RFR: 8373023: [REDO] Remove the default value of InitialRAMPercentage [v2]

[Stefan Karlsson]

On Wed, 3 Dec 2025 16:30:04 GMT, Joel Sikström <jsikstro at openjdk.org> wrote:

>> Hello,
>> 
>> We backed out this change in https://github.com/openjdk/jdk/pull/28617 after issues in found in testing. Moving forward, we've decided to problem list a few tests that assumes that a GC does not run during their execution. When the initial heap size becomes very low with this change, it may trigger additional GCs, which may interfere with a few tests.
>> 
>> We are planning to follow up on the problem lists with test fixes in:
>> https://github.com/openjdk/jdk/pull/28638, https://github.com/openjdk/jdk/pull/28637
>> 
>> From the original RFE: 
>> 
>> This RFE changes the default of `InitialRAMPercentage` to 0, effectively removing its default value. Please see the CSR for specific details on this change.
>> 
>> Changing the default value to 0 results in the behavior that the initial heap size (InitialHeapSize) is set to the minimum heap size (MinHeapSize). This is because of the following lines in `Arguments::set_heap_size()`, which takes the MAX value of MinHeapSize as well:
>> 
>> uint64_t initial_memory = (uint64_t)(((double)MaxRAM * InitialRAMPercentage) / 100);
>> ...
>> 
>> reasonable_initial = MAX3(reasonable_initial, reasonable_minimum, MinHeapSize);
>> reasonable_initial = MIN2(reasonable_initial, MaxHeapSize);
>> 
>> 
>> This change improves startup performance for all GCs, but affects the time-to-peak performance in some out-of-the-box configurations for some GCs. This is mainly visible in ParallelGC.
>> 
>> Testing:
>> * Oracle's tier1-3
>
> Joel Sikström has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:
> 
>  - Remove unnecessary line
>  - 8373023: [REDO] Remove the default value of InitialRAMPercentage

Marked as reviewed by stefank (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/28641#pullrequestreview-3535924296