RFR: 8355681: G1HeapRegionManager::find_contiguous_allow_expand ignores free regions when checking regions available for allocation

[Thomas Schatzl]

On Wed, 30 Apr 2025 10:33:10 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Hi,
>> 
>> Please review this change to account for free regions when checking if we have enough regions to satisfy an allocation request. Currently, we have that a `_hrm.expand_and_allocate_humongous` call fails where an `expand_and_allocate` call succeeds for the same allocation request. 
>> 
>> Testing: Tier 1-3
>
> src/hotspot/share/gc/g1/g1CollectedHeap.cpp line 909:
> 
>> 907:     // For humongous objects, we should have expanded the heap on the first
>> 908:     // attempt_allocation_at_safepoint above.
>> 909:     result = expand_and_allocate(word_size);
> 
> Why `attempt_allocation_at_safepoint` performs expansion for humongous objs but not ordinary objs? Since `attempt_allocation_at_safepoint` doesn't contain "expand" in its name, I'd expect it not to perform expansion at all. (If expansion for humongous objs is critical, I wonder if it makes sense to branch on `is_humongous` at the beginning of this method and handle those two cases in two diff paths.)

This also seems pre-existing. Can you file an issue?

> src/hotspot/share/gc/g1/g1HeapRegionManager.cpp line 481:
> 
>> 479: uint G1HeapRegionManager::find_contiguous_allow_expand(uint num_regions) {
>> 480:   // Check if we can actually satisfy the allocation.
>> 481:   if (num_regions > (num_free_regions() + available())) {
> 
> The name "available" is too vague -- without looking at its impl, one could think free regions should be "available" as well. I wonder if sth like "num_uncommitted" is more precise.

This seems to be a pre-existing issue. Filed https://bugs.openjdk.org/browse/JDK-8355976.

Afair this member has always been called that way.

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PR Review Comment: https://git.openjdk.org/jdk/pull/24915#discussion_r2068523076
PR Review Comment: https://git.openjdk.org/jdk/pull/24915#discussion_r2068522375