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

[David Holmes]

On Fri, 24 Nov 2023 17:31:32 GMT, Oli Gillespie <ogillespie 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

Not really a review but a couple of passing comments.

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?

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`

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PR Review: https://git.openjdk.org/jdk/pull/16813#pullrequestreview-1749611285
PR Review Comment: https://git.openjdk.org/jdk/pull/16813#discussion_r1405621428
PR Review Comment: https://git.openjdk.org/jdk/pull/16813#discussion_r1405621163