Huh, interesting.
I have:
(defn foo' [x]
(if (> x 0)
(inc x)
(let [res (locking o (dec x))] res)))
(defn foo'' [x]
(if (> x 0)
(inc x)
(locking o
(dec x))))
foo' is fast, but foo'' is slow. So something about wrapping the locking
clause in a let makes it fast. Still no idea why.
On Sunday, November 3, 2013 9:30:45 AM UTC-8, Michał Marczyk wrote:
>
> You have a typo in foo -- monitor-exit's argument is 0 (zero) rather
> than o (the sentinel object).
>
> Besides that, in foo both monitor-enter and monitor-exit get their
> arguments from a Var. Rewriting to use locking, which first puts the
> object whose monitor will be used in a local (that is, (let [lockee o]
> ...), where ... performs the locking using the newly introduced
> local), gives timings identical to those of bar and baz:
>
> (defn foo' [x]
> (if (> x 0)
> (inc x)
> (let [res (locking o (dec x))] res)))
>
> So this is one reason not to use monitor-enter and monitor-exit
> directly. Another reason is that locking guarantees that the monitor
> will be released (by using try / finally, and of course by preventing
> situations where the matching monitor-enter & monitor-exit operate on
> different objects).
>
> In fact, both monitor-enter and monitor-exit carry docstrings which
> explicitly say that they should not be used in user code and point to
> locking as the user-facing equivalent to Java's synchronized.
>
> Cheers,
> Michał
>
>
> On 1 November 2013 19:34, Michael Blume <[email protected] <javascript:>>
> wrote:
> > https://github.com/MichaelBlume/perf-test
> >
> > (ns perf-test
> > (:use (criterium core))
> > (:gen-class))
> >
> > (def o (Object.))
> >
> > (defn foo [x]
> > (if (> x 0)
> > (inc x)
> > (do
> > (monitor-enter o)
> > (let [res (dec x)]
> > (monitor-exit 0)
> > res))))
> >
> > (defn bar [x]
> > (if (> x 0)
> > (inc x)
> > (dec x)))
> >
> > (defn locking-part [x l]
> > (monitor-enter l)
> > (let [res (dec x)]
> > (monitor-exit l)
> > res))
> >
> > (defn baz [x]
> > (if (> x 0)
> > (inc x)
> > (locking-part x o)))
> >
> > (defn -main []
> > (println "benching foo")
> > (bench (foo 5) :verbose)
> > (println "benching bar")
> > (bench (bar 5) :verbose)
> > (println "benching baz")
> > (bench (baz 5) :verbose)
> > (println "done benching"))
> >
> >
> >
> > I'm only ever calling these functions with positive values, so the
> > monitor-enter branch should never be entered. Nevertheless, the
> performance
> > of foo is much worse than bar or baz.
> >
> > The best guess I've got is that the fact that lock-taking is involved
> > somehow changes how the function is compiled, somehow making the
> function
> > slower. If the practical upshot is that I shouldn't write functions that
> > only sometimes lock -- that the locking part of a function should always
> be
> > its own function -- then I can do that, but I'm curious why.
> >
> > $ lein uberjar
> > Compiling perf-test
> > Created /Users/mike/perf-test/target/perf-test-0.1.0-SNAPSHOT.jar
> > Created
> /Users/mike/perf-test/target/perf-test-0.1.0-SNAPSHOT-standalone.jar
> > $ java -jar -server target/perf-test-0.1.0-SNAPSHOT-standalone.jar
> > benching foo
> > WARNING: Final GC required 1.5974571326266802 % of runtime
> > x86_64 Mac OS X 10.8.3 4 cpu(s)
> > Java HotSpot(TM) 64-Bit Server VM 24.0-b28
> > Runtime arguments:
> > Evaluation count : 391582560 in 60 samples of 6526376 calls.
> > Execution time sample mean : 167.426696 ns
> > Execution time mean : 167.459429 ns
> > Execution time sample std-deviation : 4.079466 ns
> > Execution time std-deviation : 4.097819 ns
> > Execution time lower quantile : 160.742869 ns ( 2.5%)
> > Execution time upper quantile : 175.453376 ns (97.5%)
> > Overhead used : 1.634996 ns
> >
> > Found 2 outliers in 60 samples (3.3333 %)
> > low-severe 2 (3.3333 %)
> > Variance from outliers : 12.5602 % Variance is moderately inflated by
> > outliers
> > benching bar
> > x86_64 Mac OS X 10.8.3 4 cpu(s)
> > Java HotSpot(TM) 64-Bit Server VM 24.0-b28
> > Runtime arguments:
> > Evaluation count : 2174037300 in 60 samples of 36233955 calls.
> > Execution time sample mean : 26.068923 ns
> > Execution time mean : 26.066422 ns
> > Execution time sample std-deviation : 0.887937 ns
> > Execution time std-deviation : 0.916861 ns
> > Execution time lower quantile : 23.996763 ns ( 2.5%)
> > Execution time upper quantile : 27.911936 ns (97.5%)
> > Overhead used : 1.634996 ns
> >
> > Found 3 outliers in 60 samples (5.0000 %)
> > low-severe 1 (1.6667 %)
> > low-mild 1 (1.6667 %)
> > high-mild 1 (1.6667 %)
> > Variance from outliers : 22.1874 % Variance is moderately inflated by
> > outliers
> > benching baz
> > x86_64 Mac OS X 10.8.3 4 cpu(s)
> > Java HotSpot(TM) 64-Bit Server VM 24.0-b28
> > Runtime arguments:
> > Evaluation count : 2270676660 in 60 samples of 37844611 calls.
> > Execution time sample mean : 25.834142 ns
> > Execution time mean : 25.837429 ns
> > Execution time sample std-deviation : 0.718382 ns
> > Execution time std-deviation : 0.729431 ns
> > Execution time lower quantile : 24.837925 ns ( 2.5%)
> > Execution time upper quantile : 27.595781 ns (97.5%)
> > Overhead used : 1.634996 ns
> >
> > Found 4 outliers in 60 samples (6.6667 %)
> > low-severe 2 (3.3333 %)
> > low-mild 2 (3.3333 %)
> > Variance from outliers : 15.7591 % Variance is moderately inflated by
> > outliers
> > done benching
> >
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