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 <itagaki.takahiro@oss.ntt.co.jp> 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
>
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Bruce Momjian bruce@momjian.us
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