What is the fastest Garbage Collector in Java 8?

OpenJDK 8 has several Garbage Collector algorithms, such as Parallel GC, CMS and G1. Which one is the fastest?
What will happen if the default GC changes from Parallel GC in Java 8 to G1 in Java 9 (as currently proposed)?
Let’s benchmark it.

Benchmark methodology

  • Run the same code 6 times with a different VM argument (-XX:+UseSerialGC, -XX:+UseParallelGC, -XX:+UseConcMarkSweepGC, -XX:ParallelCMSThreads=2, -XX:ParallelCMSThreads=4, -XX:+UseG1GC).

  • Each run takes about 55 minutes.

  • Other VM arguments: -Xmx2048M -server
    OpenJDK version: 1.8.0_51 (currently the latest version)
    Software: Linux version 4.0.4-301.fc22.x86_64
    Hardware: Intel® Core™ i7-4790 CPU @ 3.60GHz

  • Each run solves 13 planning problems with OptaPlanner.
    Each planning problem runs for 5 minutes. It starts with a 30 second JVM warm up which is discarded.

  • Solving a planning problem involves no IO (except a few milliseconds during startup to load the input).
    A single CPU is completely saturated.
    It constantly creates many short lived objects, and the GC collects them afterwards.

  • The benchmarks measure the number of scores that can be calculated per millisecond. Higher is better.
    Calculating a score for a proposed planning solution is non-trivial:
    it involves many calculations, including checking for conflicts between every entity and every other entity.

  • To reproduce these benchmarks locally, build optaplanner from source
    and run the main class
    GeneralOptaPlannerBenchmarkApp.

Benchmark results

Executive summary

For your convenience, I’ve compared each Garbage Collector type to the default in Java 8 (Parallel GC).

garbageCollectorTypesJava8

The results are clear: That default (Parallel GC) is the fastest.

Raw benchmark numbers

Relative benchmark numbers

Garbage CollectorCloud balancing 200cCloud balancing 800cMachine reassignment B1Machine reassignment B10Course scheduling c7Course scheduling c8Exam scheduling s2Exam scheduling s3Nurse rostering m1Nurse rostering mh1Sport scheduling nl14

-XX:+UseSerialGC

121211

102072

239278

54637

10821

14370

17095

10130

7389

6667

2234

-XX:+UseParallelGC (default Java 8)

126248

107991

282055

59944

10919

14517

17843

10564

7459

6676

2228

-XX:+UseConcMarkSweepGC

123150

106889

255775

59087

10142

13180

16346

9903

6738

6018

2142

-XX:ParallelCMSThreads=2

128591

106992

279968

59406

10530

13621

16591

10082

7148

6319

2276

-XX:ParallelCMSThreads=4

124415

104328

276401

58234

10738

13918

16952

10072

7180

6320

2270

-XX:+UseG1GC (default Java 9?)

97146

83952

259981

39522

9803

11965

15195

9410

5961

4985

2062

Dataset scale

120k

1920k

500k

250000k

217k

145k

1705k

1613k

18k

12k

4k

Should Java 9 default to G1?

There is a proposal to make G1 the default Garbage Collector in OpenJDK9 for servers.
My first reaction is to reject this proposal:

  • G1 is 17.60% is slower on average.

  • G1 is consistently slower on every use case for every dataset.

  • On the biggest dataset (Machine Reassignment B10), which dwarfs any of the other datasets in size,
    G1 is 34.07% is slower.

  • If the default GC differs between developer machines and servers, then developer benchmarks become less trustworthy.

On the other hand, there are a few nuances to note:

  • G1 focuses on limiting GC pauses, instead of throughput.
    For these use cases (with heavy calculations) GC pause length mostly doesn’t matter.

  • This was an (almost) single threaded benchmark.
    Further benchmarking with multiple solvers in parallel or multithreaded solving might influence results.

  • G1 is recommended for a heap size of at least 6 GB.
    This benchmark used a heap size of only 2 GB and even that size is only needed for the biggest dataset (Machine Reassignment B10).

Heavy calculations is just one of the many things that OpenJDK is used for: it’s just 1 stakeholder in this community wide debate.
If other stakeholders (such as web services) prove otherwise, maybe it’s worth changing the default GC.
But show me the benchmarks on real projects first!

Conclusion

In Java 8, the default Garbage Collector (Parallel GC) is generally the best choice for OptaPlanner use cases.

Garbage CollectorAverageCloud balancing 200cCloud balancing 800cMachine reassignment B1Machine reassignment B10Course scheduling c7Course scheduling c8Exam scheduling s2Exam scheduling s3Nurse rostering m1Nurse rostering mh1Sport scheduling nl14

-XX:+UseSerialGC

-4.05%

-3.99%

-5.48%

-15.17%

-8.85%

-0.90%

-1.01%

-4.19%

-4.11%

-0.94%

-0.13%

+0.27%

-XX:+UseParallelGC (default Java 8)

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

-XX:+UseConcMarkSweepGC

-6.23%

-2.45%

-1.02%

-9.32%

-1.43%

-7.12%

-9.21%

-8.39%

-6.26%

-9.67%

-9.86%

-3.86%

-XX:ParallelCMSThreads=2

-2.67%

+1.86%

-0.93%

-0.74%

-0.90%

-3.56%

-6.17%

-7.02%

-4.56%

-4.17%

-5.35%

+2.15%

-XX:ParallelCMSThreads=4

-2.94%

-1.45%

-3.39%

-2.00%

-2.85%

-1.66%

-4.13%

-4.99%

-4.66%

-3.74%

-5.33%

+1.89%

-XX:+UseG1GC (default Java 9?)

-17.60%

-23.05%

-22.26%

-7.83%

-34.07%

-10.22%

-17.58%

-14.84%

-10.92%

-20.08%

-25.33%

-7.45%

Dataset scale

120k

1920k

500k

250000k

217k

145k

1705k

1613k

18k

12k

4k

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