[swift-dev] Profiling ARC

[Michael Gottesman]

I am familiar with it, just not all of the benchmarks by name.


> On Feb 17, 2017, at 3:04 PM, David P Grove <groved at us.ibm.com> wrote:
> 
> Sorry, I shouldn't have assumed that everyone know what the Computer Language Benchmark Game was.
> 
> There's a set of 10 toy benchmarks written in 30 different languages that are available here: http://benchmarksgame.alioth.debian.org/ <http://benchmarksgame.alioth.debian.org/> The webpage also lets you see the results of regular performance runs and see how the various programs stack up against each other.
> 
> As usual with small benchmarks, there are lots of ways in which the programs aren't realistic. 
> 
> After dispatch became available on Linux with Swift 3, I had a side project of going through the Swift implementations and adding concurrency to a few Swift programs that didn't have it already.
> 
> regex-dna is the main outlier for Swift (which is why I had profiled it). There's enough ARC overhead that using dispatch actually made it slower...
> The sequential version for of regex-dna for Swift is: http://benchmarksgame.alioth.debian.org/u64q/program.php?test=regexdna&lang=swift&id=2 <http://benchmarksgame.alioth.debian.org/u64q/program.php?test=regexdna&lang=swift&id=2>
> My slower concurrent version is: http://benchmarksgame.alioth.debian.org/u64q/program.php?test=regexdna&lang=swift&id=3 <http://benchmarksgame.alioth.debian.org/u64q/program.php?test=regexdna&lang=swift&id=3>
> 
> I suspect that the main fix for improving the performance of this program is actually doing something in CoreFoundation, but I got sidetracked and didn't finish looking into it.
> 
> --dave
> 
> <graycol.gif>Michael Gottesman ---02/17/2017 05:32:03 PM---> On Feb 17, 2017, at 11:30 AM, David P Grove via swift-dev <swift-dev at swift.org> wrote: >
> 
> From: Michael Gottesman <mgottesman at apple.com>
> To: David P Grove/Watson/IBM at IBMUS
> Cc: Jiho Choi <jray319 at gmail.com>, swift-dev <swift-dev at swift.org>
> Date: 02/17/2017 05:32 PM
> Subject: Re: [swift-dev] Profiling ARC
> Sent by: mgottesman at apple.com
> 
> 
> 
> 
> On Feb 17, 2017, at 11:30 AM, David P Grove via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> 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.  
> 
> 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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