RFR: 8337828: CDS: Trim down minimum GC region alignment [v2]

[Aleksey Shipilev]

On Thu, 15 Aug 2024 21:34:19 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> CDS currently follows G1's minimum region size to guess which alignment to use when dumping the heap. The comment near the constant rightfully recognizes it would be convenient for Shenandoah to trim the alignment down to 256K (Shenandoah's min region size). If we do this, we will improve the heap sizes [JDK-8293650](https://bugs.openjdk.org/browse/JDK-8293650) can operate at.
>> 
>> Unless I am missing something else, trimming down the min region alignment has impact on the size of the objects we can store in CDS archive. Conveniently, `-Xlog:cds+heap` prints the object size stats for us, and it looks we are way under the 256K limit:
>> 
>> 
>> $ build/macosx-aarch64-server-fastdebug/images/jdk/bin/java -XX:-UseCompressedOops -Xshare:dump -Xlog:cds+heap
>> ...
>> [0.921s][info][cds,heap]        0 objects are <= 8      bytes (total        0 bytes, avg      0.0 bytes)
>> [0.921s][info][cds,heap]     2550 objects are <= 16     bytes (total    40800 bytes, avg     16.0 bytes)
>> [0.921s][info][cds,heap]    14325 objects are <= 32     bytes (total   431896 bytes, avg     30.1 bytes)
>> [0.921s][info][cds,heap]     6572 objects are <= 64     bytes (total   301304 bytes, avg     45.8 bytes)
>> [0.921s][info][cds,heap]     1225 objects are <= 128    bytes (total   113112 bytes, avg     92.3 bytes)
>> [0.921s][info][cds,heap]     2173 objects are <= 256    bytes (total   384024 bytes, avg    176.7 bytes)
>> [0.921s][info][cds,heap]      143 objects are <= 512    bytes (total    47720 bytes, avg    333.7 bytes)
>> [0.921s][info][cds,heap]       40 objects are <= 1024   bytes (total    26872 bytes, avg    671.8 bytes)
>> [0.921s][info][cds,heap]       19 objects are <= 2048   bytes (total    29656 bytes, avg   1560.8 bytes)
>> [0.921s][info][cds,heap]        9 objects are <= 4096   bytes (total    20744 bytes, avg   2304.9 bytes)
>> [0.921s][info][cds,heap]        4 objects are <= 8192   bytes (total    20536 bytes, avg   5134.0 bytes)
>> [0.921s][info][cds,heap]        3 objects are <= 16384  bytes (total    30168 bytes, avg  10056.0 bytes)
>> [0.921s][info][cds,heap]        2 objects are <= 32768  bytes (total    32800 bytes, avg  16400.0 bytes)
>> [0.921s][info][cds,heap]        0 objects are <= 65536  bytes (total        0 bytes, avg      0.0 bytes)
>> [0.921s][info][cds,heap]        1 objects are <= 131072 bytes (total    66848 bytes, avg  66848.0 bytes)
>> [0.921s][info][cds,heap]        0 objects are <= 262144 bytes (total        0 bytes, avg      0.0 bytes)
>> [0.921s][info][cds...
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:
> 
>  - Merge branch 'master' into JDK-8337828-cds-min-alignment
>  - Work

Need a re-review after merge :) Thanks.

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PR Comment: https://git.openjdk.org/jdk/pull/20469#issuecomment-2296075672