RFR: 8338440: Parallel: Improve fragmentation mitigation in Full GC

[Ivan Walulya]

On Thu, 15 Aug 2024 06:59:35 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Extend `SplitInfo` to support more fine-grained splitting to mitigate the fragmentation issue during full GC. Added comments and diagrams in the process.
> 
> For easier review, it's best to start with `SplitInfo` and then proceed to see how it is constructed in `summarize_split_space` and consumed in `first_src_addr`. The accompanying diagrams should help create a clear mental image.
> 
> Test: tier1-8, systemgc micro bm, CacheStress, dacapo, specjbb2005, specjvm2008

"The effect of this fragmentation can be observed using `runtime/ClassInitErrors/TestOutOfMemoryDuringInit.java`. The Parallel collector takes significantly longer than other collectors, around 30 seconds compared to about 8 seconds. By adding `-Xlog:gc`, one can see that the Parallel collector runs approximately 47 full GCs, whereas others run around 12."

Any details on the improvements observed with this patch for this test?

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PR Comment: https://git.openjdk.org/jdk/pull/20590#issuecomment-2304491910