[RFC] Add GNU_PROPERTY_1_NEEDED_THP_ON_[READ_ONLY_SEGMENTS|MALLOC]

[WANG Rui]

On Fri, Apr 17, 2026 at 4:43 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> Transparent Huge Pages (THP) has been enabled in glibc to use THP on
> THP eligible read-only segments and malloc to improve application
> performance.  But there are some common issues & trade-offs of THP:
>
> 1. Latency Spikes: The background process (khugepaged) that consolidates
> pages can cause temporary system freezes or high CPU usage.
> 2. Memory Waste: Using a 2MB page for only 4KB of data can lead to
> internal fragmentation and higher memory consumption.
> 3. Database Incompatibility: Many databases (e.g., Oracle, MongoDB,
> SAP ASE) recommend disabling THP because their non-contiguous access
> patterns often trigger performance degradations.
>
> THP shouldn't be always enabled on THP eligible read-only segments nor
> in malloc automatically regardless if THP usage will improve or hurt
> application performance:
>
> https://sourceware.org/bugzilla/show_bug.cgi?id=34081
>
> To enable developers per-application control over THP usage under madvise
> THP mode, add the following bits to GNU_PROPERTY_1_NEEDED:
>
> 1. GNU_PROPERTY_1_NEEDED_THP_ON_READ_ONLY_SEGMENTS
>
>  #define GNU_PROPERTY_1_NEEDED_THP_ON_READ_ONLY_SEGMENTS (1U << 1)
>
> to opt-in THP usage on read-only segments in an application.  When this
> bit is set on executable, run-time loader should call madvise with
> MADV_HUGEPAGE on all Transparent Huge Pages (THP) eligible read-only
> segments in executable and its loaded shared libraries after they are
> mapped into memory under madvise THP mode.
>
> 2. GNU_PROPERTY_1_NEEDED_THP_ON_MALLOC
>
>  #define GNU_PROPERTY_1_NEEDED_THP_ON_MALLOC (1U << 2)
>
> to opt-in THP usage in malloc in an application.  When this bit is set on
> executable, malloc should call madvise with MADV_HUGEPAGE if the
> allocated size is greater than or equal to THP page size after calling
> mmap to allocate memory under madvise THP mode.
>
> Linker will set these bits in application if these bits are set in any
> relocatable inputs.

Linux also provides another per-process control for THP [1]. But
relying on an ELF property, or on the application explicitly opting
in, does not help existing binaries unless they can be rebuilt or
modified.

glibc tunables are more flexible here. The same program may prefer THP
in performance-sensitive cases, but want it disabled under memory
pressure.

Also, with mTHP support, lack of benefit from PMD-sized THP for a
workload does not imply that other THP sizes are also unhelpful. In
that sense, a static opt-in flag seems too limiting?

[1] https://docs.kernel.org/admin-guide/mm/transhuge.html#process-thp-controls

Thanks,
Rui

>
>
> --
> H.J.