RFR: 8319969: os::large_page_init() turns off THPs for ZGC

[Stefan Karlsson]

On Thu, 16 Nov 2023 13:30:48 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> There is code in `os::large_page_init()` that checks `/sys/kernel/mm/transparent_hugepage/enabled` and forcefully turns off `UseTransparentHugePages` if anonymous THPs are disabled in the OS:
> 
> 
>   if (UseTransparentHugePages && !HugePages::supports_thp()) {
>     if (!FLAG_IS_DEFAULT(UseTransparentHugePages)) {
>       log_warning(pagesize)("UseTransparentHugePages disabled, transparent huge pages are not supported by the operating system.");
>     }
>     UseLargePages = UseTransparentHugePages = false;
>     return;
>   }
> 
> 
> This is problematic because ZGC doesn't use the `/sys/kernel/mm/transparent_hugepage/enabled` THPs, but instead the `/sys/kernel/mm/transparent_hugepage/shmem_enabled` THPs. So, with the following settings:
> 
> /sys/kernel/mm/transparent_hugepage/enabled: never
> /sys/kernel/mm/transparent_hugepage/shmem_enabled: advise
> 
> 
> the above code will force ZGC to run without THPs.
> 
> This PR is a proposal for how to work around this in the ZGC code without disturbing the the rest of the JVM too much. The patch:
> 
> 1) remembers the initial values for UseLargePages and UseTransparentHugePages and saves those so that ZGC can continue using THPs even though they have been disabled for the rest of the JVM.
> 
> 2) adds better logic to figure out if ZGC is actually going to get THPs for the heap or not. This is then used to more accurately log the current situation and allows for a precise usage of `madvise + MADV_HUGEPAGE`.
> 
> 3) tweaks the generic pagesize logging to better reflect the situation when anonymous THPs are disabled but shared memory THPs are enabled and ZGC is used.
> 
> The result of this change can be seen in these tables:
> 
> ZGC large pages log output:
> 
> E (T)     = Enabled (Transparent)
> E (T, OS) = Enabled (Transparent, OS enforced)
> D         = Disabled
> D         = Disabled (OS enforced)
> 
> -XX:+UseTransparentHugePages
> 
> shem \ anon | always | madvise | never
> ------------+--------+---------+-------
> always      | E (T)  | E (T)   | E (T)
> within_size | E (T)  | E (T)   | E (T)
> advise      | E (T)  | E (T)   | E (T)
> never       | D (OS) | D (OS)  | D (OS)
> deny        | D (OS) | D (OS)  | D (OS)
> force       | E (T)  | E (T)   | E (T)
> 
> -XX:-UseTransparentHugePages
> 
> shem \ anon | always    | madvise   | never
> ------------+-----------+-----------+-------
> always      | E (T, OS) | E (T, OS) | E (T, OS)
> within_size | E (T, OS) | E (T, OS) | E (T, OS)
> advise      | D         | D         | D
> never       | D         | D         | D
> deny        | D         | D         | D
> force   ...

src/hotspot/os/linux/os_linux.cpp line 2886:

> 2884: 
> 2885: void os::pd_realign_memory(char *addr, size_t bytes, size_t alignment_hint) {
> 2886:   if (HugePages::should_madvise_anonymous_thps() && alignment_hint > vm_page_size()) {

The use of `HugePages::should_madvise_anonymous_thps()` adds a change in behavior. By using it instead of `UseTransparentHugepages`, we only call `madvise` when the OS is configured to care about `madvise`. I've been using this in my testing, but I can revert back to using `UseTransparentHugepages`, and then we can change this separately with [JDK-8312468](https://bugs.openjdk.org/browse/JDK-8312468).

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PR Review Comment: https://git.openjdk.org/jdk/pull/16690#discussion_r1411859149