On Sat, Jul 31, 2010 at 12:32 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Sat, Jul 31, 2010 at 12:02 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> I'm tempted to suggest that making RBNode be a hidden struct containing >>> a pointer to somebody else's datum is fundamentally the wrong way to >>> go about things, because the extra void pointer is pure overhead, >>> and we aren't ever going to be using these things in a context where >>> memory usage isn't of concern. If we refactored the API so that RBNode >>> was intended to be the first field of some larger struct, as is done in >>> dynahash tables for instance, we could eliminate the void pointer and >>> the palloc inefficiency. > >> Even if we do that, is it still going to be too much of a performance >> regression overall? > > Looking back, EntryAccumulator was 56 bytes on 64-bit machines in 8.4 > (it should have been smaller, but a poor choice of field ordering left > a lot of pad space). Right now it's 32 bytes, and if we stick an RBNode > field in the front it'd be 64. So that'd be a 14% penalty compared to > 8.4, as opposed to the present situation which is a 100% penalty (32+80 > bytes per entry). On 32-bit machines the numbers are 32 bytes (8.4) > versus 20+40 (HEAD) versus 36 bytes (my proposal), so 12.5% penalty > versus 87.5%. (All of these numbers should be discounted by whatever > space you want to assume the pass-by-reference key datum takes.) > > So it'd definitely be a lot better than now. Maybe there'd be some > remaining performance gap for non-pathological cases, but I think we > would be willing to pay that in order to avoid bad behavior for the > pathological cases. It's difficult to say for sure of course > without going to the trouble of coding and testing it.
Well, it sounds like a reasonable thing to try, then. You going to take a crack at it? -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise Postgres Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers