As discussed earlier, using POSIX shared memory can solve a few issues,
On Mac OS X and other BSD's, the default System V shared memory limits are often very low and require adjustment for acceptable performance. Particularly, when Postgres is included as part of larger end-user friendly software products, these kernel settings are often difficult to change for 2 reasons:1. The (arbitrarily) limited resources must be shared by all programs that use System V shared memory. For example on my Mac OS X computer, I have Postgres running a standalone database, but also as part of Apple Remote Desktop. Without manual adjustment, running both simultaneously causes one of them to fail. Correcting this in any robust way is challenging to automate for consumer-style (i.e. Mac) installers.2. On these BSD's, this System V shared memory is wired down and cannot be swapped out for any reason. If Postgres is running as part of another software program or is a lower priority, other programs cannot use the potentially limited memory. This places the user or developer in a tricky position of having to minimize overall system impact, while permitting enough shared memory for Postgres to perform well.
Also, the SysV code is complex since it needs to deal with the (probable) likelihood that a shmid will collide with another program or postmaster.
Here is a new patch that uses the POSIX api's. It encodes the canonical path (see 'man realpath') of the database's data directory into the shared memory segment name using an strong hash function to make it fit in the shared memory segment name under all cases, without risk of key collision.
I have taken a new, simpler approach to handling databases that have been kill -9 or crashed. It is described in the comments, but essentially since all collisions in shared memory key must be from orphaned backends or crashed postmasters from the current data directory, they can be freed. A 2 character identifier field is prepended to the data directory hash, which is incremented after freeing an orphan, so that the new postmaster need not wait for the backends to die. This approach also works equally well on Windows as it does on Unixen. The comments also describe some of the portability concerns (which have been handled). Please see the code (PGSharedMemoryCreate and its helpers) for more information on this point.
To build/test this, place the attached file in src/backend/port/ and change the symbolic link pg_shmem.c to point to this file. If this gets used on BSD's, keep in mind that shared memory is no longer drawn from the SysV pool, so the SysV settings (SHMMAX, etc.) can be set to their default values to recover the memory that was wired down for the SysV pool.
I don't have access to any Linux machines to test this. Thanks for your feedback, Chris Marcellino
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