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