mallinfo(3) man page for review

[KOSAKI Motohiro]

(5/5/12 5:50 PM), Michael Kerrisk (man-pages) wrote:
> Motohiro,
>
> Thank you for taking the time to look at this.
>
> On Sun, May 6, 2012 at 9:25 AM, KOSAKI Motohiro
> <kosaki.motohiro@gmail.com>  wrote:
>> Hi
>>
>> On Fri, May 4, 2012 at 8:08 PM, Michael Kerrisk (man-pages)
>> <mtk.manpages@gmail.com>  wrote:
>>> I've written the man page page below to document glibc's mallinfo(3).
>>> Review comments welcome!
>>>
>>> Cheers,
>>>
>>> Michael
>>
>> I don't find any issue. a few nit are below.
>>
>>
>>> .TP 10
>>> .I arena
>>> The total amount of memory allocated by means other than
>>> .BR mmap (2)
>>> (i.e., memory allocated on the heap).
>>> This figure includes both in-use blocks and blocks on the free list.
>>> .TP
>>> .I ordblks
>>> The number of ordinary (i.e., non-fastbin) free blocks.
>>> .TP
>>> .I smblks
>>> The number of fastbin free blocks (see
>>> .BR mallopt (3)).
>>
>> It would be nice if a meaning of fastbin was described here.
>> It is allocation cache (for increasing cpu cache hit ratio) from
>> point of bird view.
>
> In the new mallopt(3) that is part of the next man-pages release,
> there is this text on fastbins:
>
>         M_MXFAST (since glibc 2.3)
>                Set the upper limit for memory allocation requests  that
>                are  satisfied  using "fastbins".  (The measurement unit
>                for this parameter  is  bytes.)   Fastbins  are  storage
>                areas that hold deallocated blocks of memory of the same
>                size without merging adjacent free  blocks.   Subsequent
>                reallocation  of  blocks of the same size can be handled
>                very quickly by allocating from  the  fastbin,  although
>                memory fragmentation and the overall memory footprint of
>                the program can increase.  The default  value  for  this
>                parameter  is  64*sizeof(size_t)/4  (i.e.,  64 on 32-bit
>                architectures).  The range for this parameter  is  0  to
>                80*sizeof(size_t)/4.  Setting M_MXFAST to 0 disables the
>                use of fastbins.
>
> Probably this is sufficient?

Yes, very good. can you please refer mallopt here?


>>> .TP
>>> .I hblks
>>> The number of blocks currently allocated using
>>> .BR mmap (2).
>>
>> It would be nice if glibc malloc try to allocate memory by using mmap directly
>> if size is greater than MMAP_THRESHOLD.
>>
>> Also nice if refer mallopt and MMAP_THRESHOLD.
>
> Good idea. I added:
>
> [[
> (See the discussion of
> .B M_MMAP_THRESHOLD
> in
> .BR mallopt (3).)
> ]]
>
> In the new mallopt(3) page, there is then this text:
>
>         M_MMAP_THRESHOLD
>                For allocations greater than or equal to the limit spec‐
>                ified  (in bytes) by M_MMAP_THRESHOLD that can't be sat‐
>                isfied from the free list, the  memory-allocation  func‐
>                tions  employ  mmap(2) instead of increasing the program
>                break using sbrk(2).
>
>                Allocating memory  using  mmap(2)  has  the  significant
>                advantage that the allocated memory blocks can always be
>                independently released back to  the  system.   (By  con‐
>                trast,  the  heap can be trimmed only if memory is freed
>                at the top end.)  On the other hand, there are some dis‐
>                advantages  to  the use of mmap(2): deallocated space is
>                not placed on the free list for reuse by  later  alloca‐
>                tions;  memory may be wasted because mmap(2) allocations
>                must be page-aligned; and the kernel  must  perform  the
>                expensive  task  of  zeroing  out  memory  allocated via
>                mmap(2).  Balancing these factors  leads  to  a  default
>                setting of 128*1024 for the M_MMAP_THRESHOLD parameter.
>
>                The  lower  limit  for  this  parameter is 0.  The upper
>                limit is DEFAULT_MMAP_THRESHOLD_MAX: 512*1024 on  32-bit
>                systems or 4*1024*1024*sizeof(long) on 64-bit systems.
>
>                Note:  Nowadays,  glibc uses a dynamic mmap threshold by
>                default.   The  initial  value  of  the   threshold   is
>                128*1024,  but  when  blocks  larger  than  the  current
>                threshold and less than or equal to DEFAULT_MMAP_THRESH‐
>                OLD_MAX  are freed, the threshold is adjusted upwards to
>                the size of the freed block.  When dynamic mmap  thresh‐
>                olding is in effect, the threshold for trimming the heap
>                is also dynamically adjusted to  be  twice  the  dynamic
>                mmap  threshold.  Dynamic adjustment of the mmap thresh‐
>                old  is  disabled  if  any  of   the   M_TRIM_THRESHOLD,
>                M_TOP_PAD, M_MMAP_THRESHOLD, or M_MMAP_MAX parameters is
>                set.

very nice!