8368954: G1: Document why G1 uses TLS storage for the current card table reference

[tschatzl]

Hi all,

please review some documentation update why G1 now uses TLS storage to get the current card table base value for the card mark.

TLDR: it's the overall fastest currently available way.

Testing: local compilation, this is just a trivial documentation change.

Thanks,
Thomas

---

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Issue

• JDK-8368954: G1: Document why G1 uses TLS storage for the current card table reference (Enhancement - P4)

Reviewers

• Roberto Castañeda Lozano (@robcasloz - Reviewer) Review applies to bf0f553e
• Ivan Walulya (@walulyai - Reviewer) Review applies to bf0f553e
• Albert Mingkun Yang (@albertnetymk - Reviewer)

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[openjdk]

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8368954: G1: Document why G1 uses TLS storage for the current card table reference

Reviewed-by: ayang, rcastanedalo, iwalulya

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At the time when this comment was updated there had been 8 new commits pushed to the master branch:

• fe9dbcc: 8368599: ShenandoahConcurrentMark could use ThreadsClaimTokenScope
• 07ecc93: 8367692: RISC-V: Align post call nop
• 6b4b102: 8368809: JFR: Remove events from testSettingConfiguration in TestActiveSettingEvent
• ... and 5 more: https://git.openjdk.org/jdk/compare/ba0a6eed1a6a22bd4c1d159592b62e054afa401a...master

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Webrevs

• 01: Full - Incremental (bfc4d1e9)
• 00: Full (bf0f553e)

[albertnetymk]

Not sure if I misunderstood this text or not, but Serial/Parallel doesn't embed the "address" into the code stream -- they embed the actual value in the code stream, since the value (card-tabe-base) is constant. G1 can't/doesn't do that, since card-table-base is not constant.

[tschatzl]

But G1 can embed the address of the value in the code stream, similar to Serial/Parallel (which embed the actual base address). The emphasis should be on the embedding, not what is embedded.

[tschatzl]

maybe just remove the "an address" to avoid the confusion?

[albertnetymk]

It's my misunderstanding then. How about just dropping "similar to Serial/Parallel"? I don't see any benefit of referencing them here.

[albertnetymk]

maybe just remove the "an address" to avoid the confusion?

I think you need the "address" to connect with the latter "loading from that" -- "that" refers to the "address", doesn't it?

[tschatzl]

Thanks @robcasloz @walulyai @albertnetymk for your reviews
/integrate

[openjdk]

Going to push as commit 1188ca5.
Since your change was applied there have been 13 commits pushed to the master branch:

• 394eb80: 8368957: Remove metaprogramming/logical.hpp in favor of C++17 facilities
• 8c3ca02: 8368817: Convert JDK_Version::to_string to use stringStream instead of jio_snprintf-chain
• 17d8fa8: 8367026: Reorder the timeout failure handler commands to have jstack run before the rest
• ... and 10 more: https://git.openjdk.org/jdk/compare/ba0a6eed1a6a22bd4c1d159592b62e054afa401a...master

Your commit was automatically rebased without conflicts.

[openjdk]

@tschatzl Pushed as commit 1188ca5.

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