Re: [Patch] Optimize dropping of relation buffers using dlist

On Fri, Nov 27, 2020 at 11:36 AM Kyotaro Horiguchi
<horikyota(dot)ntt(at)gmail(dot)com> wrote:
>
> At Fri, 27 Nov 2020 02:19:57 +0000, "k(dot)jamison(at)fujitsu(dot)com" <k(dot)jamison(at)fujitsu(dot)com> wrote in
> > > From: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
> > > Hello, Kirk. Thank you for the new version.
> >
> > Hi, Horiguchi-san. Thank you for your very helpful feedback.
> > I'm updating the patches addressing those.
> >
> > > + if (!smgrexists(rels[i], j))
> > > + continue;
> > > +
> > > + /* Get the number of blocks for a relation's fork */
> > > + blocks[i][numForks] = smgrnblocks(rels[i], j,
> > > NULL);
> > >
> > > If we see a fork which its size is not cached we must give up this optimization
> > > for all target relations.
> >
> > I did not use the "cached" flag in DropRelFileNodesAllBuffers and use InRecovery
> > when deciding for optimization because of the following reasons:
> > XLogReadBufferExtended() calls smgrnblocks() to apply changes to relation page
> > contents. So in DropRelFileNodeBuffers(), XLogReadBufferExtended() is called
> > during VACUUM replay because VACUUM changes the page content.
> > OTOH, TRUNCATE doesn't change the relation content, it just truncates relation pages
> > without changing the page contents. So XLogReadBufferExtended() is not called, and
> > the "cached" flag will always return false. I tested with "cached" flags before, and this
>
> A bit different from the point, but if some tuples have been inserted
> to the truncated table, XLogReadBufferExtended() is called for the
> table and the length is cached.
>
> > always return false, at least in DropRelFileNodesAllBuffers. Due to this, we cannot use
> > the cached flag in DropRelFileNodesAllBuffers(). However, I think we can still rely on
> > smgrnblocks to get the file size as long as we're InRecovery. That cached nblocks is still
> > guaranteed to be the maximum in the shared buffer.
> > Thoughts?
>
> That means that we always think as if smgrnblocks returns "cached" (or
> "safe") value during recovery, which is out of our current
> consensus. If we go on that side, we don't need to consult the
> "cached" returned from smgrnblocks at all and it's enough to see only
> InRecovery.
>
> I got confused..
>
> We are relying on the "fact" that the first lseek() call of a
> (startup) process tells the truth. We added an assertion so that we
> make sure that the cached value won't be cleared during recovery. A
> possible remaining danger would be closing of an smgr object of a live
> relation just after a file extension failure. I think we are thinking
> that that doesn't happen during recovery. Although it seems to me
> true, I'm not confident.
>

Yeah, I also think it might not be worth depending upon whether smgr
close has been done before or not. I feel the current idea of using
'cached' parameter is relatively solid and we should rely on that.
Also, which means that in DropRelFileNodesAllBuffers() we should rely
on the same, I think doing things differently in this regard will lead
to confusion. I agree in some cases we might not get benefits but it
is more important to be correct and keep the code consistent to avoid
introducing bugs now or in the future.

--
With Regards,
Amit Kapila.