Re: [HACKERS] Resurrecting per-page cleaner for btree

Tom, I ran your tests with fsync off (as you did), and saw numbers
bouncing between 400-700 tps without my patch, and sticking at 700 tps
with my patch.

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Bruce Momjian wrote:
>
> The attached patch requires the new row to fit, and 10% to be free on
> the page. Would someone test that?
>
> ---------------------------------------------------------------------------
>
> Tom Lane wrote:
> > ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)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(at)momjian(dot)us
> EnterpriseDB http://www.enterprisedb.com
>
> + If your life is a hard drive, Christ can be your backup. +

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--
Bruce Momjian bruce(at)momjian(dot)us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +