Re: [COMMITTERS] pgsql: Introduce dynamic shared memory areas.

On Mon, Dec 5, 2016 at 7:18 AM, Michael Paquier
<michael.paquier@gmail.com> wrote:
> On Fri, Dec 2, 2016 at 9:36 AM, Robert Haas <rhaas@postgresql.org> wrote:
>> Introduce dynamic shared memory areas.
>>
>> Programmers discovered decades ago that it was useful to have a simple
>> interface for allocating and freeing memory, which is why malloc() and
>> free() were invented.  Unfortunately, those handy tools don't work
>> with dynamic shared memory segments because those are specific to
>> PostgreSQL and are not necessarily mapped at the same address in every
>> cooperating process.  So invent our own allocator instead.  This makes
>> it possible for processes cooperating as part of parallel query
>> execution to allocate and free chunks of memory without having to
>> reserve them prior to the start of execution.  It could also be used
>> for longer lived objects; for example, we could consider storing data
>> for pg_stat_statements or the stats collector in shared memory using
>> these interfaces, rather than writing them to files.  Basically,
>> anything that needs shared memory but can't predict in advance how
>> much it's going to need might find this useful.
>>
>> Thomas Munro and Robert Haas.  The original code (of mine) on which
>> Thomas based his work was actually designed to be a new backend-local
>> memory allocator for PostgreSQL, but that hasn't gone anywhere - or
>> not yet, anyway.  Thomas took that work and performed major
>> refactoring and extensive modifications to make it work with dynamic
>> shared memory, including the addition of appropriate locking.
>
> This commit is generating a warning when compiling on my Win7 dev box:
> "C:\Users\ioltas\git\postgres\pgsql.sln" (default target) (1) ->
> "C:\Users\ioltas\git\postgres\postgres.vcxproj" (default target) (2) ->
> (ClCompile target) ->
>   src/backend/utils/mmgr/dsa.c(1921): warning C4334: '<<' : result of 32-bit sh
> ift implicitly converted to 64 bits (was 64-bit shift intended?) [C:\Users\iolt
> as\git\postgres\postgres.vcxproj]
>
>     1 Warning(s)
>     0 Error(s)

Hmm, I'm not sure I understand that warning.  I think the complaint is
about this line of code:
       Size        threshold = 1 << (bin - 1);

"bin" is declared as "Size", and threshold is also declared as "Size",
so what's the problem?  Is it unhappy that the "1" being shifted isn't
declared as 1L?  Is this a 32-bit system or a 64-bit system?  Or maybe
(Size) 1 << (bin - 1) would be safer?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company