Re: Incorrect formula for SysV IPC parameters

On Fri, Feb 5, 2016 at 2:17 PM, Kyotaro HORIGUCHI
<horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> At Thu, 4 Feb 2016 21:43:04 +0900, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote in <CAHGQGwHgBsat29_ZqK3aXg4a5Lsa0JUv579VkGWX3R_g0KOncw(at)mail(dot)gmail(dot)com>
>> On Wed, Feb 3, 2016 at 12:51 PM, Kyotaro HORIGUCHI
>> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> > Hello, I found that the formulas to calculate SEMMNI and SEMMNS
>> > are incorrect in 9.2 and later.
>> >
>> > http://www.postgresql.org/docs/9.5/static/kernel-resources.html
>> >
>> > But actually the number of semaphores PostgreSQL needs is
>> > calculated as following in 9.4 and later.
> ...
>> > So, the formula for SEMMNI should be
>> >
>> > ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16)
>> >
>> > and SEMMNS should have the same fix.
>> >
>> >
>> > In 9.3 and 9.2, the documentation says the same thing but
> ...
>> > ceil((max_connections + autovacuum_max_workers + 5) / 16)
>> >
>> > In 9.1, NUM_AUXILIARY_PROCS is 3 so the documentations is correct.
>>
>> Good catch!
>
> Thanks.
>
>> ISTM that you also need to change the descriptions about SEMMNI and SEMMNS
>> under the Table 17-1.
>
> Oops! Thank you for pointing it out.
>
> The original description doesn't mention the magic-number ('5' in
> the above formulas, which previously was '4') so I haven't added
> anything about it.
>
> Process of which the number is limited by max_worker_processes is
> called 'background process' (not 'backend worker') in the
> documentation so I used the name to mention it in the additional
> description.
>
> The difference in the body text for 9.2, 9.3 is only a literal
> '4' to '5' in the formula.

Thanks for updating the patches!

They look good to me except that the formulas don't include the number of
background processes requesting shared memory access, i.e.,
GetNumShmemAttachedBgworkers(), in 9.3. Isn't it better to use the following
formula in 9.3?

ceil((max_connections + autovacuum_max_workers + number of
background proceses + 5) / 16)

Attached patch uses the above formula for 9.3. I'm thinking to push your
patches to 9.2, 9.4, 9.5, master, also push the attached one to 9.3.
Comments?

Regards,

--
Fujii Masao