Garbage-first collector

Garbage-First (G1) collector is a server-style garbage collector, targeted for multiprocessors with large memories, that meets a soft real-time goal with high probability, while achieving high-throughput.{{cite book |year=2004 |first1=David |last1=Detlefs |first2=Christine |last2=Flood |first3=Steve |last3=Heller |first4=Tony |last4=Printezis |title=Proceedings of the 4th international symposium on Memory management |chapter=Garbage-first garbage collection |pages=37–48 |doi=10.1145/1029873.1029879 |isbn=1581139454 |citeseerx=10.1.1.63.6386|s2cid=3330886 }} G1 preferentially collects regions with the least amount of live data, or "garbage first".{{cite web |url=https://www.redhat.com/en/about/blog/part-1-introduction-g1-garbage-collector |title=Part 1: Introduction to the G1 Garbage Collector |website=www.redhat.com}} G1 is the long term replacement of CMS. Whole-heap operations, such as global marking, are performed concurrently with the application threads, to prevent interruptions proportional to heap or live-data size. Concurrent marking provides both collection completeness and identifies regions ripe for reclamation via compacting evacuation. This evacuation is performed in parallel on multiprocessors, to decrease pause times and increase throughput.

G1 was first introduced as an experimental option in Java SE 6 Update 14,{{cite web |url=http://www.drdobbs.com/jvm/g1-javas-garbage-first-garbage-collector/219401061 |title=G1: Java's Garbage First Garbage Collector |publisher=Dr Dobbs}} where it can be enabled with the following two command-line parameters: -XX:+UnlockExperimentalVMOptions and -XX:+UseG1GC With JDK 7, G1 was planned to replace CMS in the Hotspot JVM. There are two major differences between CMS and G1. The first is that G1 is a compacting collector.{{cite web |url=http://www.fasterj.com/articles/G1.shtml |title=The Garbage First Collector |website=www.fasterj.com}} G1 compacts sufficiently to completely avoid the use of fine-grain free lists for allocation, which considerably simplifies parts of the collector and mostly eliminates potential fragmentation issues. As well as compacting, G1 offers more predictable garbage collection pauses than the CMS collector and allows users to set their desired pause targets.

In Java 9 G1 was made the default garbage collector,{{cite web |url=https://stackoverflow.com/questions/46377561/why-g1-is-default-garbage-collector-for-java-9 |title=Why G1 is default garbage collector for Java 9?}} in spite of Google counter proposing the well-known CMS as the standard, claiming the modified CMS it uses performs better than G1.{{cite web |url=https://jaxenter.com/java-9s-new-garbage-collector-whats-changing-whats-staying-118313.html |title=Java 9's new garbage collector: What's changing? What's staying? - JAXenter |date=26 June 2015 |publisher=jaxenter.com}} Since then, Oracle has greatly improved G1's throughput, latency and memory footprint.{{cite web |url=https://inside.java/2021/10/11/p99-g1-to-infinity-and-beyond/ |title=G1: To Infinity and Beyond |first=Stefan |last=Johansson |website=inside.java |date=11 October 2021}}

Guaranteed real-time behavior even with garbage collection requires a real-time garbage collector such as those that come with Sun's Java RTShttp://www.oracle.com/technetwork/articles/javase/index-138577.html An Introduction to Real-Time Java Technology: Part 2, Garbage Collection and the Sun Java Real-Time System (Java RTS) or IBM’s WebSphere RT.http://www.ibm.com/developerworks/java/library/j-rtj4/index.html?S_TACT=105AGX02&S_CMP=EDU Real time Garbage Collection

• Mark-compact algorithm

{{Reflist}}

• [https://tschatzl.github.io HotSpot G1/Parallel/Serial GCs team blog]
• [http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.63.6386&rep=rep1&type=pdf Garbage-First Garbage collection]
• [https://stackoverflow.com/questions/2881827/how-does-the-garbage-first-garbage-collector-work How does the Garbage-First Garbage Collector work?]
• [https://plumbr.eu/handbook/garbage-collection-algorithms-implementations/g1 JAVA GARBAGE COLLECTION HANDBOOK G1 – Garbage First] {{Webarchive|url=https://web.archive.org/web/20170605201055/https://plumbr.eu/handbook/garbage-collection-algorithms-implementations/g1 |date=2017-06-05 }}
• [https://www.infoq.com/articles/G1-One-Garbage-Collector-To-Rule-Them-All G1: One Garbage Collector To Rule Them All]
• [https://www.safaribooksonline.com/library/view/java-performance-companion/9780133796896/ch02.html 1. Garbage First Overview]

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