RFR: 8319795: Static huge pages are not used for CodeCache

[Evgeny Astigeevich]

On Thu, 9 Nov 2023 15:20:26 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

> This is a fix of a regression introduced by [JDK-8261894](https://bugs.openjdk.org/browse/JDK-8261894).
> After JDK-8261894, `os::can_execute_large_page_memory()` returns `true` only if `UseTransparentHugePages` is true. As CodeCache uses `os::can_execute_large_page_memory()` when it detects a page size, CodeCache cannot use static huge pages (`UseTransparentHugePages` is `false`) anymore after the change.
> Before JDK-8261894, `os::can_execute_large_page_memory()` returned `true`  when either `UseTransparentHugePages` or `UseHugeTLBFS` was true.
> 
> After JDK-8261894, `XX:+UseLargePages XX:-UseTransparentHugePages` means to use static huge pages: aka `UseHugeTLBFS` is `true`. If `UseLargePages` is not set to `true` via the option, it will be set to `true` if `UseTransparentHugePages` is `true`.
> 
> `os::can_execute_large_page_memory()` is modified to return `UseLargePages`. A regression gtest  is added.
> 
> Tested fastdebug and release builds:
> - [x] tier1
> - [x] gtest
> - [x] test/hotspot/jtreg/gtest/LargePageGtests.java

Hm...
I looked into `CodeCache::page_size`, its history and uses. The function smells. It has an interesting hack for the large page case:

  if (os::can_execute_large_page_memory()) {
    if (InitialCodeCacheSize < ReservedCodeCacheSize) {
      // Make sure that the page size allows for an incremental commit of the reserved space
      min_pages = MAX2(min_pages, (size_t)8);
    }
``` 

The uses are:
- 2 of `page_size(false, 8)`
- 1 of `page_size(false, 1)`
- 1 of `page_size(true, 1)`

This looks strange to me. I need to if everything is correct.

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

PR Comment: https://git.openjdk.org/jdk/pull/16582#issuecomment-1804853205