Why

Simple experiment to test the effects of different techniques and options on application start up time

Class Data Sharing (CDS)

The goal of CDS is to reduce the startup time of the JVM by loading from a pre-processed archive of Java classes and JVM metadata that is used during the initialization process. https://dev.java/learn/jvm/cds-appcds/

Clojure compiler options

see

Meta elision

To decrease class size and make classloading faster, meta can be elided.

Direct linking

Direct linking can be used to replace this indirection with a direct static invocation of the function instead. This will result in faster var invocation. Additionally, the compiler can remove unused vars from class initialization and direct linking will make many more vars unused. Typically this results in smaller class sizes and faster startup times.

Technique

Clone Sean Corfield's user manager example: https://github.com/seancorfield/usermanager-example
Hack the uber opts to conditionally add direct linking and elide meta compiler options
Run and measure with hyperfine
- Tested with openjdk 17 on AMD CPU

Results

The table below displays hyperfine results for mean application start up time with different options enabled and speed up relative to the baseline

cds	direct	elide	mean ms	stddev ms	speedup%
false	false	false	1431	18	0
false	false	true	1422	33	.63
false	true	false	1388	18	3
false	true	true	1379	17	3.6
true	false	false	992.9	16.4	30
true	false	true	992.4	20.1	30
true	true	false	972.6	12.5	32
true	true	true	959.4	12.3	33

Discussion

CDS has significant flat speed up effect. Consider using it if you have to shave off your start up time

For big applications direct linking and meta elision have small effect on start up time.

They have other performance benefits, especially direct linking, on application stready state performance.

The effort required to enjoy these improvements was very little

Implementation

hack uber opts

(defn- uber-opts [{:keys [elide direct] :as opts}]
  (-> opts
      (assoc
       :lib lib :main main
       :uber-file (format "target/%s-standalone.jar" lib)
       :basis (b/create-basis {})
       :class-dir class-dir
       :src-dirs ["src"])
      (cond->
          direct (assoc-in [:compile-opts :direct-linking] true)
          elide (assoc-in [:compile-opts :elide-meta] [:doc :file :line]))))

hack main

(defn -main
  [& [port]]
  (let [port (or port (get (System/getenv) "PORT" 8080))
        port (cond-> port (string? port) Integer/parseInt)]
    (println "Starting up on port" port)
    ;; start the web server and application:
    (-> (component/start (new-system port false))
        ;; then put it into the atom so we can get at it from a REPL
        ;; connected to this application:
        (->> (reset! repl-system))
        ;; then wait "forever" on the promise created:
        #_#_#_:web-server :shutdown deref))
  (System/exit 0))

run script

#!/usr/bin/env sh

for direct in true false;
do
    for elide in true false;
    do
        echo direct $direct elide $elide
        clojure -T:build ci :elide $elide :direct $direct && \
            java \
                -XX:ArchiveClassesAtExit=archive.jsa \
                -jar target/usermanager/example-standalone.jar && \
            hyperfine 'java -XX:SharedArchiveFile=archive.jsa -jar target/usermanager/example-standalone.jar' && \
            hyperfine 'java -jar target/usermanager/example-standalone.jar'
    done
done

bsless/README.md

Why