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:

<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:

mvn release:prepare
mvn release:perform

Benchmarks

Read more about the benchmarks at bramp.net

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.

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modification, are permitted provided that the following conditions are met:

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  this list of conditions and the following disclaimer in the documentation
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