RFR: 8367993: G1: Speed up ConcurrentMark initialization [v8]

[Stefan Johansson]

On Tue, 27 Jan 2026 18:42:51 GMT, Leo Korinth <lkorinth at openjdk.org> wrote:

>> This change moves almost all of the ConcurrentMark initialisation from its constructor to the method `G1ConcurrentMark::fully_initialize()`.  Thus, creation time of the VM can be slightly improved by postponing creation of ConcurrentMark. Most time is saved postponing creation of statistics buffers and threads.
>> 
>> It is not obvious that this is the best solution. I have earlier experimented with lazily allocating statistics buffers _only_. One could also initialise a little bit more eagerly (for example the concurrent mark thread) and maybe get a slightly cleaner change. However IMO it seems better to not have ConcurrentMark "half initiated" with a created mark thread, but un-initialised worker threads.
>> 
>> This change is depending on the integration of https://bugs.openjdk.org/browse/JDK-8373253.
>> 
>> I will be out for vacation, and will be back after new year (and will not answer questions during that time), but I thought I get the pull request out now so that you can have a look.
>
> Leo Korinth has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 21 commits:
> 
>  - Revert "Stefan J 4"
>    
>    This reverts commit 203d11979524126add9ee5d04174bde07a5a3f5a.
>  - Revert "remove commented out code"
>    
>    This reverts commit 9d9df671574be7a15a30ef0af452629330cb4fd1.
>  - Merge branch 'master' into _8367993
>  - Merge branch '_master_jdk' into _8367993
>  - Merge branch '_8373253' into _8367993
>  - rename of method
>  - Proposal by Stefan J
>  - wip
>  - Revert "8373253: Re-work InjectGCWorkerCreationFailure for future changes"
>    
>    This reverts commit d45ea8817ab2303b2decd8cbb2cd1bf5280aa181.
>  - Revert "Fixup after comment from Ivan."
>    
>    This reverts commit 2aa8aa4b68027b62a8d4be1b86720fadfa48dda5.
>  - ... and 11 more: https://git.openjdk.org/jdk/compare/90b54692...c5a7e2bb

Changes requested by sjohanss (Reviewer).

src/hotspot/share/gc/g1/g1YoungCollector.cpp line 1128:

> 1126: 
> 1127: void G1YoungCollector::collect() {
> 1128:   _g1h->_cm->fully_initialize();

Why did you revert the call to this location? Instead of having it in G1CollectedHeap::do_collection_pause_at_safepoint()?

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PR Review: https://git.openjdk.org/jdk/pull/28723#pullrequestreview-3716074772
PR Review Comment: https://git.openjdk.org/jdk/pull/28723#discussion_r2736113065