Re: shared-memory based stats collector

Hi,

Please find v60 of my version of Horiguchi-san's patch attached. It's now
pretty reasonably split, I think.

Major changes:

- several precursor patches committed

- Split pgstat.c into separate files (see below)

- split into reasonable-ish sized invidual commits, in particular no code is
moved around in the same commit with function changes

- stats for newly created objects are now dropped on abort

- a good bit of comment, correctness, ... cleanup

- quite a few AFIXME addition denoting places that would need to be fixed
before commit

I've spent most of the last 2 1/2 weeks on this now. Unfortunately I think
that, while it has gotten a lot closer, it's still about a week's worth of
work away from being committable.

My main concerns are:

- Test Coverage:

I've added a fair bit of tests, but it's still pretty bad. There were a lot
of easy-to-hit bugs in earlier versions that nevertheless passed the test
just fine.

Due to the addition of pg_stat_force_next_flush(), and that there's no need
to wait for the stats collector to write out files, it's now a lot more
realistic to have proper testing of a lot of the pgstat.c code.

- Architectural Review

I rejiggered the patchset pretty significantly, and I think it needs more
review than I see as realistic in the next two days. In particular I don't
think

- Performance Testing

I did a small amount, but given that this touches just about every query
etc, I think that's not enough. My changes unfortunately are substantial
enough to invalidate Horiguchi-san's earlier tests.

- Currently there's a corner case in which function (but not table!) stats
for a dropped function may not be removed. That possibly is not too bad,

- Too many FIXMEs still open

It is quite disappointing to not have the patch go into v14 :(. But I just
don't quite see the path right now. But maybe I am just too tired right now,
and it'll look better tomorrow (err today, in a few hours).

On 2021-04-02 10:27:23 -0700, Andres Freund wrote:
> On 2021-04-02 15:34:54 +0900, Kyotaro Horiguchi wrote:
> > At Thu, 1 Apr 2021 19:44:25 -0700, Andres Freund <andres(at)anarazel(dot)de> wrote in
> > > Hi,
> > >
> > > I spent quite a bit more time working on the patch. There's are large
> > > changes:
> > >
> > > - postmaster/pgstats.c (which is an incorrect location now that it's not
> > > a subprocess anymore) is split into utils/activity/pgstat.c and
> > > utils/activity/pgstat_kind.c. I don't love the _kind name, but I
> > > couldn't come up with anything better.
> >
> > The place was not changed to keep footprint smaller. I agree that the
> > old place is not appropriate. pgstat_kind... How about changin
> > pgstat.c to pgstat_core.c and pgstat_kind.c to pgstat.c?
>
> I don't really like that split over what I chose.

I now changed the split so that there is
utils/activity/pgstat_{database,functions,global,relation}.c

For me that makes the code a lot more readable. Before this change I found it
really hard to know where code should best be put etc, or where to find
code. I found it to be pretty nice to work with after the new split.

The only sad thing about the split is that pgstat_kind_infos now is defined in
pgstat.c, necessitating to expose the callback functions. Seems worth it.

Greetings,

Andres Freund