G1 has been around for quite some time now and since JDK 9 it is the default garbage collector in OpenJDK. The community working on G1 is big and the contributions over the last few years have made a significant impact on the overall performance. This talk will focus on some of these features and how they have improved G1 in various ways, including smaller memory footprint and shorter P99 pause times. We will also take a brief look at what features we have lined up for the future.

1. Brought to you by
G1: To Infinity and Beyond
Stefan Johansson
OpenJDK GC Engineer, Oracle

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Agenda
■ GC in OpenJDK
■ Short intro to G1
■ Progress since JDK 8
■ The future

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GC in OpenJDK

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Introduction
GC in OpenJDK
● Fast allocation
● Efficient reclamation
● Serving different use cases

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Strategies
GC in OpenJDK
● Tracing the object graph
● Stop-the-world or concurrent
● Generational collections

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Focus and tradeoffs
GC in OpenJDK
● Throughput
● Latency
● Footprint

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Current Collectors
GC in OpenJDK
Collector name Focus area Concepts
Serial Memory footprint Generational, single threaded STW collections
Parallel Throughput Generational, multi-threaded STW collections
G1 Balanced performance Generational, STW collections, concurrent marking
ZGC Low latency Concurrent collections
Shenandoah Low latency Concurrent collections

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Short intro to G1

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Basic idea
Short intro to G1
● Balance between throughput and latency
● Region based
● Concurrent marking
● Mixed collections
● Main tuning knob: pause-time goal

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Current status
Short intro to G1
● Supported since JDK 7u4
● Stable and mature
● Default collector since JDK 9
● Actively maintained and improved

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Progress since JDK 8

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Overview
Progress since JDK 8
● 1000+ enhancements since JDK 8
● Significant improvements
● Across all areas
● Cut away old tradeoffs

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Throughput
Progress since JDK 8
● Improved NUMA awareness
● More efficient concurrent work
● Parallel Full Collection

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Throughput
Progress since JDK 8
● SPECjbb® 2015 results
● 16 GB heap
● Not only GC improvements
● More time spent running Java
SPEC® and the benchmark name SPECjbb® are registered
trademarks of the Standard Performance Evaluation Corporation.
For more information about SPECjbb, see www.spec.org/jbb2015/.

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Latency
Progress since JDK 8
● Improved parallelism
● More efficient processing
● Better pause time predictions
● Abortable mixed collections

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Latency
Progress since JDK 8
● SPECjbb® 2015 results
● 16 GB heap
● Mostly GC improvements
● Trade throughput for latency
SPEC® and the benchmark name SPECjbb® are registered
trademarks of the Standard Performance Evaluation Corporation.
For more information about SPECjbb, see www.spec.org/jbb2015/.

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Latency
Progress since JDK 8
● SPECjbb® 2015 fixed load
● 16 GB heap
● Pause times
JDK 8 JDK 11 JDK 17
Average (ms) 154 98 84
P99 (ms) 186 126 111

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Latency
Progress since JDK 8
● SPECjbb® 2015 fixed load
● 16 GB heap
● Pause times
JDK 8 JDK 11 JDK 17 JDK 17 50ms
Average (ms) 154 98 84 47
P99 (ms) 186 126 111 60

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Latency
Progress since JDK 8
● Bonus slide: ZGC
● Sub-millisecond pause goal
● All heavy work done concurrently
JDK 8 JDK 11 JDK 17 JDK 17 ZGC
Average (ms) 154 98 84 0.029
P99 (ms) 186 126 111 0.186

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Footprint
Progress since JDK 8
● Rebuilding remembered sets concurrently
● Improved remembered set ergonomics
● Return heap memory to the OS

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Footprint
Progress since JDK 8
● BigRamTester
● 16GB heap

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The future

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Keep improving
The future
● Usability improvements
● Improved write barriers
● Footprint reductions

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Key takeaways
To Infinity and Beyond
● Massive improvements since JDK 8
● Exciting features in the pipe

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Brought to you by
Stefan Johansson
Twitter: @kstefanj
Blog: kstefanj.github.io