RFR: 8268388: Update large pages information in Java manpage

[Stefan Johansson]

On Wed, 9 Jun 2021 09:46:17 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>     * vm.nr_hugepages is not the only way to do this setup, since we have vm.nr_overcommit_hugepages, which are a bit more flexible to use. Is the java man page supposed to be complete in explaining this, or should it only show one possible way of many?
> 
I think mentioning one way to configure it is fine, we don't want to try to cover every possibility. I mean there are multiple ways of specifying each of the options as well. Most advanced users will likely refer to the kernel documentation for more details. I see this as a short explanation of one way to enable large pages.

>     * most users probably just use UseLargePages and are not aware of the different flavors. So they would not know what "explicit" means. How about, instead of mentioning UseSHM and UseHugeTLBFS, not just: "if you use large pages but don't use transparent huge pages (UseTransparentHugePages), large pages need to be preallocated...".
> 
Something like: 

- When using explicit large pages (options `-XX:+UseSHM` or `-XX:+UseHugeTLBFS`), the number of...
+ When using large pages and not enabling transparent huge pages (option `-XX:+UseTransparentHugePages`), the number of...

>     * Otherwise, at least reverse the order of the two options? UseSHM is mentioned first, which is maybe historical, but TLBFS is the standard.
Good point, I reversed the order of configuring the tow but forgot it here. But going with the above is better I thing.

-------------

PR: https://git.openjdk.java.net/jdk/pull/4425