[RFC] Add GNU_PROPERTY_1_NEEDED_THP_ON_[READ_ONLY_SEGMENTS|MALLOC]

[H.J. Lu]

On Fri, Apr 17, 2026 at 9:36 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 17.04.2026 10:42, H.J. Lu 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.
>
> Why would one (the main) executable control behavior for all involved
> shared libraries? Shouldn't each binary get its own say?

Normally library developers don't know if THP usage in the library will
improve or hurt application performance.

> > 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.
>
> Things are yet more fuzzy here, as you can't really control this by
> origin of a call to an allocation function.
>

This will be checked by malloc in glibc to decide if madvise should be
called.

-- 
H.J.