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

["Michael Kerrisk (man-pages)" <mtk dot manpages at gmail dot com>]

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/