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malloc: Improve documentation of malloc tunables

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Update default for tcache_count tunable.  Remove existing documentation and
mention removal of fastbins in mxfast tunable.  Improve wording of hugetlb
tunable, including default for AArch64.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
This commit is contained in:
Wilco Dijkstra
2026-06-03 13:10:58 +00:00
parent 4a7805d05c
commit b7e256c32c
1 changed files with 6 additions and 14 deletions
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manual/tunables.texi
+6 -14
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@@ -239,31 +239,23 @@ disabled.
The approximate maximum overhead of the per-thread cache is thus equal
to the number of bins times the chunk count in each bin times the size
of each chunk. With defaults, the approximate maximum overhead of the
per-thread cache is approximately 236 KB on 64-bit systems and 118 KB
on 32-bit systems.
of each chunk.
@end deftp
@deftp Tunable glibc.malloc.mxfast
One of the optimizations @code{malloc} uses is to maintain a series of ``fast
bins'' that hold chunks up to a specific size. The default and
maximum size which may be held this way is 80 bytes on 32-bit systems
or 160 bytes on 64-bit systems. Applications which value size over
speed may choose to reduce the size of requests which are serviced
from fast bins with this tunable. Note that the value specified
includes @code{malloc}'s internal overhead, which is normally the size of one
pointer, so add 4 on 32-bit systems or 8 on 64-bit systems to the size
passed to @code{malloc} for the largest bin size to enable.
This tunable has no effect since the ``fastbins'' have been removed.
@end deftp
@deftp Tunable glibc.malloc.hugetlb
This tunable controls the usage of Huge Pages on @code{malloc} calls. The
default value is @code{0}, which disables any additional support on
@code{malloc}.
default value is @code{0} on most targets. Using @code{0} disables support
that improves use of huge pages in @code{malloc}. However huge pages may
still be created depending on the OS settings.
Setting its value to @code{1} enables the use of @code{madvise} with
@code{MADV_HUGEPAGE} after memory allocation with @code{mmap}. It is enabled
only if the system supports Transparent Huge Page (currently only on Linux).
This is the default used for AArch64.
Setting its value to @code{2} enables the use of Huge Page directly with
@code{mmap} with the use of @code{MAP_HUGETLB} flag. The huge page size