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

[Rical Jasan <ricaljasan at pacific dot net>]

Hi Siddhesh,

Most of this is formatting or cleanup/improvement of the pre-existing
chunks that were moved, but the comment on M_ARENA_MAX regards content.

On 10/24/2016 07:07 AM, 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 | 122 +++++++++++++++++++++++++++--------------------------
>  2 files changed, 62 insertions(+), 61 deletions(-)
> 
> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index ef04360..a849901 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 6f33455..198a933 100644
> --- a/manual/memory.texi
> +++ b/manual/memory.texi
> @@ -162,6 +162,8 @@ special to @theglibc{} and GNU Compiler.
>  
>  @menu
>  * Memory Allocation and C::     How to get different kinds of allocation in C.
> +* The GNU allocator::		An overview of the GNU @code{malloc}

I think "Allocator" should be capitalized.  The manual is inconsistent
on @subsection capitalization, but the majority of it is (capitalized).
Also, all the other info menu entries visible in the context here are
fully capitalized.

> +				implementation.
>  * Unconstrained Allocation::    The @code{malloc} facility allows fully general
>  		 		 dynamic allocation.
>  * Allocation Debugging::        Finding memory leaks and not freed memory.
> @@ -258,6 +260,43 @@ address of the space.  Then you can use the operators @samp{*} and
>  @}
>  @end smallexample
>  
> +@node The GNU allocator
> +@subsection The GNU allocator

Allocator

> +@cindex gnu allocator
> +
> +The @code{malloc} implementation in @theglibc{} is derived from ptmalloc
> +(pthreads malloc), which in turn is derived from dlmalloc (Doug Lea malloc).
> +This malloc may allocate memory in two different ways depending on their size
> +and certain parameters that may be controlled by users. The most common way is
> +to allocate portions of memory (called chunks) from a large contiguous area of
> +memory and manage these areas to optimize their use and reduce wastage in the
> +form of unusable chunks. Traditionally the system heap was set up to be the one
> +large memory area but @theglibc{} @code{malloc} implementation maintains

This should be "the @glibcadj{} @code{malloc}".

> +multiple such areas to optimize their use in multi-threaded applications.  Each
> +such area is internally referred to as an @code{arena}.

@dfn{arena}

Unless this is a function name (or literal string one would be using in
code, for example), this is simply a term we use to describe the
concept.  On first use, where we define the term, it should have @dfn{},
and otherwise it doesn't need to be stylized at all, such as...

> +
> +As opposed to other versions, the @code{malloc} in @theglibc{} does not round
> +up chunk sizes to powers of two, neither for large nor for small sizes.
> +Neighboring chunks can be coalesced on a @code{free} no matter what their size
> +is.  This makes the implementation suitable for all kinds of allocation
> +patterns without generally incurring high memory waste through fragmentation.
> +The presence of multiple @code{arenas} allows multiple threads to allocate

here (no @code{}).

> +memory simultaneously in their own separate arenas, thus improving performance.
> +
> +The other way of memory allocation is for very large blocks, i.e. much larger
> +than a page. These requests are allocated with @code{mmap} (anonymous or via
> +@code{/dev/zero}). This has the great advantage that these chunks are returned

Should be @file{}.

A reference to mmap would be good.  Something like, "... (anonymous or
via @file{/dev/zero}; @pxref{Memory-mapped I/O})."

> +to the system immediately when they are freed.  Therefore, it cannot happen
> +that a large chunk becomes ``locked'' in between smaller ones and even after
> +calling @code{free} wastes memory.  The size threshold for @code{mmap} to be
> +used is dynamic and gets adjusted according to allocation patterns of the
> +program.  This can also be statically adjusted with @code{mallopt}.  The use of

A reference to mallopt would be good here.

> +@code{mmap} can also be disabled completely.

Should briefly say how and/or give a reference.  I believe mallopt
applies to both, so maybe something like, "@code{mallopt} can be used to
statically adjust the threshold using @code{M_MMAP_THRESHOLD}, and the
use of @code{mmap} can be disabled completely with @code{M_MMAP_MAX};
@pxref{Malloc Tunable Parameters}."

> +
> +A more detailed technical description of the GNU allocator is maintained in
> +@theglibc{} wiki. See

the @glibcadj{}

> +@uref{https://sourceware.org/glibc/wiki/MallocInternals}.
> +
>  @node Unconstrained Allocation
>  @subsection Unconstrained Allocation
>  @cindex unconstrained memory allocation
> @@ -278,8 +317,6 @@ any time (or never).
>  				 bigger or smaller.
>  * Allocating Cleared Space::    Use @code{calloc} to allocate a
>  				 block and clear it.
> -* Efficiency and Malloc::       Efficiency considerations in use of
> -				 these functions.
>  * Aligned Memory Blocks::       Allocating specially aligned memory.
>  * Malloc Tunable Parameters::   Use @code{mallopt} to adjust allocation
>                                   parameters.
> @@ -867,59 +904,6 @@ But in general, it is not guaranteed that @code{calloc} calls
>  @code{malloc}/@code{realloc}/@code{free} outside the C library, it
>  should always define @code{calloc}, too.
>  
> -@node Efficiency and Malloc
> -@subsubsection Efficiency Considerations for @code{malloc}
> -@cindex efficiency and @code{malloc}
> -
> -
> -
> -
> -@ignore
> -
> -@c No longer true, see below instead.
> -To make the best use of @code{malloc}, it helps to know that the GNU
> -version of @code{malloc} always dispenses small amounts of memory in
> -blocks whose sizes are powers of two.  It keeps separate pools for each
> -power of two.  This holds for sizes up to a page size.  Therefore, if
> -you are free to choose the size of a small block in order to make
> -@code{malloc} more efficient, make it a power of two.
> -@c !!! xref getpagesize
> -
> -Once a page is split up for a particular block size, it can't be reused
> -for another size unless all the blocks in it are freed.  In many
> -programs, this is unlikely to happen.  Thus, you can sometimes make a
> -program use memory more efficiently by using blocks of the same size for
> -many different purposes.
> -
> -When you ask for memory blocks of a page or larger, @code{malloc} uses a
> -different strategy; it rounds the size up to a multiple of a page, and
> -it can coalesce and split blocks as needed.
> -
> -The reason for the two strategies is that it is important to allocate
> -and free small blocks as fast as possible, but speed is less important
> -for a large block since the program normally spends a fair amount of
> -time using it.  Also, large blocks are normally fewer in number.
> -Therefore, for large blocks, it makes sense to use a method which takes
> -more time to minimize the wasted space.
> -
> -@end ignore
> -
> -As opposed to other versions, the @code{malloc} in @theglibc{}
> -does not round up block sizes to powers of two, neither for large nor
> -for small sizes.  Neighboring chunks can be coalesced on a @code{free}
> -no matter what their size is.  This makes the implementation suitable
> -for all kinds of allocation patterns without generally incurring high
> -memory waste through fragmentation.
> -
> -Very large blocks (much larger than a page) are allocated with
> -@code{mmap} (anonymous or via @code{/dev/zero}) by this implementation.
> -This has the great advantage that these chunks are returned to the
> -system immediately when they are freed.  Therefore, it cannot happen
> -that a large chunk becomes ``locked'' in between smaller ones and even
> -after calling @code{free} wastes memory.  The size threshold for
> -@code{mmap} to be used can be adjusted with @code{mallopt}.  The use of
> -@code{mmap} can also be disabled completely.
> -
>  @node Aligned Memory Blocks
>  @subsubsection Allocating Aligned Memory Blocks
>  
> @@ -1105,10 +1089,6 @@ parameter to be set, and @var{value} the new value to be set.  Possible
>  choices for @var{param}, as defined in @file{malloc.h}, are:
>  
>  @table @code
> -@comment TODO: @item M_ARENA_MAX
> -@comment       - Document ARENA_MAX env var.
> -@comment TODO: @item M_ARENA_TEST
> -@comment       - Document ARENA_TEST env var.
>  @comment TODO: @item M_CHECK_ACTION
>  @item M_MMAP_MAX
>  The maximum number of chunks to allocate with @code{mmap}.  Setting this
> @@ -1169,6 +1149,28 @@ value is set statically to the provided input.
>  
>  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 parameter specifies the number of arenas that can be created before the
> +test on the limit to the number of arenas is conducted. The value is ignored if
> +@code{M_ARENA_MAX} is set.
> +
> +The default value of this parameter is 2 on 32-bit systems and 8 on 64-bit
> +systems.
> +
> +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.
> +
> +The default value of this tunable is @code{0}, meaning that the limit on the
> +number of arenas is determined by the number of CPU cores online. For 32-bit
> +systems the limit is twice the number of cores online and on 64-bit systems, it
> +is eight times the number of cores online.

Even though I had followed the thread, I immediately jumped to the same
confusion with M_ARENA_TEST that was resolved in [1].  Explicitly
stating here that 2 and 8 are not derived from M_ARENA_TEST defaults
might save a lot of general confusion down the road.

> +
> +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
> @@ -1511,7 +1513,7 @@ This is the total size of memory allocated with @code{sbrk} by
>  This is the number of chunks not in use.  (The memory allocator
>  internally gets chunks of memory from the operating system, and then
>  carves them up to satisfy individual @code{malloc} requests; see
> -@ref{Efficiency and Malloc}.)
> +@ref{The GNU allocator}.)

Allocator

Also, making this an @pxref{} now would be nice, since this renders
wrong in info as-is.

>  
>  @item int smblks
>  This field is unused.
> 

Rical

[1] https://sourceware.org/ml/libc-alpha/2016-10/msg00310.html