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

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

Siddhesh Poyarekar-9
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.

        * 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.
 @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.
 @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.
 @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.
 @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.
 @end table
 
 @end deftypefun
--
2.7.4

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[PATCH 2/2] Document the M_ARENA_* mallopt parameters

Siddhesh Poyarekar-9
The M_ARENA_* mallopt parameters are in wide use in production to
control the number of arenas that a long lived process creates and
hence there is no point in stating that this interface is non-public.
Document this interface and remove the obsolete comment.

        * manual/memory.texi (M_ARENA_TEST): Add documentation.
        (M_ARENA_MAX): Likewise.
        * malloc/malloc.c: Remove obsolete comment.
---
 malloc/malloc.c    |  1 -
 manual/memory.texi | 21 +++++++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 09e004b..0011a6d 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1718,7 +1718,6 @@ static struct malloc_par mp_ =
 };
 
 
-/*  Non public mallopt parameters.  */
 #define M_ARENA_TEST -7
 #define M_ARENA_MAX  -8
 
diff --git a/manual/memory.texi b/manual/memory.texi
index 222f126..b98dcf2 100644
--- a/manual/memory.texi
+++ b/manual/memory.texi
@@ -1153,6 +1153,27 @@ 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.
+
+@item M_ARENA_TEST
+An arena is a memory pool that is allocated to act as a heap alongside the
+system heap that the kernel creates for the process.  This is to provide
+multiple heap structures for multiple threads of a process to reduce contention
+between them.  The limit to the number of such arenas per process is determined
+by the number of cores of the system and whether it is a 32-bit or a 64-bit
+processor.
+
+The value is ignored if @code{M_ARENA_MAX} is set either via @code{mallopt} or
+via the @code{MALLOC_ARENA_MAX} environment variable.
+
+This parameter can also be set for the process at startup by setting the
+environment variable @code{MALLOC_ARENA_TEST} to the desired value.
+
+@item M_ARENA_MAX
+This parameter sets the number of arenas to use regardless of the number of
+cores in the system.
+
+This parameter can also be set for the process at startup by setting the
+environment variable @code{MALLOC_ARENA_MAX} to the desired value.
 @end table
 
 @end deftypefun
--
2.7.4

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

DJ Delorie-2
In reply to this post by Siddhesh Poyarekar-9

These variables all end with "_", most likely the original intention is
that they are not official and may be removed or changed at any time.

By making them official, we lock in that ABI.  Is that your intention?

(likewise your other patch).
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Re: [PATCH 1/2] Add note on MALLOC_MMAP_* environment variables

Siddhesh Poyarekar-9
On Monday 10 October 2016 11:03 PM, DJ Delorie wrote:
> These variables all end with "_", most likely the original intention is
> that they are not official and may be removed or changed at any time.
>
> By making them official, we lock in that ABI.  Is that your intention?
>
> (likewise your other patch).

I don't see the point in not doing so because they're too widely in use
for us to take them back anyway.

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

Michael Kerrisk (man-pages)
In reply to this post by DJ Delorie-2
On Mon, Oct 10, 2016 at 7:33 PM, DJ Delorie <[hidden email]> wrote:
>
> These variables all end with "_", most likely the original intention is
> that they are not official and may be removed or changed at any time.
>
> By making them official, we lock in that ABI.  Is that your intention?

I think the notion that because one does not document something, it's
not an official part of the ABI is at best highly dubious. Especially
when so many "official" parts of the ABI are also not documented.

The only way that documentation might be able to help in such
situations is where pieces are clearly and loudly documented right
from the beginning, in the official documentation, as "not supported,
may disappear at any moment in the future, use at your own risk", but
even then people are likely to ignore the documentation or be unaware
of it.

And in any case, these environment vars have long been documented in
various "unofficial" places including (since 2012)
http://man7.org/linux/man-pages/man3/mallopt.3.html

Cheers,

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

DJ Delorie-2

Michael Kerrisk <[hidden email]> writes:
> I think the notion that because one does not document something, it's
> not an official part of the ABI is at best highly dubious.

I wasn't here for the origin of those variables, I'm just thinking that
we use a leading underscore elsewhere to mean "not official" so the
trailing underscore here might have similar intent.

FTR I have no problem with documenting unsupported behavior, I just want
to make sure we understand how this patch affects the officialness of
this ABI.

> The only way that documentation might be able to help in such
> situations is where pieces are clearly and loudly documented right
> from the beginning, in the official documentation, as "not supported,
> may disappear at any moment in the future, use at your own risk", but
> even then people are likely to ignore the documentation or be unaware
> of it.

That would be fine too.  My concern is: if we intended for those
variables to be unofficial before, do we want to (1) make them official
now, or (2) be careful not to *accidentally* make them official?

And if we decide they're official (and/or always have been), should we
add in variants without underscores to be the official ones?

So I guess the next step is to have someone in authority (or consensus?)
decide if those variables are "official" or not, and if they should
become so if not.

Based on that we may want to tweak the documentation patch, or the code.
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Re: [PATCH 1/2] Add note on MALLOC_MMAP_* environment variables

Michael Kerrisk (man-pages)
On 10/11/2016 08:18 PM, DJ Delorie wrote:

>
> Michael Kerrisk <[hidden email]> writes:
>> I think the notion that because one does not document something, it's
>> not an official part of the ABI is at best highly dubious.
>
> I wasn't here for the origin of those variables, I'm just thinking that
> we use a leading underscore elsewhere to mean "not official" so the
> trailing underscore here might have similar intent.
>
> FTR I have no problem with documenting unsupported behavior, I just want
> to make sure we understand how this patch affects the officialness of
> this ABI.

My thought is: not very much. The ABI is out there and is well known.
There's not really anyway to argue to users that it is not "official",
no matter what the original developers might once have thought about
these interfaces.

Cheers,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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Re: [PATCH 1/2] Add note on MALLOC_MMAP_* environment variables

Kalle Olavi Niemitalo
In reply to this post by Siddhesh Poyarekar-9
Siddhesh Poyarekar <[hidden email]> writes:

> +This parameter can also be set for the process at startup by setting the
> +environment variable @code{MALLOC_MMAP_MAX_} to the desired value.

startup.texi has e.g. "@cindex @code{TZ} environment variable".
Could add similar index entries for MALLOC_MMAP_MAX_ and others.

Inconsistently, @vindex is used for the PWD, LANGUAGE, and
COREFILE environment variables.
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Re: [PATCH 1/2] Add note on MALLOC_MMAP_* environment variables

Siddhesh Poyarekar-9
In reply to this post by DJ Delorie-2
On Tuesday 11 October 2016 11:48 PM, DJ Delorie wrote:
> I wasn't here for the origin of those variables, I'm just thinking that
> we use a leading underscore elsewhere to mean "not official" so the
> trailing underscore here might have similar intent.

I agree, that may have been the intent.  However what happened
afterwards (i.e. their widespread use and the lack of communication to
the contrary from the then maintainers) have invalidated that intent
IMO.  What's more, other important sources (the man pages, Red Hat
product documentation) document these environment variables and that
gives them as much a sense of credibility as anything else.

> And if we decide they're official (and/or always have been), should we
> add in variants without underscores to be the official ones?

I'll be adding in tunables, which would become the 'official' versions
of these envvars.  Tunables come with the disclaimer that they may
disappear/reappear in releases, but I don't think we will do that for
the malloc tunables in practice.

> So I guess the next step is to have someone in authority (or consensus?)
> decide if those variables are "official" or not, and if they should
> become so if not.

It is usually consensus.

Siddhesh
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[PING][PATCH 1/2] Add note on MALLOC_MMAP_* environment variables

Siddhesh Poyarekar-8
In reply to this post by Siddhesh Poyarekar-9
Ping!

On Monday 10 October 2016 10:56 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.
>
> * 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.
>  @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.
>  @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.
>  @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.
>  @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.
>  @end table
>  
>  @end deftypefun
>
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[PING][PATCH 2/2] Document the M_ARENA_* mallopt parameters

Siddhesh Poyarekar-8
In reply to this post by Siddhesh Poyarekar-9
Ping!

On Monday 10 October 2016 10:56 PM, Siddhesh Poyarekar wrote:

> The M_ARENA_* mallopt parameters are in wide use in production to
> control the number of arenas that a long lived process creates and
> hence there is no point in stating that this interface is non-public.
> Document this interface and remove the obsolete comment.
>
> * manual/memory.texi (M_ARENA_TEST): Add documentation.
> (M_ARENA_MAX): Likewise.
> * malloc/malloc.c: Remove obsolete comment.
> ---
>  malloc/malloc.c    |  1 -
>  manual/memory.texi | 21 +++++++++++++++++++++
>  2 files changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index 09e004b..0011a6d 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -1718,7 +1718,6 @@ static struct malloc_par mp_ =
>  };
>  
>  
> -/*  Non public mallopt parameters.  */
>  #define M_ARENA_TEST -7
>  #define M_ARENA_MAX  -8
>  
> diff --git a/manual/memory.texi b/manual/memory.texi
> index 222f126..b98dcf2 100644
> --- a/manual/memory.texi
> +++ b/manual/memory.texi
> @@ -1153,6 +1153,27 @@ 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.
> +
> +@item M_ARENA_TEST
> +An arena is a memory pool that is allocated to act as a heap alongside the
> +system heap that the kernel creates for the process.  This is to provide
> +multiple heap structures for multiple threads of a process to reduce contention
> +between them.  The limit to the number of such arenas per process is determined
> +by the number of cores of the system and whether it is a 32-bit or a 64-bit
> +processor.
> +
> +The value is ignored if @code{M_ARENA_MAX} is set either via @code{mallopt} or
> +via the @code{MALLOC_ARENA_MAX} environment variable.
> +
> +This parameter can also be set for the process at startup by setting the
> +environment variable @code{MALLOC_ARENA_TEST} to the desired value.
> +
> +@item M_ARENA_MAX
> +This parameter sets the number of arenas to use regardless of the number of
> +cores in the system.
> +
> +This parameter can also be set for the process at startup by setting the
> +environment variable @code{MALLOC_ARENA_MAX} to the desired value.
>  @end table
>  
>  @end deftypefun
>
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Re: [PATCH 2/2] Document the M_ARENA_* mallopt parameters

Carlos O'Donell-6
In reply to this post by Siddhesh Poyarekar-9
On 10/10/2016 01:26 PM, Siddhesh Poyarekar wrote:

> The M_ARENA_* mallopt parameters are in wide use in production to
> control the number of arenas that a long lived process creates and
> hence there is no point in stating that this interface is non-public.
> Document this interface and remove the obsolete comment.
>
> * manual/memory.texi (M_ARENA_TEST): Add documentation.
> (M_ARENA_MAX): Likewise.
> * malloc/malloc.c: Remove obsolete comment.
> ---
>  malloc/malloc.c    |  1 -
>  manual/memory.texi | 21 +++++++++++++++++++++
>  2 files changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index 09e004b..0011a6d 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -1718,7 +1718,6 @@ static struct malloc_par mp_ =
>  };
>  
>  
> -/*  Non public mallopt parameters.  */
>  #define M_ARENA_TEST -7
>  #define M_ARENA_MAX  -8
>  
> diff --git a/manual/memory.texi b/manual/memory.texi
> index 222f126..b98dcf2 100644
> --- a/manual/memory.texi
> +++ b/manual/memory.texi
> @@ -1153,6 +1153,27 @@ 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.
> +
> +@item M_ARENA_TEST

This description of an arena doesn't belong here, it belongs higher up in the
manual, and in fact I would suggest the following:

- Move 3.2.2.6 "Efficiency considerations for malloc" up to just under
  "Dynamic Memory allocation" and put it under a new subsection "The GNU allocator".

- Under "The GNU allocator" give a brief 1 paragraph description of how
  the allocator works e.g. https://sourceware.org/glibc/wiki/MallocInternals
  and talk about arenas.

> +An arena is a memory pool that is allocated to act as a heap alongside the
> +system heap that the kernel creates for the process.  This is to provide
> +multiple heap structures for multiple threads of a process to reduce contention
> +between them.  The limit to the number of such arenas per process is determined
> +by the number of cores of the system and whether it is a 32-bit or a 64-bit
> +processor.

You don't say what M_ARENA_TEST is for?

> +
> +The value is ignored if @code{M_ARENA_MAX} is set either via @code{mallopt} or
> +via the @code{MALLOC_ARENA_MAX} environment variable.
> +
> +This parameter can also be set for the process at startup by setting the
> +environment variable @code{MALLOC_ARENA_TEST} to the desired value.

What is the default?

> +
> +@item M_ARENA_MAX
> +This parameter sets the number of arenas to use regardless of the number of
> +cores in the system.
> +
> +This parameter can also be set for the process at startup by setting the
> +environment variable @code{MALLOC_ARENA_MAX} to the desired value.

What is the default?

>  @end table
>  
>  @end deftypefun
>


--
Cheers,
Carlos.
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Re: [PATCH 1/2] Add note on MALLOC_MMAP_* environment variables

Carlos O'Donell-6
In reply to this post by Siddhesh Poyarekar-9
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.
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Re: [PATCH 1/2] Add note on MALLOC_MMAP_* environment variables

Siddhesh Poyarekar-9
On Monday 17 October 2016 09:43 PM, Carlos O'Donell wrote:
> 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 :-)

Grrr...

> 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.

No I agree that is a good point.  I'll work on it based on your comments
on the MALLOC_ARENA_* doc patch, i.e. first describe the GNU allocator
and then use that information to explain the envvars/mallopt params and
also add in defaults.

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

Carlos O'Donell-6
In reply to this post by Michael Kerrisk (man-pages)
On 10/11/2016 02:20 AM, Michael Kerrisk wrote:

> On Mon, Oct 10, 2016 at 7:33 PM, DJ Delorie <[hidden email]> wrote:
>>
>> These variables all end with "_", most likely the original intention is
>> that they are not official and may be removed or changed at any time.
>>
>> By making them official, we lock in that ABI.  Is that your intention?
>
> I think the notion that because one does not document something, it's
> not an official part of the ABI is at best highly dubious. Especially
> when so many "official" parts of the ABI are also not documented.
>
> The only way that documentation might be able to help in such
> situations is where pieces are clearly and loudly documented right
> from the beginning, in the official documentation, as "not supported,
> may disappear at any moment in the future, use at your own risk", but
> even then people are likely to ignore the documentation or be unaware
> of it.
>
> And in any case, these environment vars have long been documented in
> various "unofficial" places including (since 2012)
> http://man7.org/linux/man-pages/man3/mallopt.3.html

The things that DJ is worried about are the arena ones
e.g. M_ARENA_MAX, and M_ARENA_TEST and their env vars.

In 2013 I brought up the discussion about ABI implications:
https://sourceware.org/ml/libc-alpha/2013-03/msg00376.html

Only Siddhesh and I commented, and we both agreed it was a forgone
conclusion that these were stable parts of the ABI/API.

In 2015 I documented them in the linux man pages project:
https://www.sourceware.org/ml/libc-alpha/2015-08/msg00991.html
~~~
Consensus among Siddhesh and myself was that they should be public,
and in fact they were already in the public header. Therefore there
may already be applications uses these constants and expecting them
to work. At best we could limit mallopt's acceptance of the options,
but that seems like a bad solution that could lead to unexpected
behaviour for user applications. A quick google search shows that
there are packages relying on these constants to tune the glibc
malloc implementation.
~~~

They are part of the ABI and public, and should be documented
in the manual as Siddhesh's patches do.

We should however be very very circumspect about adding more
of these mallopt tunables without first seeing that such tunables
are stable and long-term useful as generic tunables (via the tunables
interface). So go review Siddhesh's patches on tunables :-)

--
Cheers,
Carlos.
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Re: [PATCH 2/2] Document the M_ARENA_* mallopt parameters

Michael Kerrisk (man-pages)
In reply to this post by Carlos O'Donell-6
On Mon, Oct 17, 2016 at 6:08 PM, Carlos O'Donell <[hidden email]> wrote:

> On 10/10/2016 01:26 PM, Siddhesh Poyarekar wrote:
>> The M_ARENA_* mallopt parameters are in wide use in production to
>> control the number of arenas that a long lived process creates and
>> hence there is no point in stating that this interface is non-public.
>> Document this interface and remove the obsolete comment.
>>
>>       * manual/memory.texi (M_ARENA_TEST): Add documentation.
>>       (M_ARENA_MAX): Likewise.
>>       * malloc/malloc.c: Remove obsolete comment.
>> ---
>>  malloc/malloc.c    |  1 -
>>  manual/memory.texi | 21 +++++++++++++++++++++
>>  2 files changed, 21 insertions(+), 1 deletion(-)
>>
>> diff --git a/malloc/malloc.c b/malloc/malloc.c
>> index 09e004b..0011a6d 100644
>> --- a/malloc/malloc.c
>> +++ b/malloc/malloc.c
>> @@ -1718,7 +1718,6 @@ static struct malloc_par mp_ =
>>  };
>>
>>
>> -/*  Non public mallopt parameters.  */
>>  #define M_ARENA_TEST -7
>>  #define M_ARENA_MAX  -8
>>
>> diff --git a/manual/memory.texi b/manual/memory.texi
>> index 222f126..b98dcf2 100644
>> --- a/manual/memory.texi
>> +++ b/manual/memory.texi
>> @@ -1153,6 +1153,27 @@ 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.
>> +
>> +@item M_ARENA_TEST
>
> This description of an arena doesn't belong here, it belongs higher up in the
> manual, and in fact I would suggest the following:
>
> - Move 3.2.2.6 "Efficiency considerations for malloc" up to just under
>   "Dynamic Memory allocation" and put it under a new subsection "The GNU allocator".
>
> - Under "The GNU allocator" give a brief 1 paragraph description of how
>   the allocator works e.g. https://sourceware.org/glibc/wiki/MallocInternals
>   and talk about arenas.
>
>> +An arena is a memory pool that is allocated to act as a heap alongside the
>> +system heap that the kernel creates for the process.  This is to provide
>> +multiple heap structures for multiple threads of a process to reduce contention
>> +between them.  The limit to the number of such arenas per process is determined
>> +by the number of cores of the system and whether it is a 32-bit or a 64-bit
>> +processor.
>
> You don't say what M_ARENA_TEST is for?
>
>> +
>> +The value is ignored if @code{M_ARENA_MAX} is set either via @code{mallopt} or
>> +via the @code{MALLOC_ARENA_MAX} environment variable.
>> +
>> +This parameter can also be set for the process at startup by setting the
>> +environment variable @code{MALLOC_ARENA_TEST} to the desired value.
>
> What is the default?

So my reading from malloc/malloc.c:

#define NARENAS_FROM_NCORES(n) ((n) * (sizeof (long) == 4 ? 2 : 8))
  .arena_test = NARENAS_FROM_NCORES (1)

So, the default value for this parameter is 2 on systems where
sizeof(long) is 4; otherwise the default value is 8.

>> +
>> +@item M_ARENA_MAX
>> +This parameter sets the number of arenas to use regardless of the number of
>> +cores in the system.
>> +
>> +This parameter can also be set for the process at startup by setting the
>> +environment variable @code{MALLOC_ARENA_MAX} to the desired value.
>
> What is the default?

So, IIUC, the default value of this parameter is 0, meaning that there
is no limit on the number of arenas that can be created. Do you
confirm, Siddhesh?

Siddhesh, in case you want to use any of my wordings (assuming they
are correct), I place them in the public domain. (The text will also
land in the mallopt(3) manual page.)

Cheers,

Michael
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Re: [PATCH 2/2] Document the M_ARENA_* mallopt parameters

Siddhesh Poyarekar-9
On Tuesday 18 October 2016 12:45 PM, Michael Kerrisk wrote:
> So my reading from malloc/malloc.c:
>
> #define NARENAS_FROM_NCORES(n) ((n) * (sizeof (long) == 4 ? 2 : 8))
>   .arena_test = NARENAS_FROM_NCORES (1)
>
> So, the default value for this parameter is 2 on systems where
> sizeof(long) is 4; otherwise the default value is 8.

That is correct.

> So, IIUC, the default value of this parameter is 0, meaning that there
> is no limit on the number of arenas that can be created. Do you
> confirm, Siddhesh?

No, the default is a function of the number of cores using the
NARENAS_FROM_NCORES macro.  That is, 2 * (number of cores) where
sizeof(long) == 4 and 8 * (number of cores) otherwise.  If the number of
cores is not available (i.e. we can't read this info for some reason)
then we default to 4 and 16 respectively, i.e. assume 2 cores.

Note that this default only comes into force once the number of arenas
cross arena_test.

> Siddhesh, in case you want to use any of my wordings (assuming they
> are correct), I place them in the public domain. (The text will also
> land in the mallopt(3) manual page.)

Thanks.

Siddhesh
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Re: [PATCH 2/2] Document the M_ARENA_* mallopt parameters

Michael Kerrisk (man-pages)
Hi Siddhesh,

On 10/18/2016 12:07 PM, Siddhesh Poyarekar wrote:

> On Tuesday 18 October 2016 12:45 PM, Michael Kerrisk wrote:
>> So my reading from malloc/malloc.c:
>>
>> #define NARENAS_FROM_NCORES(n) ((n) * (sizeof (long) == 4 ? 2 : 8))
>>   .arena_test = NARENAS_FROM_NCORES (1)
>>
>> So, the default value for this parameter is 2 on systems where
>> sizeof(long) is 4; otherwise the default value is 8.
>
> That is correct.

Thanks for the confirmation.

[You've over trimmed the mail for this next piece. To be clear, below,
we are talking about M_ARENA_MAX; or at least I was.]

>> So, IIUC, the default value of this parameter is 0, meaning that there
>> is no limit on the number of arenas that can be created. Do you
>> confirm, Siddhesh?
>
> No, the default is a function of the number of cores using the
> NARENAS_FROM_NCORES macro.  That is, 2 * (number of cores) where
> sizeof(long) == 4 and 8 * (number of cores) otherwise.  If the number of
> cores is not available (i.e. we can't read this info for some reason)
> then we default to 4 and 16 respectively, i.e. assume 2 cores.
>
> Note that this default only comes into force once the number of arenas
> cross arena_test.

I don't think you're correct here. 'arena_max' is a field in a static
structure that is not otherwise initialized, AFAICT. So, it has the
value zero. (Some dirty hacking with a program that uses
malloc_get_state() and inspects the internal data structure seems
to confirm this.)

And then in malloc/arena.c we have

          if (mp_.arena_max != 0)
            narenas_limit = mp_.arena_max;
          else if (narenas > mp_.arena_test)
            {
              int n = __get_nprocs ();
             
              if (n >= 1)
                narenas_limit = NARENAS_FROM_NCORES (n);
              else
                /* We have no information about the system.  Assume two
                   cores.  */
                narenas_limit = NARENAS_FROM_NCORES (2);

So, I believe my original statement about M_ARENA_MAX is correct.
Have I missed something?

Cheers,

Michael

>> Siddhesh, in case you want to use any of my wordings (assuming they
>> are correct), I place them in the public domain. (The text will also
>> land in the mallopt(3) manual page.)
>
> Thanks.
>
> Siddhesh
>


--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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Re: [PATCH 2/2] Document the M_ARENA_* mallopt parameters

Siddhesh Poyarekar-9
On Tuesday 18 October 2016 07:20 PM, Michael Kerrisk (man-pages) wrote:

> Hi Siddhesh,
>
> On 10/18/2016 12:07 PM, Siddhesh Poyarekar wrote:
>> On Tuesday 18 October 2016 12:45 PM, Michael Kerrisk wrote:
>>> So my reading from malloc/malloc.c:
>>>
>>> #define NARENAS_FROM_NCORES(n) ((n) * (sizeof (long) == 4 ? 2 : 8))
>>>   .arena_test = NARENAS_FROM_NCORES (1)
>>>
>>> So, the default value for this parameter is 2 on systems where
>>> sizeof(long) is 4; otherwise the default value is 8.
>>
>> That is correct.
>
> Thanks for the confirmation.
>
> [You've over trimmed the mail for this next piece. To be clear, below,
> we are talking about M_ARENA_MAX; or at least I was.]
>
>>> So, IIUC, the default value of this parameter is 0, meaning that there
>>> is no limit on the number of arenas that can be created. Do you
>>> confirm, Siddhesh?
>>
>> No, the default is a function of the number of cores using the
>> NARENAS_FROM_NCORES macro.  That is, 2 * (number of cores) where
>> sizeof(long) == 4 and 8 * (number of cores) otherwise.  If the number of
>> cores is not available (i.e. we can't read this info for some reason)
>> then we default to 4 and 16 respectively, i.e. assume 2 cores.
>>
>> Note that this default only comes into force once the number of arenas
>> cross arena_test.
>
> I don't think you're correct here. 'arena_max' is a field in a static
> structure that is not otherwise initialized, AFAICT. So, it has the
> value zero. (Some dirty hacking with a program that uses
> malloc_get_state() and inspects the internal data structure seems
> to confirm this.)
>
> And then in malloc/arena.c we have
>
>           if (mp_.arena_max != 0)
>             narenas_limit = mp_.arena_max;
>           else if (narenas > mp_.arena_test)
>             {
>               int n = __get_nprocs ();
>              
>               if (n >= 1)
>                 narenas_limit = NARENAS_FROM_NCORES (n);
>               else
>                 /* We have no information about the system.  Assume two
>                    cores.  */
>                 narenas_limit = NARENAS_FROM_NCORES (2);
>
> So, I believe my original statement about M_ARENA_MAX is correct.
> Have I missed something?

You're right in that the variable arena_max is initialized to 0.
However you also concluded that there is no limit to the number of
arenas that can be created when arena_max is 0, which is incorrect.  As
the code snippet you pasted above shows that if arena_max is 0, once we
cross arena_test arenas, the narenas_limit static variable is set to a
default upper limit based on the number of cores.  That acts as the
upper limit to the number of arenas that can be created when arena_max is 0.

Siddhesh
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Re: [PATCH 2/2] Document the M_ARENA_* mallopt parameters

Michael Kerrisk (man-pages)
Hi Siddhesh,

On 10/18/2016 04:30 PM, Siddhesh Poyarekar wrote:

> On Tuesday 18 October 2016 07:20 PM, Michael Kerrisk (man-pages) wrote:
>> Hi Siddhesh,
>>
>> On 10/18/2016 12:07 PM, Siddhesh Poyarekar wrote:
>>> On Tuesday 18 October 2016 12:45 PM, Michael Kerrisk wrote:
>>>> So my reading from malloc/malloc.c:
>>>>
>>>> #define NARENAS_FROM_NCORES(n) ((n) * (sizeof (long) == 4 ? 2 : 8))
>>>>   .arena_test = NARENAS_FROM_NCORES (1)
>>>>
>>>> So, the default value for this parameter is 2 on systems where
>>>> sizeof(long) is 4; otherwise the default value is 8.
>>>
>>> That is correct.
>>
>> Thanks for the confirmation.
>>
>> [You've over trimmed the mail for this next piece. To be clear, below,
>> we are talking about M_ARENA_MAX; or at least I was.]
>>
>>>> So, IIUC, the default value of this parameter is 0, meaning that there
>>>> is no limit on the number of arenas that can be created. Do you
>>>> confirm, Siddhesh?
>>>
>>> No, the default is a function of the number of cores using the
>>> NARENAS_FROM_NCORES macro.  That is, 2 * (number of cores) where
>>> sizeof(long) == 4 and 8 * (number of cores) otherwise.  If the number of
>>> cores is not available (i.e. we can't read this info for some reason)
>>> then we default to 4 and 16 respectively, i.e. assume 2 cores.
>>>
>>> Note that this default only comes into force once the number of arenas
>>> cross arena_test.
>>
>> I don't think you're correct here. 'arena_max' is a field in a static
>> structure that is not otherwise initialized, AFAICT. So, it has the
>> value zero. (Some dirty hacking with a program that uses
>> malloc_get_state() and inspects the internal data structure seems
>> to confirm this.)
>>
>> And then in malloc/arena.c we have
>>
>>           if (mp_.arena_max != 0)
>>             narenas_limit = mp_.arena_max;
>>           else if (narenas > mp_.arena_test)
>>             {
>>               int n = __get_nprocs ();
>>              
>>               if (n >= 1)
>>                 narenas_limit = NARENAS_FROM_NCORES (n);
>>               else
>>                 /* We have no information about the system.  Assume two
>>                    cores.  */
>>                 narenas_limit = NARENAS_FROM_NCORES (2);
>>
>> So, I believe my original statement about M_ARENA_MAX is correct.
>> Have I missed something?
>
> You're right in that the variable arena_max is initialized to 0.

Okay.

> However you also concluded that there is no limit to the number of
> arenas that can be created when arena_max is 0, which is incorrect.  As
> the code snippet you pasted above shows that if arena_max is 0, once we
> cross arena_test arenas, the narenas_limit static variable is set to a
> default upper limit based on the number of cores.  That acts as the
> upper limit to the number of arenas that can be created when arena_max is 0.

D'oh!  Yes, of course. Thanks for that. Not sure how I managed to
misread that code :-}. So a better formulation would be something like:

    The default value of this parameter is 0, meaning that the limit on
    the number of arenas is determined according to the setting of
    M_ARENA_TEST.

Seem okay?

Cheers,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
12