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

[Stefan Johansson]

On Wed, 25 Nov 2020 15:10:56 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>>> So Thomas in what cases do we need the `ReservedSpace` to be able to handle multiple page-sizes? A simpler approach would be to aim for only having one page size per `ReservedSpace`. For example, in G1 it would be problematic or at least inefficient to have multiple page sizes per mapping.
>> 
>> Happens if you reserve a space with a size not aligned to the underlying large page size. See reserve_memory_special_huge_tlbfs_mixed() on Linux. In that case, we try to be smart and fill in the ends with small pages rather than failing.
>
> Of which the Intel approach with the 2M pages could be seen as an expansion (e.g. if you have a heap of 1.5G, allocate 1G huge page, and fill in the ends with 2M pages, and if necessary with 4K pages).

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.

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