RFR: 8338912: CDS: Segmented roots array

[Aleksey Shipilev]

On Thu, 5 Sep 2024 05:03:28 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Attempt to drop the min region alignment with [JDK-8337828](https://bugs.openjdk.org/browse/JDK-8337828) highlights an interesting trouble. The roots array we are creating during the dump time can easily be larger than the min region alignment. We are currently "lucky" none of our tests hit this limit. AFAICS, about 128K classes would be enough to hit the current 1M min region alignment. Dropping the min region alignment to 256K starts to fail the test with "only" 30K classes, [JDK-8338856](https://bugs.openjdk.org/browse/JDK-8338856).
>> 
>> We can slice that heap root array, and thus untie the roots count from the min region alignment. I am submitting something that works, but this might not be the final form for it. I would like @iklam to poke holes in this approach :)
>> 
>> Additional testing:
>>  - [x] macos-aarch64-server-fastdebug, `runtime/cds`
>>  - [ ] linux-aarch64-server-fastdebug, `all`
>>  - [ ] linux-x86_64-server-fastdebug, `all`
>
> src/hotspot/share/cds/archiveHeapWriter.cpp line 212:
> 
>> 210:   int segment_beg = 0;
>> 211:   while (segment_beg < length) {
>> 212:     // Guess the size of next segment.
> 
> nit: it's not a guess when you can't be wrong. I think this comment is not needed.
> 
> Also, can you use `HeapRoots::length_for_segment()` here?

Yeah. I'll do these in new commit.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20858#discussion_r1747222357