[swift-dev] Profiling ARC

Michael Gottesman mgottesman at apple.com
Fri Feb 17 16:31:53 CST 2017

> On Feb 17, 2017, at 11:30 AM, David P Grove via swift-dev <swift-dev at swift.org> wrote:
> swift-dev-bounces at swift.org wrote on 02/16/2017 09:48:28 PM:
> > 
> > I was curious about the overhead of ARC and started profiling some 
> > benchmarks found in the Computer Language Benchmark Game (http://
> > benchmarksgame.alioth.debian.org/u64q/measurements.php?lang=swift). 
> > So far, it seems that ARC sequence optimization is surprisingly good
> > and most benchmarks don't have to perform ARC operations as often as
> > I expected.  I have some questions regarding this finding.
> > 
> > I compiled all benchmarks with "-O -wmo" flags and counted the 
> > number of calls to ARC runtime (e.g., swift_rt_swift_retain) using Pin.
> > 
> > 1. Reference counting is considered to have high overhead due to 
> > frequent counting operations which also have to be atomic.  At least
> > for the benchmarks I tested, it is not the case and there is almost 
> > no overhead.  Is it expected behavior?  Or is it because the 
> > benchmarks are too simple (they are all single-file programs)?  How 
> > do you estimate the overhead of ARC would be?
> > 
> hmm,  I wonder if your method of profiling is really finding all the ARC operations.  The Swift version of regex-dna is about 25x slower than the Java version (on Linux).  I looked at some prof profiles about a month ago and at the time roughly 80% of all execution samples were attributed to swift_retain/swift_release operations coming from CoreFoundation's regex implementation.  
Question. Where is this regex-dna benchmark, is it in the swift benchmark suite?
> --dave
> (See attached file: regex-dna.svg)
> <regex-dna.svg>_______________________________________________
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