8351011: Serial GC: Use a single VirtualSpace for both young and tenured generations

[swesonga]

JEP draft JDK-8350152 (Automatic Heap Sizing for the Serial Garbage Collector) proposes dynamically sizing the young generation using GC overhead and adding support for committing and uncommitting memory depending on global memory pressure. The current implementation of the serial collector reserves a single block of memory for the entire heap then splits it into two ReservedSpaces, one for the young generation and another for the tenured generation. Each of the generations then individually commits and uncommits memory.

One drawback of this approach is that with dynamic sizing of the generations, fixed limits of the reserved spaces for each generation will not be compatible with the newly computed region sizes. A possible workaround is to reserve twice the maximum heap size to enable each generation to grow to the dynamically computed sizes. However, there would now be two memory regions to manage when committing and uncommitting memory.

This PR adds an experimental flag, -XX:+SharedSerialGCVirtualSpace, to enable the use of a single reserved space and dynamically partitioning its committed memory into tenured and young regions for the automatic heap sizing scenario.

This PR builds on #23853 by adding to it commit 98f1bfc

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
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Warnings

 ⚠️ Patch contains a binary file (src/java.desktop/share/classes/java/awt/doc-files/BorderLayout-1.gif)
 ⚠️ Patch contains a binary file (src/java.desktop/share/classes/java/awt/doc-files/FlowLayout-1.gif)
 ⚠️ Patch contains a binary file (src/java.desktop/share/classes/java/awt/doc-files/GridBagLayout-1.gif)
 ⚠️ Patch contains a binary file (src/java.desktop/share/classes/java/awt/doc-files/GridBagLayout-2.gif)
 ⚠️ Patch contains a binary file (src/java.desktop/share/classes/java/awt/doc-files/GridLayout-1.gif)
 ⚠️ Patch contains a binary file (src/java.desktop/share/classes/java/awt/doc-files/GridLayout-2.gif)
 ⚠️ Patch contains a binary file (test/hotspot/jtreg/runtime/ClassFile/JsrRewritingTestCase.jar)
 ⚠️ Patch contains a binary file (test/hotspot/jtreg/runtime/ClassFile/testcase.jar)

Issue

• JDK-8351011: Serial GC: Use a single VirtualSpace for both young and tenured generations (Enhancement - P4)

Reviewing

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@swesonga this pull request can not be integrated into master due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout single-serialgc-virtualspace
git fetch https://git.openjdk.org/jdk.git master
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge master"
git push

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