[swift-dev] Performance issues in automatic reference counting (ARC)?

[Michael Gottesman]

> On Dec 17, 2016, at 11:13 AM, Brian Gesiak via swift-dev <swift-dev at swift.org> wrote:
> 
> Hello all!
> 
> I really enjoyed Chris Lattner's slides from his talk at IBM <http://researcher.watson.ibm.com/researcher/files/us-lmandel/lattner.pdf <http://researcher.watson.ibm.com/researcher/files/us-lmandel/lattner.pdf>>. 
> 
> The speaker notes mention ARC:
> 
> "There are two principle downsides to ARC that people cite: one is the need for atomic increment/decrements, which can be slow." [...] "The performance problems it can cause are real in some important cases"
> 
> Can someone point me to a good resource that explains these problems? I guess atomic reference count changes create overhead in multithreaded applications? Are there more detailed explorations into this topic?

With a proper concurrency model, I believe you can make most reference counting operations local (my opinion). I have done some explorations in this area in the past using what I call thread local vs global reference counts and using marked concurrency boundaries to mediate transitions in between them (moving from thread local -> atomic of course if one escapes in an undefined way).

If you are interested in the perf difference with ARC atomics, Roman recently added a mode to the compiler called -assume-single-threaded that uses non-atomic reference counts anywhere.

There are some interesting optimizations in this area as well, specifically even today, COW gives a nice guarantee of thread localness allowing you to eliminate atomic reference counts once you have a uniqued cow data structure.

Michael

> 
> Thanks!
> 
> - Brian Gesiak
> 
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