RFR: 8369111: G1: Determining concurrent start uses inconsistent predicates [v2]

[Thomas Schatzl]

> Hi all,
> 
>   please review this change that fixes an inconsistency between requesting a concurrent start garbage collection during humongous object allocation and then actually starting it.
> 
> I.e. in `G1CollectedHeap::attempt_allocation_humongous` we check whether the allocation would cross the IHOP threshold taking the current allocation into account, and if so, see if G1 should start a concurrent marking, eventually starting a GC pause.
> 
> That GC pause did not take the prospective allocation into account, so we could do that GC for nothing (i.e. not start a concurrent marking although we already knew that the allocation would cause one).
> 
> This, in conjunction with JDK-8368959 can cause hundreds of extra GCs for the test in the CR (without eager reclaim of humongous arrays with references); otherwise it could cause the marking starting too late.
> 
> There is a second bug in the calculation whether G1 crossed the threshold: for humongous objects it only takes the actual size into account, not the size that is needed for allocating it. The same issue existed for determining to start a concurrent mark after any other collection too.
> 
> The change also tries to unify naming of the parameter to pass the allocation size (`alloc_word_size` -> `allocation_word_size`) and the parameter order where this size is passed along in multiple related methods.
> 
> Testing: mentioned test case now behaving correctly, tier1-5
> 
> Thanks,
>   Thomas

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  * walulyai review

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27789/files
  - new: https://git.openjdk.org/jdk/pull/27789/files/c8aff5cb..b42f9b20

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27789&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27789&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27789.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27789/head:pull/27789

PR: https://git.openjdk.org/jdk/pull/27789