RFR: JDK-8313782: Add user-facing warning if THPs are enabled but cannot be used

[Thomas Stuefe]

On Fri, 4 Aug 2023 14:27:39 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> We don't warn the user if `+UseTransparentHugePages ` were specified (and nothing else, but this implies `+UseLargePages`), but the system does not support THPs.
> 
> We used to have warnings in the code, but I checked and they never really worked (I tried older versions of the JDK). The reason is that THP detection had been broken. The broken detection always reported success even if THPs are disabled, so the user would never have seen this warning even when we had it. 
> 
> THP detection has since been fixed with  [JDK-8310233](https://bugs.openjdk.org/browse/JDK-8310233), but we still don't warn.
> 
> The patch re-introduces the warning, with the same text as it used to have, and using the warning level of UL with the "pagesize" tag - that's what we do for static hugepage warnings too.
> 
> Before:
> 
> thomas at starfish$ ./images/jdk/bin/java -XX:+UseTransparentHugePages -version
> openjdk version "20-ea" 2023-03-21
> OpenJDK Runtime Environment SapMachine (build 20-ea+32)
> OpenJDK 64-Bit Server VM SapMachine (build 20-ea+32, mixed mode, sharing)
> 
> 
> Now:
> 
> thomas at starfish$ ./images/jdk/bin/java -XX:+UseTransparentHugePages -version
> [0.001s][warning][pagesize] TransparentHugePages is not supported by the operating system.
> openjdk version "22-internal" 2024-03-19
> OpenJDK Runtime Environment (fastdebug build 22-internal-adhoc.thomas.source)
> OpenJDK 64-Bit Server VM (fastdebug build 22-internal-adhoc.thomas.source, mixed mode, sharing)

src/hotspot/os/linux/os_linux.cpp line 3805:

> 3803:   }
> 3804:   if (!UseTransparentHugePages && HugePages::supports_static_hugepages() == false) {
> 3805:     warn_no_large_pages_configured();

Reviewer note: `warn_no_large_pages_configured()` is the warning for *static* hugepage errors, so of no use for THPs.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15158#discussion_r1284501416