[PATCH v2] malloc: Remove dynamic mmap/trim threshold [BZ #30769]

Tue Apr 7 17:39:06 GMT 2026

On 02/04/26 10:56, Wilco Dijkstra wrote:
> v2: Update documentation
> 
> Whenever a large mmap is released the mmap and trim thresholds are updated.
> As a result these thresholds grow ever larger which means huge allocations
> are always served by arenas rather than mmap.  The thresholds can end up as
> large as an arena, which completely stops all trimming of the top block.
> Remove the code completely - the default thresholds seem way too low for
> modern 64-bit targets, but they can be increased seperately.
> 
> Passes regress - OK for commit?

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> 
> ---
> 
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index 6a888b0eb7de53ae7b814275e86d2bd2f06b5e53..8b7a414ab8d2c0f84ba7be7449a7909d1930d325 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -1770,10 +1770,6 @@ struct malloc_par
>    int n_mmaps;
>    int n_mmaps_max;
>    int max_n_mmaps;
> -  /* the mmap_threshold is dynamic, until the user sets
> -     it manually, at which point we need to disable any
> -     dynamic behavior. */
> -  int no_dyn_threshold;
>  
>    /* Statistics */
>    INTERNAL_SIZE_T mmapped_mem;
> @@ -4291,18 +4287,6 @@ _int_free_chunk (mstate av, mchunkptr p, INTERNAL_SIZE_T size, int have_lock)
>      /* Preserve errno in case munmap sets it.  */
>      int err = errno;
>  
> -    /* See if the dynamic brk/mmap threshold needs adjusting.
> -       Dumped fake mmapped chunks do not affect the threshold.  */
> -    if (!mp_.no_dyn_threshold
> -        && chunksize_nomask (p) > mp_.mmap_threshold
> -        && chunksize_nomask (p) <= DEFAULT_MMAP_THRESHOLD_MAX)
> -      {
> -        mp_.mmap_threshold = chunksize (p);
> -        mp_.trim_threshold = 2 * mp_.mmap_threshold;
> -        LIBC_PROBE (memory_mallopt_free_dyn_thresholds, 2,
> -		    mp_.mmap_threshold, mp_.trim_threshold);
> -      }
> -
>      munmap_chunk (p);
>  
>      __set_errno (err);
> @@ -4901,40 +4885,32 @@ __malloc_stats (void)
>  static __always_inline int
>  do_set_trim_threshold (size_t value)
>  {
> -  LIBC_PROBE (memory_mallopt_trim_threshold, 3, value, mp_.trim_threshold,
> -	      mp_.no_dyn_threshold);
> +  LIBC_PROBE (memory_mallopt_trim_threshold, 2, value, mp_.trim_threshold);
>    mp_.trim_threshold = value;
> -  mp_.no_dyn_threshold = 1;
>    return 1;
>  }
>  
>  static __always_inline int
>  do_set_top_pad (size_t value)
>  {
> -  LIBC_PROBE (memory_mallopt_top_pad, 3, value, mp_.top_pad,
> -	      mp_.no_dyn_threshold);
> +  LIBC_PROBE (memory_mallopt_top_pad, 2, value, mp_.top_pad);
>    mp_.top_pad = value;
> -  mp_.no_dyn_threshold = 1;
>    return 1;
>  }
>  
>  static __always_inline int
>  do_set_mmap_threshold (size_t value)
>  {
> -  LIBC_PROBE (memory_mallopt_mmap_threshold, 3, value, mp_.mmap_threshold,
> -	      mp_.no_dyn_threshold);
> +  LIBC_PROBE (memory_mallopt_mmap_threshold, 2, value, mp_.mmap_threshold);
>    mp_.mmap_threshold = value;
> -  mp_.no_dyn_threshold = 1;
>    return 1;
>  }
>  
>  static __always_inline int
>  do_set_mmaps_max (int32_t value)
>  {
> -  LIBC_PROBE (memory_mallopt_mmap_max, 3, value, mp_.n_mmaps_max,
> -	      mp_.no_dyn_threshold);
> +  LIBC_PROBE (memory_mallopt_mmap_max, 2, value, mp_.n_mmaps_max);
>    mp_.n_mmaps_max = value;
> -  mp_.no_dyn_threshold = 1;
>    return 1;
>  }
>  
> diff --git a/manual/memory.texi b/manual/memory.texi
> index 4f0ef51514057136c14ceb6811face0117e69272..f8b6be6ee781cafda1774598585b50524d6cdca0 100644
> --- a/manual/memory.texi
> +++ b/manual/memory.texi
> @@ -1333,10 +1333,7 @@ that the memory for these chunks can be returned to the system on
>  @code{free}.  Note that requests smaller than this threshold might still
>  be allocated via @code{mmap}.
>  
> -If this parameter is not set, the default value is set as 128 KiB and the
> -threshold is adjusted dynamically to suit the allocation patterns of the
> -program. If the parameter is set, the dynamic adjustment is disabled and the
> -value is set statically to the input value.
> +If this parameter is not set, the default value is set as 128 KiB.
>  
>  This parameter can also be set for the process at startup by setting the
>  environment variable @env{MALLOC_MMAP_THRESHOLD_} to the desired value.
> @@ -1371,10 +1368,7 @@ environment variable @env{MALLOC_TOP_PAD_} to the desired value.
>  This is the minimum size (in bytes) of the top-most, releasable chunk
>  that will trigger a system call in order to return memory to the system.
>  
> -If this parameter is not set, the default value is set as 128 KiB and the
> -threshold is adjusted dynamically to suit the allocation patterns of the
> -program. If the parameter is set, the dynamic adjustment is disabled and the
> -value is set statically to the provided input.
> +If this parameter is not set, the default value is set as 128 KiB.
>  
>  This parameter can also be set for the process at startup by setting the
>  environment variable @env{MALLOC_TRIM_THRESHOLD_} to the desired value.
> diff --git a/manual/probes.texi b/manual/probes.texi
> index 23340e8e0774ae080bc6cecf0a916902c490f629..82d234f4b8ee0be489ef56918b536fedf252531d 100644
> --- a/manual/probes.texi
> +++ b/manual/probes.texi
> @@ -161,37 +161,33 @@ value, and @var{$arg2} is the previous value of this @code{malloc}
>  parameter.
>  @end deftp
>  
> -@deftp Probe memory_mallopt_trim_threshold (int @var{$arg1}, int @var{$arg2}, int @var{$arg3})
> +@deftp Probe memory_mallopt_trim_threshold (int @var{$arg1}, int @var{$arg2})
>  This probe is triggered shortly after the @code{memory_mallopt} probe,
>  when the parameter to be changed is @code{M_TRIM_THRESHOLD}.  Argument
>  @var{$arg1} is the requested value, @var{$arg2} is the previous value of
> -this @code{malloc} parameter, and @var{$arg3} is nonzero if dynamic
> -threshold adjustment was already disabled.
> +this @code{malloc} parameter.
>  @end deftp
>  
> -@deftp Probe memory_mallopt_top_pad (int @var{$arg1}, int @var{$arg2}, int @var{$arg3})
> +@deftp Probe memory_mallopt_top_pad (int @var{$arg1}, int @var{$arg2})
>  This probe is triggered shortly after the @code{memory_mallopt} probe,
>  when the parameter to be changed is @code{M_TOP_PAD}.  Argument
>  @var{$arg1} is the requested value, @var{$arg2} is the previous value of
> -this @code{malloc} parameter, and @var{$arg3} is nonzero if dynamic
> -threshold adjustment was already disabled.
> +this @code{malloc} parameter.
>  @end deftp
>  
> -@deftp Probe memory_mallopt_mmap_threshold (int @var{$arg1}, int @var{$arg2}, int @var{$arg3})
> +@deftp Probe memory_mallopt_mmap_threshold (int @var{$arg1}, int @var{$arg2})
>  This probe is triggered shortly after the @code{memory_mallopt} probe,
>  when the parameter to be changed is @code{M_MMAP_THRESHOLD}, and the
>  requested value is in an acceptable range.  Argument @var{$arg1} is the
>  requested value, @var{$arg2} is the previous value of this @code{malloc}
> -parameter, and @var{$arg3} is nonzero if dynamic threshold adjustment
> -was already disabled.
> +parameter.
>  @end deftp
>  
> -@deftp Probe memory_mallopt_mmap_max (int @var{$arg1}, int @var{$arg2}, int @var{$arg3})
> +@deftp Probe memory_mallopt_mmap_max (int @var{$arg1}, int @var{$arg2})
>  This probe is triggered shortly after the @code{memory_mallopt} probe,
>  when the parameter to be changed is @code{M_MMAP_MAX}.  Argument
>  @var{$arg1} is the requested value, @var{$arg2} is the previous value of
> -this @code{malloc} parameter, and @var{$arg3} is nonzero if dynamic
> -threshold adjustment was already disabled.
> +this @code{malloc} parameter.
>  @end deftp
>  
>  @deftp Probe memory_mallopt_perturb (int @var{$arg1}, int @var{$arg2})
> @@ -217,12 +213,6 @@ requested value, and @var{$arg2} is the previous value of this
>  @code{malloc} parameter.
>  @end deftp
>  
> -@deftp Probe memory_mallopt_free_dyn_thresholds (int @var{$arg1}, int @var{$arg2})
> -This probe is triggered when function @code{free} decides to adjust the
> -dynamic brk/mmap thresholds.  Argument @var{$arg1} and @var{$arg2} are
> -the adjusted mmap and trim thresholds, respectively.
> -@end deftp
> -
>  @deftp Probe memory_tunable_tcache_max_bytes (int @var{$arg1}, int @var{$arg2})
>  This probe is triggered when the @code{glibc.malloc.tcache_max}
>  tunable is set.  Argument @var{$arg1} is the requested value, and
> 