RFR: 8320710: Adjust heap size when close to compressed oops limit

[Oli Gillespie]

On Mon, 27 Nov 2023 05:05:59 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> When the heap is between 0 and 2% larger than the compressed oops limit, lower it enough to enable compressed oops. See [issue](https://bugs.openjdk.org/browse/JDK-8320710) for more details.
>> 
>> This is just one way of doing it to show the idea, any other ideas welcome.
>> 
>> Before:
>> 
>> java -Xmx32G -XX:+UseCompressedOops -version
>> OpenJDK 64-Bit Server VM warning: Max heap size too large for Compressed Oops
>> 
>> After:
>> 
>> java -Xmx32G -XX:+UseCompressedOops -version
>> OpenJDK 64-Bit Server VM warning: Heap size lowered from 34359738368 to 33822867456 to accommodate Compressed Oops
>
> src/hotspot/share/runtime/arguments.cpp line 1484:
> 
>> 1482: 
>> 1483:   double ratio = max_heap_size / (double) max_heap_for_compressed_oops();
>> 1484:   if (ratio > 1 && ratio < 1.02) {
> 
> Shouldn't this only be done if `UseCompressedOops` has not been explicitly disabled?

Good point, thanks.

> src/hotspot/share/runtime/arguments.cpp line 1488:
> 
>> 1486:     // would benefit from a small reduction to allow enabling compressed oops. This is easily done by
>> 1487:     // accident, for example setting -Xmx32G is a tiny amount over the limit.
>> 1488:     warning("Heap size lowered from %lu to %lu to accommodate Compressed Oops", max_heap_size, max_heap_for_compressed_oops());
> 
> I think it would be much clearer to show the old/new sizes in a human friendly format rather than bytes e.g. `from 32G to 31.8G`

Good idea :)

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PR Review Comment: https://git.openjdk.org/jdk/pull/16813#discussion_r1405940935
PR Review Comment: https://git.openjdk.org/jdk/pull/16813#discussion_r1405941337