Re: Posix Shared Mem patch

On Tue, Jun 26, 2012 at 6:25 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Josh Berkus <josh@agliodbs.com> writes:
>> So let's fix the 80% case with something we feel confident in, and then
>> revisit the no-sysv interlock as a separate patch.  That way if we can't
>> fix the interlock issues, we still have a reduced-shmem version of Postgres.
>
> Yes.  Insisting that we have the whole change in one patch is a good way
> to prevent any forward progress from happening.  As Alvaro noted, there
> are plenty of issues to resolve without trying to change the interlock
> mechanism at the same time.

So, here's a patch.  Instead of using POSIX shmem, I just took the
expedient of using mmap() to map a block of MAP_SHARED|MAP_ANONYMOUS
memory.  The sysv shm is still allocated, but it's just a copy of
PGShmemHeader; the "real" shared memory is the anonymous block.  This
won't work if EXEC_BACKEND is defined so it just falls back on
straight sysv shm in that case.

There are obviously some portability issues here - this is documented
not to work on Linux <= 2.4, but it's not clear whether it fails with
some suitable error code or just pretends to work and does the wrong
thing.  I tested that it does compile and work on both Linux 3.2.6 and
MacOS X 10.6.8.  And the comments probably need work and... who knows
what else is wrong.  But, thoughts?

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