Re: [PATCHES] Resurrecting per-page cleaner for btree - Mailing list pgsql-hackers

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.

---------------------------------------------------------------------------

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.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
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 4: Have you searched our list archives?
> >
> >                http://archives.postgresql.org
>
> --
>   Bruce Momjian   bruce@momjian.us
>   EnterpriseDB    http://www.enterprisedb.com
>
>   + If your life is a hard drive, Christ can be your backup. +


>
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster

--
  Bruce Momjian   bruce@momjian.us
  EnterpriseDB    http://www.enterprisedb.com

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