Re: same-address mappings vs. relative pointers

On 12/05/2013 06:32 AM, Robert Haas wrote:
> During development of the dynamic shared memory facility, Noah and I
> spent a lot of time arguing about whether it was practical to ensure
> that a dynamic shared memory segment got mapped at the same address in
> every backend that used it.

My vote goes for not trying to map at same address. I don't see how you 
could do that reliably, and I don't see much need for it anyway.

That said, it naturally depends on what you're going to use the dynamic 
shared memory facility for. It's the same problem I have with reviewing 
the already-committed DSM patch and the message queue patch. The patches 
look fine as far as they go, but I have the nagging feeling that there 
are a bunch of big patches coming up later that use the facilities, and 
I can't tell if the facilities are over-engineered for what's actually 
needed, or not sufficient.

As a side-note, I've been thinking that we don't really need 
same-address mapping for shared_buffers either. Getting rid of it 
wouldn't buy us anything right now, but if we wanted e.g to make 
shared_buffers changeable without a restart, that would be useful.

- Heikki