RFR: 8243315: ParallelScavengeHeap::initialize() passes GenAlignment as page size to os::trace_page_sizes instead of actual page size

[Thomas Stuefe]

On Wed, 25 Nov 2020 15:22:27 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> Sorry was a bit unclear, I know about this case but I was more thinking do we need to support it. Could we just always align correctly.
>
> The model would be so much nicer if a `ReservedSpace` always just had one page-size.

Yes, but I think it makes sense to allow the os layer to mix page sizes for large paged areas, for performance reasons. The fact that this coding exists, and that Intel wants to further complicate it and add 2M pages, means we have a need here. 

Trying to avoid this just means that people add patches sideways to satisfy specific needs, which hurts maintainability. Also, I don't like to discourage first time contributors with lots of concerns, therefore I'd like a cleaner, more flexible os layer.

But no-one forces you to accept multi-page-sized-areas. If you really want just one page size, you can query the largest page size available beforehand and align the reservation size accordingly, and with my proposed change you could now assert the result and log it correctly.

But if one just generally wants large pages without caring for the precise layout, one could let os::reserve_memory_special() do its best, and would now get the information about what reserve_memory_special() did. 

For example, were I to re-introduce large pages for Metaspace, I would like to have the luxury of just calling os::reserve_memory_special() without having to think about specific geometry - if the space is large enough for large pages, it should stitch the area together as best as it can.

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PR: https://git.openjdk.java.net/jdk/pull/1161