Ensure System.gc() triggers a GC and runs finalization

#1161

Updated the Garbage collection topic with the new behavior of the System.gc call. incorporated the feedback.

Closes #1161
Signed-off-by: Sreekala Gopakumar <[email protected]>

docs/gc_overview.md

@@ -34,6 +34,8 @@ A global GC cycle can be triggered explicitly or implicitly according to the fol
 - A global GC cycle is triggered implicitly if it occurs because of internal mechanisms, such as an allocation failure or a *taxation* threshold being reached.
 - A global GC cycle is triggered explicitly if it is started directly by an application calling `System.gc()` or indirectly, for example when requesting a heap dump.
 
+ The `System.gc()` call triggers the GC cycle twice internally to clear unreachable objects that were not identified during the first GC cycle. This call also triggers finalization to release resources that were used by the unreachable objects so that the resources can be reused.
+
 Partial GC cycles are triggered only implicitly under the control of a particular GC policy. For more information about the GC policies available with Eclipse OpenJ9™, see [Garbage collection policies](gc.md).
 
 The GC process is designed to operate without intervention from an application. Developers should not attempt to predict GC behavior, for example, by making calls to `System.gc()` to trigger a cycle or by using finalizers to clean up objects in memory. Such actions might degrade the performance of an application.

docs/version0.42.md

@@ -27,6 +27,7 @@ The following new features and notable changes since version 0.41.0 are included
 
 - [New binaries and changes to supported environments](#binaries-and-supported-environments)
 - ![Start of content that applies to Java 21 (LTS) and later](cr/java21plus.png) [OpenJ9 `jextract` removed](#openj9-jextract-removed) ![End of content that applies to Java 21 (LTS) and later](cr/java_close_lts.png)
+- [Change in the `System.gc()` call behavior](#change-in-the-systemgc-call-behavior)
 
 ## Features and changes
 
@@ -42,6 +43,14 @@ To learn more about support for OpenJ9 releases, including OpenJDK levels and pl
 
 The dump extractor tool, OpenJ9 `jextract`, that was deprecated since the [0.26.0 release](version0.26.md) is now removed from Java 21 and later. The [`jpackcore`](tool_jextract.md) tool replaced the OpenJ9 `jextract` tool after its deprecation. ![End of content that applies to Java 21 (LTS) and later](cr/java_close_lts.png)
 
+### Change in the `System.gc()` call behavior
+
+The `System.gc` call triggers a global garbage collection (GC) cycle, which is a stop-the-world GC. If the `System.gc()` call was made during an active GC cycle where the concurrent operation was in progress, the status of objects might not be updated. An object that was reachable when the concurrent operation started might be considered as reachable even if the object is no longer in use later. The `System.gc()` call had to be called twice explicitly to ensure that all unreachable objects are identified and cleared.
+
+Now, the `System.gc()` call triggers the GC cycle twice internally to clear the unreachable objects that were not identified during the first GC cycle. The call also triggers finalization of the objects in the Finalization queues.
+
+For more information, see [Garbage collection](gc_overview.md).
+
 ## Known problems and full release information
 
 To see known problems and a complete list of changes between Eclipse OpenJ9 v0.41.0 and v0.42.0 releases, see the [Release notes](https://github.com/eclipse-openj9/openj9/blob/master/doc/release-notes/0.42/0.42.md).