Add documentation for -Xgc suballocator options

[ThanHenderson]

Issue or pull request number:
eclipse-openj9/openj9#19872

Overview:
eclipse-openj9/openj9#19872 add -Xgc options:

• suballocatorIncrementSize
• suballocatorQuickAllocEnable
• suballocatorQuickAllocDisable

We should document:

• suballocatorInitialSize
• suballocatorIncrementSize
• suballocatorCommitSize
• suballocatorQuickAllocEnable
• suballocatorQuickAllocDisable

Release target:
Eclipse OpenJ9 0.49.0

Applies to the following JDK versions:
All

Applies to the following platforms:
All

For new command line options:

• Option name and syntax: -Xgc:suballocatorInitialSize=<bytes>
• If values are set:
  • Units for setting: bytes
  • Range: Positive non-zero integer
  • Default value: 200 MB
• Purpose: Sets the initial size of the heap used for compressed references.

---

• Option name and syntax: -Xgc:suballocatorIncrementSize=<bytes>
• If values are set:
  • Units for setting: bytes
  • Range: Positive non-zero integer
  • Default value: 8 MB for all platforms except AIX (256 MB)
• Purpose: Sets the reservation increment size of the heap used for compressed references.

---

• Option name and syntax: -Xgc:suballocatorCommitSize=<bytes>
• If values are set:
  • Units for setting: bytes
  • Range: Positive non-zero integer
  • Default value: 50 MB
• Purpose: Sets the commit size of the heap used for compressed references.

---

• Option name and syntax: -Xgc:suballocatorQuickAllocEnable
• Purpose: Enables mmap-based allocation for compressed references (affects Linux only).

---

• Option name and syntax: -Xgc:suballocatorQuickAllocDisable
• Purpose: Disables mmap-based allocation for compressed references (affects Linux only).

[ThanHenderson]

PR: #1417

[Sreekala-Gopakumar]

Option name and syntax: -Xgc:suballocatorQuickAllocDisable
Purpose: Disables mmap-based allocation for compressed references (affects Linux only).

@ThanHenderson - Is there any similar option that enables or disables mmap-based allocation in other platforms? Is mmap-based allocation applicable only for Linux? Thanks!

[ThanHenderson]

Is there any similar option that enables or disables mmap-based allocation in other platforms?

The options are generic in that they can be passed in on other platforms, but the mmap-based allocation is only applicable on Linux and thus will only have an affect on Linux builds. On non-Linux builds it is effectively a no-op.