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 15:09:29 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.
>
>> 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).

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