[RFC] Add GNU_PROPERTY_1_NEEDED_THP_ON_[READ_ONLY_SEGMENTS|MALLOC]

[H.J. Lu]

On Mon, Apr 20, 2026 at 1:56 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 18.04.2026 01:24, H.J. Lu wrote:
> > 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.
>
> "Normally" as in what exactly? An entirely private shared library may be
> used by both an application wanting THP and one that doesn't. It
> containing, say, huge tables of data, it may very well benefit from THP
> itself, irrespective of the application's choice.

Application developers should evaluate the overall impact of THP on the
application to decide if THP should be used for it.

> >>> 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.
>
> "This" being what here? The new property? If so, then yes, sure, but how
> does that relate to the point I made?
>

The malloc implementation in glibc checks the
GNU_PROPERTY_1_NEEDED_THP_ON_MALLOC bit
on the application.  The malloc implementation will use
madvise if the bit is set.

-- 
H.J.