[swift-dev] Profiling ARC

Michael Gottesman mgottesman at apple.com
Mon Feb 20 17:20:18 CST 2017

Are you talking about this one (there are two)?

http://benchmarksgame.alioth.debian.org/u64q/program.php?test=binarytrees&lang=swift&id=1 <http://benchmarksgame.alioth.debian.org/u64q/program.php?test=binarytrees&lang=swift&id=1>


> On Feb 20, 2017, at 2:24 PM, Jiho Choi via swift-dev <swift-dev at swift.org> wrote:
> You are right that regex has many ARC operations from libFoundation.  Another outlier in terms of the number of ARC operations is binary-tree.  In this case, ARC operations are from the user code, and the optimizer couldn't make much difference.
> Other than these two, the optimizer seems working pretty well in removing ARC operations.
> On Fri, Feb 17, 2017 at 1:34 PM David P Grove <groved at us.ibm.com <mailto:groved at us.ibm.com>> wrote:
> swift-dev-bounces at swift.org <mailto: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 <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.  
> --dave
> (See attached file: regex-dna.svg)
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