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

Mark Lacey mark.lacey at apple.com
Sat Dec 17 13:47:22 CST 2016

> 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?

This might be a good starter post: https://fgiesen.wordpress.com/2014/08/18/atomics-and-contention

> I guess atomic reference count changes create overhead in multithreaded applications?

There is overhead even in single-threaded applications, as the post above discusses. The overhead depends very much on the details of the particular microarchitecture.

> Are there more detailed explorations into this topic?

If you’re looking for something Swift- or ARC-specific I don’t know of anything.


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