Bug ID: JDK-8297639 Remove preventive GCs in G1

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JDK 21
21 b02Fixed

After obsoleting it in JDK20, remove the functionality for the next release (i.e. JDK 21). No good application for it has been found, and investing more time in fixing it seems too much work for too little gain.

The disabling and subsequent removal of preventive GC has caused issues for Elasticsearch, when running with JDK 20 and 21. The particular scenario that encounters problems is one where hundreds of documents are retrieved from a search, each with a large(ish) 4MB size. These documents comprise a ~400 MB pytorch model being transfered from a data node to an ML (Machine Learning) node. The pytorch model is chunked and stored in separate documents that are retrieved one at a time by the ML node. When repeatedly allocating these large arrays, we see memory pressure distinctly increase in JDK 20.0.1. For JDK 20.x we set -XX:+G1UsePreventiveGC. For JDK 21, since no such flag is available, we obviously do not set it, but the behaviour is slightly better ( GC exhibit's slightly better behaviour, but not as good as JDK 20 with the aforementioned flag ). Elasticsearch endeavours to workaround this issue, by 1) reducing the size of the chunks to say 1MB, and 2) reusing the memory backed by the arrays for subsequent chunks. See https://github.com/elastic/elasticsearch/issues/99592 for further details. ( there are many other related issues with Elasticsearch, but this one is the most relevant and coherent ).

13-11-2023

Changeset: 8ff2928a Author: Thomas Schatzl <tschatzl@openjdk.org> Date: 2022-12-14 17:55:15 +0000 URL: https://git.openjdk.org/jdk/commit/8ff2928a04aeec8c09ff4a1ec4e83d4c9010950e

14-12-2022

A pull request was submitted for review. URL: https://git.openjdk.org/jdk/pull/11654 Date: 2022-12-13 16:29:59 +0000

13-12-2022

Relates :	JDK-8304802 - After JDK-8297639 the flag G1UsePreventiveGC needs to be added to the obsoletion table
Relates :	JDK-8288484 - Unexpected marking cycles and full gcs in SPECjbb2015
Relates :	JDK-8293861 - G1: Disable preventive GCs by default
Relates :	JDK-8290738 - Improve predictions for preventive GC
Relates :	JDK-8272980 - Only trigger preventive collections when there is reason that it will help