I should perhaps make clear that with direct use of monitor-enter and monitor-exit with a Var it's possible for monitor-enter and monitor-exit to operate on different objects even in the absence of typos, namely if somebody rebinds the Var. To illustrate this with print at the REPL (a regular Clojure REPL, as opposed to nREPL or similar, in case there are any issues with reproducing this in different environments):
(def x 1) (future (print x) (Thread/sleep 5000) (print x)) ; prints 1 immediately, but then goes to sleep; ; in the meantime, I say (def x 2) @*2 ; and after a moment 2 is printed NB. this could also happen with alter-var-root or set! to a Var with a thread-local binding. Cheers, Michał On 3 November 2013 18:30, Michał Marczyk <michal.marc...@gmail.com> 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 <blume.m...@gmail.com> 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 >> >> -- >> -- >> You received this message because you are subscribed to the Google >> Groups "Clojure" group. >> To post to this group, send email to clojure@googlegroups.com >> Note that posts from new members are moderated - please be patient with your >> first post. >> To unsubscribe from this group, send email to >> clojure+unsubscr...@googlegroups.com >> For more options, visit this group at >> http://groups.google.com/group/clojure?hl=en >> --- >> You received this message because you are subscribed to the Google Groups >> "Clojure" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to clojure+unsubscr...@googlegroups.com. >> For more options, visit https://groups.google.com/groups/opt_out. -- -- You received this message because you are subscribed to the Google Groups "Clojure" group. 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