On Sat, Jul 31, 2010 at 12:02 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Sat, Jul 31, 2010 at 12:40 AM, Tom Lane <t...@sss.pgh.pa.us> wrote: >>> So, I would like somebody to show cause why that whole module shouldn't >>> be ripped out and the code reverted to where it was in 8.4. My >>> recollection is that the argument for adding it was to speed things up >>> in corner cases, but what I think it's actually going to do is slow >>> things down in every case. > >> I've always been a bit suspicious of this code, too, even though I >> didn't think about the memory consumption issue. But see here: >> http://archives.postgresql.org/pgsql-hackers/2010-02/msg00307.php > > I did a bit of experimentation and confirmed my fears: HEAD is willing > to eat about double the specified maintenance_work_mem. If you cut > back the setting so that its actual memory use is no more than 8.4's, > it's about 33% slower on non-pathological data (I'm testing the dataset > from Artur Dabrowski here).
That seems like a pretty serious regression. > 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. The added storage compared to what 8.4 used > would be a parent link and the iteratorState/color fields, which would > end up costing us 16 more bytes per EntryAccumulator rather than 64. > Still not great but at least it's not a 2X penalty, and the memory > allocation would become the caller's problem not rbtree's, so the > problem of tracking usage would be no different from before. Even if we do that, is it still going to be too much of a performance regression overall? -- 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
