RFR: JDK-8256155: os::Linux Populate all large_page_sizes, select smallest page size in reserve_memory_special_huge_tlbfs* [v15]

[Stefan Johansson]

On Tue, 19 Jan 2021 07:06:48 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> The "dynamic part" might make this actually being "too much logic", otherwise if feels like a pretty reasonable check. The fact that huge pages can be added during the runtime of the JVM doesn't feel like a big problem, since most large reservations are done at startup. 
>> 
>> But you might be right, since we have to handle the case of a failed mapping, it might not be to big of a problem trying all possible page sizes.
>
> I'm actually using nr_overcommit_hugepages alot (has been a tip by Per Liden) since its so convenient. As for allocation at startup, I plan on making Metaspace large-page-able again at some point in the future; that would mean larger LP allocations may happen later in VM life too.

Ok, maybe I should try it out as well :) 

Regarding allocation at startup vs later, is the plan to make new reservations during the run or supporting uncommit of large pages. Currently if a `ReservedSpace` is special (uses large pages), uncommit is disabled  and all pages are committed up front. Is your plan to change this or will it work by adding and removing `ReservedSpace`s. I have not had time to look at the new `Metaspace` implementation in detail yet.

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PR: https://git.openjdk.java.net/jdk/pull/1153