Unsafe

by Andrew Brampton (bramp.net)

GitHub | JavaDoc

This is a collection of tools that make use of the sun.misc.Unsafe class.
This Unsafe class allows direct access to memory within the JVM, which is extremely dangerous, but fun :).

• unsafe-helper - Contains some simple methods that make using sun.misc.Unsafe easier.
• unsafe-collection - An example List modelled on the ArrayList, which instead of storing reference to objects within
  the collection, instead copies the elements directly into the list. This has a few interesting properties
  • Less total memory is required for the list and contained objects. Reducing GC overheads.
  • The objects are guranteed to be contingous in memory, which may provide some good CPU cache benefits.
  • Objects are copied into the list, this copy overhead may not be worth it, and you lose many of the reference semantics you would be used t
• unsafe-unroller - At runtimes generates optomal bytecode to copy objects with the Unsafe class.
• unsafe-benchmark - Code to benchmark everything using the JMH framework.
• unsafe-tests - Some simple test classes to help with tests of the other modules.

Read about this in a series of articles:

• Part 1: sun.misc.Unsafe Helper Classes
• Part 2: Using sun.misc.Unsafe to create a contiguous array of objects
• Part 3: Benchmarking a java UnsafeArrayList

Use

Requires Java 7 or higher. To include use the following Maven dependency:

xmlCopy
<dependencies>
    <dependency>
        <groupId>net.bramp.unsafe</groupId>
        <artifactId>unsafe-helper</artifactId>
        <version>1.0</version>
    </dependency>
</dependencies>

Build

To build use maven, e.g mvn

To release to maven central, we use the Sonatype OSS repo, and
maven-release-plugin:

bashCopy
mvn release:prepare
mvn release:perform

Benchmarks

Read more about the benchmarks at bramp.net

bashCopy
cd unsafe-benchmark
mvn
java -jar target/benchmarks.jar | tee logs
...
# JMH 1.10.3 (released 9 days ago)
# VM version: JDK 1.8.0_45-internal, VM 25.45-b02

Benchmark                                       Mode     Cnt     Score          Error  Units
UnrolledCopierBenchmark.HandUnrolledState.test  thrpt    25      438819259.527  ±      14364692.101  ops/s
UnrolledCopierBenchmark.LoopState.test          thrpt    25      196408390.244  ±      2173851.339   ops/s
UnrolledCopierBenchmark.UnrolledState.test      thrpt    25      458324068.892  ±      6192069.477   ops/s

Benchmark                                  (clazz)                      (size)    Mode   Cnt    Score   Error  Units
ArrayListBenchmark.testIterate             ArrayList<LongPoint>         80000000  avgt   5      2.266   ±      0.229  s/op
ArrayListBenchmark.testIterate             ArrayList<LongPoint>         20000000  avgt   5      0.552   ±      0.019  s/op
ArrayListBenchmark.testIterate             ArrayList<LongPoint>         5000000   avgt   5      0.136   ±      0.004  s/op
ArrayListBenchmark.testSort                ArrayList<LongPoint>         80000000  avgt   5      70.310  ±      3.939  s/op
ArrayListBenchmark.testSort                ArrayList<LongPoint>         20000000  avgt   5      14.754  ±      0.541  s/op
ArrayListBenchmark.testSort                ArrayList<LongPoint>         5000000   avgt   5      3.250   ±      0.139  s/op

Benchmark                                  (clazz)                      (size)    Mode   Cnt    Score   Error  Units
ArrayListBenchmark.testIterate             UnsafeArrayList<LongPoint>   80000000  avgt   5      1.790   ±      0.030  s/op
ArrayListBenchmark.testIterate             UnsafeArrayList<LongPoint>   20000000  avgt   5      0.449   ±      0.016  s/op
ArrayListBenchmark.testIterate             UnsafeArrayList<LongPoint>   5000000   avgt   5      0.112   ±      0.001  s/op
ArrayListBenchmark.testListIterateInPlace  UnsafeArrayList<LongPoint>   80000000  avgt   5      0.442   ±      0.023  s/op
ArrayListBenchmark.testListIterateInPlace  UnsafeArrayList<LongPoint>   20000000  avgt   5      0.110   ±      0.003  s/op
ArrayListBenchmark.testListIterateInPlace  UnsafeArrayList<LongPoint>   5000000   avgt   5      0.028   ±      0.002  s/op
ArrayListBenchmark.testSort                UnsafeArrayList<LongPoint>   80000000  avgt   5      18.690  ±      3.158  s/op
ArrayListBenchmark.testSort                UnsafeArrayList<LongPoint>   20000000  avgt   5      3.414   ±      0.034  s/op
ArrayListBenchmark.testSort                UnsafeArrayList<LongPoint>   5000000   avgt   5      0.682   ±      0.014  s/op

Benchmark                                  (clazz)                      (size)    Mode   Cnt    Score   Error  Units
ArrayListBenchmark.testIterate             ArrayList<FourLongs>         80000000  avgt   5      2.277   ±      0.211  s/op
ArrayListBenchmark.testIterate             ArrayList<FourLongs>         20000000  avgt   5      0.557   ±      0.023  s/op
ArrayListBenchmark.testIterate             ArrayList<FourLongs>         5000000   avgt   5      0.140   ±      0.007  s/op
ArrayListBenchmark.testSort                ArrayList<FourLongs>         80000000  avgt   5      79.673  ±      6.119  s/op
ArrayListBenchmark.testSort                ArrayList<FourLongs>         20000000  avgt   5      16.705  ±      1.353  s/op
ArrayListBenchmark.testSort                ArrayList<FourLongs>         5000000   avgt   5      3.673   ±      0.156  s/op

Benchmark                                  (clazz)                      (size)    Mode   Cnt    Score   Error  Units
ArrayListBenchmark.testIterate             UnsafeArrayList<FourLongs>   80000000  avgt   5      2.126   ±      0.019  s/op
ArrayListBenchmark.testIterate             UnsafeArrayList<FourLongs>   20000000  avgt   5      0.533   ±      0.004  s/op
ArrayListBenchmark.testIterate             UnsafeArrayList<FourLongs>   5000000   avgt   5      0.133   ±      0.002  s/op
ArrayListBenchmark.testListIterateInPlace  UnsafeArrayList<FourLongs>   80000000  avgt   5      0.648   ±      0.019  s/op
ArrayListBenchmark.testListIterateInPlace  UnsafeArrayList<FourLongs>   20000000  avgt   5      0.163   ±      0.005  s/op
ArrayListBenchmark.testListIterateInPlace  UnsafeArrayList<FourLongs>   5000000   avgt   5      0.040   ±      0.006  s/op
ArrayListBenchmark.testSort                UnsafeArrayList<FourLongs>   80000000  avgt   5      24.822  ±      0.790  s/op
ArrayListBenchmark.testSort                UnsafeArrayList<FourLongs>   20000000  avgt   5      4.843   ±      0.075  s/op
ArrayListBenchmark.testSort                UnsafeArrayList<FourLongs>   5000000   avgt   5      1.020   ±      0.017  s/op

Benchmark                                  (clazz)                      (size)    Mode   Cnt    Score   Error  Units
ArrayListBenchmark.testIterate             ArrayList<EightLongs>        80000000  avgt   5      2.792   ±      0.072  s/op
ArrayListBenchmark.testIterate             ArrayList<EightLongs>        20000000  avgt   5      0.564   ±      0.022  s/op
ArrayListBenchmark.testIterate             ArrayList<EightLongs>        5000000   avgt   5      0.138   ±      0.007  s/op
ArrayListBenchmark.testSort                ArrayList<EightLongs>        80000000  avgt   5      97.687  ±      4.860  s/op
ArrayListBenchmark.testSort                ArrayList<EightLongs>        20000000  avgt   5      20.084  ±      1.124  s/op
ArrayListBenchmark.testSort                ArrayList<EightLongs>        5000000   avgt   5      4.474   ±      0.248  s/op

Benchmark                                  (clazz)                      (size)    Mode   Cnt    Score   Error  Units
ArrayListBenchmark.testIterate             UnsafeArrayList<EightLongs>  80000000  avgt   5      2.672   ±      0.322  s/op
ArrayListBenchmark.testIterate             UnsafeArrayList<EightLongs>  20000000  avgt   5      0.688   ±      0.014  s/op
ArrayListBenchmark.testIterate             UnsafeArrayList<EightLongs>  5000000   avgt   5      0.171   ±      0.003  s/op
ArrayListBenchmark.testListIterateInPlace  UnsafeArrayList<EightLongs>  80000000  avgt   5      0.941   ±      0.032  s/op
ArrayListBenchmark.testListIterateInPlace  UnsafeArrayList<EightLongs>  20000000  avgt   5      0.236   ±      0.008  s/op
ArrayListBenchmark.testListIterateInPlace  UnsafeArrayList<EightLongs>  5000000   avgt   5      0.058   ±      0.002  s/op
ArrayListBenchmark.testSort                UnsafeArrayList<EightLongs>  80000000  avgt   5      40.697  ±      0.743  s/op
ArrayListBenchmark.testSort                UnsafeArrayList<EightLongs>  20000000  avgt   5      7.608   ±      0.267  s/op
ArrayListBenchmark.testSort                UnsafeArrayList<EightLongs>  5000000   avgt   5      1.729   ±      0.101  s/op

Further Reading

• Java Objects Memory Structure
• sun.misc.Unsafe Javadoc
• OpenJDK Unsafe source
• The infamous sun.misc.Unsafe explained
• Dangerous Code: How to be Unsafe with Java Classes & Objects in Memory

JMH

• Introduction to JMH
• Hashmap Tests

Licence (Simplified BSD License)

Copyright (c) 2016, Andrew Brampton
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright notice, this
  list of conditions and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright notice,
  this list of conditions and the following disclaimer in the documentation
  and/or other materials provided with the distribution.

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