Re: [PATCH 1/2] Add note on MALLOC_MMAP_* environment variables

[Carlos O'Donell <carlos@redhat.com>]

On 10/10/2016 01:26 PM, Siddhesh Poyarekar wrote:
> The mallopt parameters manual does not mention the environment
> variables that can be used to set these parameters at program startup.
> Mention those environment variables for completeness.

Please don't get upset with me :-)

I'm asking you to document the default for these env var tunables.

I think it's valid for users to want to know what value they would have
had if they _didn't_ set the environment variable.

Throwing our hands up in the air and saying "it's implementation defined"
isn't very useful IMO.

> 	* manual/memory.texi: Add environment variable alternatives to
> 	setting mallopt parameters.
> ---
>  manual/memory.texi | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/manual/memory.texi b/manual/memory.texi
> index 5383105..222f126 100644
> --- a/manual/memory.texi
> +++ b/manual/memory.texi
> @@ -1113,12 +1113,18 @@ choices for @var{param}, as defined in @file{malloc.h}, are:
>  @item M_MMAP_MAX
>  The maximum number of chunks to allocate with @code{mmap}.  Setting this
>  to zero disables all use of @code{mmap}.
> +
> +This parameter can also be set for the process at startup by setting the
> +environment variable @code{MALLOC_MMAP_MAX_} to the desired value.

What's the default?

>  @item M_MMAP_THRESHOLD
>  All chunks larger than this value are allocated outside the normal
>  heap, using the @code{mmap} system call.  This way it is guaranteed
>  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}.
> +
> +This parameter can also be set for the process at startup by setting the
> +environment variable @code{MALLOC_MMAP_THRESHOLD_} to the desired value.

What's the default?

>  @comment TODO: @item M_MXFAST
>  @item M_PERTURB
>  If non-zero, memory blocks are filled with values depending on some
> @@ -1128,16 +1134,25 @@ use of uninitialized or freed heap memory.  Note that this option does not
>  guarantee that the freed block will have any specific values.  It only
>  guarantees that the content the block had before it was freed will be
>  overwritten.
> +
> +This parameter can also be set for the process at startup by setting the
> +environment variable @code{MALLOC_MMAP_PERTURB_} to the desired value.

What is the default?

>  @item M_TOP_PAD
>  This parameter determines the amount of extra memory to obtain from the
>  system when a call to @code{sbrk} is required.  It also specifies the
>  number of bytes to retain when shrinking the heap by calling @code{sbrk}
>  with a negative argument.  This provides the necessary hysteresis in
>  heap size such that excessive amounts of system calls can be avoided.
> +
> +This parameter can also be set for the process at startup by setting the
> +environment variable @code{MALLOC_TOP_PAD_} to the desired value.

What's the default?

>  @item M_TRIM_THRESHOLD
>  This is the minimum size (in bytes) of the top-most, releasable chunk
>  that will cause @code{sbrk} to be called with a negative argument in
>  order to return memory to the system.
> +
> +This parameter can also be set for the process at startup by setting the
> +environment variable @code{MALLOC_TRIM_THRESHOLD_} to the desired value.

What's the default? Note: There is a limit even with dynamic trim threshold on.

This disables the dynamic trim threshold, and that's a very important thing to
mention.

>  @end table
>  
>  @end deftypefun
> 


-- 
Cheers,
Carlos.