RFR: JDK-8312492: Remove THP sanity checks at VM startup [v3]

[Thomas Stuefe]

On Fri, 28 Jul 2023 20:03:04 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> When starting the JVM with -XX:+UseTransparentHugePages, we need to know if the OS supports THPs:
>> 
>> A) We could run on a super-old kernel (very unlikely, THPs are supported since 2.6.38 which is EOL since 2011)
>> B) We could run on a kernel built without THP support, e.g. an embedded device
>> C) We could run on a system with THPs disabled by the admin
>> 
>> The JVM does a little sanity test at startup: it maps an area the size of a huge page - but in small pages - then calls madvise(MADV_HUGEPAGE) on it. The JVM assumes if the madvise() worked, all is fine.
>> 
>> That sanity test may be expensive since we allocate (reserve AND commit) an area, then signal khugepaged to fold it into a huge page. That area could be large once the kernel supports 1 GB THP pages. We may end up committing 1 GB space at startup. In addition to that, depending on the THP defrag settings, the allocation may stall. Even if it does not stall it creates unnecessary work for both the JVM and khugepaged.
>> 
>> The sanity test is also not really useful since it will happily report success on systems with THPs disabled (case (C)). Arguably, that is the majority of cases on existing Linux installations.
>> 
>> The correct way is to check the proc fs whether THPs are enabled. But we already do that since [JDK-8310233](https://bugs.openjdk.org/browse/JDK-8310233): "Fix THP detection on Linux". And that test detects all cases (A)(B)(C). Therefore we can completely remove the sanity checks for THPs.
>> 
>> I tested the patch manually in a number of settings with THPs enabled, disabled, and not compiled into the kernel at all. There is no behavioral difference because since [JDK-8310233](https://bugs.openjdk.org/browse/JDK-8310233) we catch all relevant cases before ever entering this sanity test.
>> 
>> [1] https://github.com/openjdk/jdk/blob/37ca9024ef59d99cae0bd7e25b2e6d3c1e085f97/src/hotspot/os/linux/os_linux.cpp#L3762
>
> Thomas Stuefe has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8312492-Remove-THP-sanity-checks-at-VM-startup
>  - Fix regression in tests
>  - remove-thp-sanity-tests

Friendly ping. Need a second review.

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PR Comment: https://git.openjdk.org/jdk/pull/14967#issuecomment-1656565993