On Mon, May 5, 2014 at 3:09 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Sun, May 4, 2014 at 7:50 AM, Andres Freund <and...@2ndquadrant.com> wrote: >>> Thinking about this, I think it was a mistake to not add a 'name' field >>> to dynamic shared memory's dsm_control_item. > >> Well, right now a dsm_control_item is 8 bytes. If we add a name field >> of our usual 64 bytes, they'll each be 9 times bigger. > > And the controlled shared segment is likely to be how big exactly? It's > probably not even possible for it to be smaller than a page size, 4K or > so depending on the OS. I agree with Andres that a name would be a good > idea; complaining about the space needed to hold it is penny-wise and > pound-foolish.
The control segment is sized to support a number of dynamic shared memory segments not exceeding 64 + 2 *MaxBackends. With default settings, that currently works out to 288 segments, or 2306 bytes. So, adding a 64-byte name to each of those structures would increase the size from 2k to about 20k. So, sure, that's not a lot of memory. But I'm still not convinced that's it's very useful. What I think is going to happen is that (1) most people won't be used dynamic shared memory at all, so they won't have any use for this; (2) those people who do run an extension that uses dynamic shared memory will most likely only be running one such extension, so they won't need a name to know what the segments are being used for; and (3) if and when we eventually get parallel query, dynamic shared memory segments will be widely used, but a bunch of segments that are all named "parallel_query" or "parallel_query.$PID" isn't going to be too informative. Now, all that having been said, I recognize that human-readable names are a generally useful thing, so I'm not going to hold my breath until I turn blue if other people really want this, and it may turn out to be useful someday. But if anyone is curious whether I'm *confident* that it will be useful someday: at this point, no. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
