From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
To: siddhesh@sourceware.org, Carlos O'Donell <carlos@redhat.com>
Cc: mtk.manpages@gmail.com, libc-alpha <libc-alpha@sourceware.org>
Subject: Re: [PATCH 2/2] Document the M_ARENA_* mallopt parameters
Date: Tue, 18 Oct 2016 16:03:00 -0000 [thread overview]
Message-ID: <ca2e2f8d-f889-2dc6-3c30-9894403044e2@gmail.com> (raw)
In-Reply-To: <33f30947-9f11-363e-3f35-6c593632f000@sourceware.org>
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/
next prev parent reply other threads:[~2016-10-18 16:03 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-10 17:26 [PATCH 1/2] Add note on MALLOC_MMAP_* environment variables Siddhesh Poyarekar
2016-10-10 17:26 ` [PATCH 2/2] Document the M_ARENA_* mallopt parameters Siddhesh Poyarekar
2016-10-17 14:04 ` [PING][PATCH " Siddhesh Poyarekar
2016-10-17 16:09 ` [PATCH " Carlos O'Donell
2016-10-18 7:15 ` Michael Kerrisk
2016-10-18 10:07 ` Siddhesh Poyarekar
2016-10-18 13:50 ` Michael Kerrisk (man-pages)
2016-10-18 14:30 ` Siddhesh Poyarekar
2016-10-18 16:03 ` Michael Kerrisk (man-pages) [this message]
2016-10-18 16:46 ` Siddhesh Poyarekar
2016-10-18 17:18 ` Andreas Schwab
2016-10-19 6:53 ` Michael Kerrisk (man-pages)
2016-10-19 7:09 ` Siddhesh Poyarekar
2016-10-10 17:33 ` [PATCH 1/2] Add note on MALLOC_MMAP_* environment variables DJ Delorie
2016-10-10 17:42 ` Siddhesh Poyarekar
2016-10-11 6:20 ` Michael Kerrisk
2016-10-11 18:19 ` DJ Delorie
2016-10-11 19:03 ` Michael Kerrisk (man-pages)
2016-10-12 11:57 ` Siddhesh Poyarekar
2016-10-17 19:40 ` Carlos O'Donell
2016-10-11 19:35 ` Kalle Olavi Niemitalo
2016-10-17 14:04 ` [PING][PATCH " Siddhesh Poyarekar
2016-10-17 16:13 ` [PATCH " Carlos O'Donell
2016-10-17 16:16 ` Siddhesh Poyarekar
2016-10-24 14:08 [PATCH 0/2] Malloc manual cleanups Siddhesh Poyarekar
2016-10-24 14:08 ` [PATCH 2/2] Document the M_ARENA_* mallopt parameters Siddhesh Poyarekar
2016-10-25 7:51 ` Rical Jasan
2016-10-25 21:55 ` DJ Delorie
2016-10-26 5:42 ` Siddhesh Poyarekar
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