Array cloning performance diffs about clj-fast

[joinr]

No, good writeup (and nugget of opt knowledge). Feel free to close.

[bsless]

This is weird, I can't recreate the results. Getting the exact same performance for both cases.
Which JVM and Clojure versions were you using?

[joinr]

Yeah, I'm seeing similar results. This (was) on java 8 on ubuntu's JDK, whatever variant that was. I just ran it on W10 with adopt open jdk, 1.8.0_222, and had like a 1 ns difference. Considering it anomalous for now (or jdk specific...)

[bsless]

I ran my benchmarks on Ubuntu 20.10 with Java 15.
If this was about the JDK version, 8 is considered EOL and we can happily forget about it.
If you have no objections I'm closing this issue, but do reopen it if you find it crops up again

[joinr]

, 8 is considered EOL and we can happily forget about it.

Not so. There are many many large orgs still running Java 8 for LTS (part of the reason amazon has their own, among others). Some places (including mine) are still there for the foreseeable future. Just FYI.

[bsless]

Well then, I'll try to recreate

[joinr]

I'll re-run on the original machine. an ubuntu machine on ec2.

[bsless]

Reran with:

;; CIDER 1.1.0snapshot (package: 20210416.1915), nREPL 0.8.3
;; Clojure 1.10.1, Java 1.8.0_282

Ubuntu 20.10
Got same results again, exactly same performance in both cases

[joinr]

I just re-ran it on the same setup, and reproduced:

Execution time mean : 121.353224 ns
Execution time mean : 36.097984 ns

[joinr]

openjdk version "1.8.0_282"
OpenJDK Runtime Environment (build 1.8.0_282-8u282-b08-0ubuntu1~16.04-b08)
OpenJDK 64-Bit Server VM (build 25.282-b08, mixed mode)

[bsless]

Weird, can you print out all the options and flags the JVM ran with?

[joinr]

running from lein repl with default options (no :jvm-opts entry).

[joinr]

lein 2.9.1, not that it matters.

[joinr]

Also running this on an EC2 instance; could be some throttling, but I'd expect that to be amortized over the multiple runs.

[bsless]

The JVM still has some default configurations, such as GC, server, client or tiered etc. There's a way to print out all of them, something like https://alvinalexander.com/java/how-see-jvm-parameters-arguments-from-running-java-application/

[joinr]

[:arg -Dfile.encoding=UTF-8]
[:arg -XX:-OmitStackTraceInFastThrow]
[:arg -XX:+TieredCompilation]
[:arg -XX:TieredStopAtLevel=1]
[:arg -Dclojure.compile.path=/home/tom/repos/spork/target/classes]
[:arg -Dspork.version=0.2.1.4-SNAPSHOT]
[:arg -Dclojure.debug=false]

[joinr]

this particular repo running clojure 1.10.1

[bsless]

Okay, I managed to recreate. One of these is the culprit:

"-Djdk.attach.allowAttachSelf" "-XX:+UnlockDiagnosticVMOptions" "-XX:+DebugNonSafepoints"

[bsless]

Printed the options with and without passing JVM opts flag:

;;; no jvm opts
["-Dfile.encoding=UTF-8"
 "-XX:-OmitStackTraceInFastThrow"
 "-XX:+TieredCompilation"
 "-XX:TieredStopAtLevel=1"
 "-Dclj-fast.version=0.0.10-SNAPSHOT"
 "-Dclojure.debug=false"]

;;; jvm opts
["-Dfile.encoding=UTF-8"
 "-Djdk.attach.allowAttachSelf"
 "-XX:+UnlockDiagnosticVMOptions"
 "-XX:+DebugNonSafepoints"
 "-Dclj-fast.version=0.0.10-SNAPSHOT"
 "-Dclojure.debug=false"]

I suspect XX:TieredStopAtLevel

[joinr]

with :jvm-opts ["-XX:+UnlockDiagnosticVMOptions" "-XX:+DebugNonSafepoints"]

I get 20 ns for both runs now....both in lein repl and from cider.

[bsless]

Okay, tested with a variety of flags
The culprit is -XX:TieredStopAtLevel
For anything under 4 you'll get the performance difference because C2 doesn't kick in.
It's probably used by lein to reduce start-up time for dev profile. I think lein run uses dev profile.
I tried passing empty jvm-opts vector and got the same results for both, so I'm attributing this to dev profile.

[joinr]

Alright, yet more jvm corner case magic to understand. If nothing else, the issue is documented for posterity.

[bsless]

I wouldn't call it a corner case of the JVM, more of leiningen IMO - It's an undocumented flag which is injected when youlein run and messes with performance. It's in Lein's source code
I think I'll open an issue about it.

[bsless]

I think I'll end up adding a few notes to the readme, nothing more I can do in the meanwhile

[bsless]

This is the best I could do in the meanwhile
bba3ec0

Edit suggestions?