Have we made a decision on this issue? Should I apply my patch that still forces a split unless 10% of the page has been freed?
--------------------------------------------------------------------------- Tom Lane wrote: > ITAGAKI Takahiro <[EMAIL PROTECTED]> writes: > > This is a revised patch originated by Junji TERAMOTO for HEAD. > > [BTree vacuum before page splitting] > > http://archives.postgresql.org/pgsql-patches/2006-01/msg00301.php > > I think we can resurrect his idea because we will scan btree pages > > at-atime now; the missing-restarting-point problem went away. > > I've applied this but I'm now having some second thoughts about it, > because I'm seeing an actual *decrease* in pgbench numbers from the > immediately prior CVS HEAD code. Using > pgbench -i -s 10 bench > pgbench -c 10 -t 1000 bench (repeat this half a dozen times) > with fsync off but all other settings factory-stock, what I'm seeing > is that the first run looks really good but subsequent runs tail off in > spectacular fashion :-( Pre-patch there was only minor degradation in > successive runs. > > What I think is happening is that because pgbench depends so heavily on > updating existing records, we get into a state where an index page is > about full and there's one dead tuple on it, and then for each insertion > we have > > * check for uniqueness marks one more tuple dead (the > next-to-last version of the tuple) > * newly added code removes one tuple and does a write > * now there's enough room to insert one tuple > * lather, rinse, repeat, never splitting the page. > > The problem is that we've traded splitting a page every few hundred > inserts for doing a PageIndexMultiDelete, and emitting an extra WAL > record, on *every* insert. This is not good. > > Had you done any performance testing on this patch, and if so what > tests did you use? I'm a bit hesitant to try to fix it on the basis > of pgbench results alone. > > One possible fix that comes to mind is to only perform the cleanup > if we are able to remove more than one dead tuple (perhaps about 10 > would be good). Or do the deletion anyway, but then go ahead and > split the page unless X amount of space has been freed (where X is > more than just barely enough for the incoming tuple). > > After all the thought we've put into this, it seems a shame to > just abandon it :-(. But it definitely needs more tweaking. > > regards, tom lane > > ---------------------------(end of broadcast)--------------------------- > TIP 4: Have you searched our list archives? > > http://archives.postgresql.org -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDB http://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
