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

[Siddhesh Poyarekar <siddhesh at sourceware dot org>]

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