On 21 September 2017 at 05:50, Thomas Munro <thomas.mu...@enterprisedb.com> wrote:
> On Thu, Sep 21, 2017 at 12:59 AM, Robert Haas <robertmh...@gmail.com> > wrote: > > On Wed, Sep 20, 2017 at 5:54 AM, Craig Ringer <cr...@2ndquadrant.com> > wrote: > >> By the way, dsa.c really needs a cross-reference to shm_toc.c and vice > >> versa. With a hint as to when each is appropriate. > > > > /me blinks. > > > > Aren't those almost-entirely-unrelated facilities? > > I think I see what Craig means. > > 1. A DSM segment works if you know how much space you'll need up > front so that you can size it. shm_toc provides a way to exchange > pointers into it with other backends in the form of shm_toc keys > (perhaps implicitly, in the form of well known keys or a convention > like executor node ID -> shm_toc key). Examples: Fixed sized state > for parallel-aware executor nodes, and fixed size parallel executor > infrastructure. > > 2. A DSA area is good if you don't know how much space you'll need > yet. dsa_pointer provides a way to exchange pointers into it with > other backends. Examples: A shared cache, an in-memory database > object like Gaddam Sai Ram's graph index extension, variable sized > state for parallel-aware executor nodes, the shared record typmod > registry stuff. > > Perhaps confusingly we also support DSA areas inside DSM segments, > there are DSM segments inside DSA areas. We also use DSM segments as > a kind of shared resource cleanup mechanism, and don't yet provide an > equivalent for DSA. I haven't proposed anything like that because I > feel like there may be a better abstraction of reliable scoped cleanup > waiting to be discovered (as I think Craig was also getting at). > Well said, and what I would've wanted to say if I could've figured it out well enough to express it. Hence needing some kind of README or cross reference to help people know which facility/facilities are suitable for their needs... and actually discover them. (A hint on RequestAddinShmemSpace etc pointing to DSM + DSA would be good too) -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
