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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