same size as a non-GC one, it's 
liberating.

It looks like we want to solve a PR problem more than a real one.


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