Re: Shared memory and processes

> From: Thomas Munro [mailto:thomas.munro@enterprisedb.com]
> > Does Postgres provide a convenient way for one process to pass data to
> > another using shared memory?
>
> 1.  Look at ShmemInitStruct, ShmemInitHash (as in hash table), ShmemInitQueue
> in storage/shmem.h.  These use memory that is mapped at the same address in
> every backend (process) which is convenient for sharing data structures with
> internal pointers.  The amount of memory available is fixed at cluster
> startup however.

Thanks. That limit could be an issue. The notes in shmem.c are helpful.

> 2.  Look at dsm_XXX in storage/dsm.h.  This subsystem provides segments of
> memory that is "dynamic" in the sense that it is limited only by your
> system's available virtual memory, but you have to explicitly attach these
> segment in any backend that wants to access them by passing a handle around
> and the memory may be mapped at any address in each backend, so you need to
> work harder to build data structures that reference each other (using offsets
> rather than pointers, that kind of thing).  DSM segments won't work well if
> you try to create large numbers of them, so you'll need to provide a way to
> manage the space inside a smallish number of large chunks of DSM memory
> yourself (this is a problem I'm working to fix by providing a general purpose
> allocator backed by a bunch of DSM segments -- watch this space).  LWLocks
> (our usual lock primitive for cases where spinlocks are inappropriate)
> currently don't work correctly inside DSM segments (this too will be fixed).

I've now found this through the test_shm_mq sample. Looks like an answer, if quite a bit of machinery needed.

Thanks for the pointers.


Regards
David M Bennett FACS

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