Re: Defining (and possibly skipping) useless VACUUM operations

On Tue, Dec 14, 2021 at 6:05 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> I think this is a reasonable line of thinking, but I think it's a
> little imprecise. In general, we could be vacuuming a relation to
> advance relfrozenxid, but we could also be vacuuming a relation to
> advance relminmxid, or we could be vacuuming a relation to fight
> bloat, or set pages all-visible. It is possible that there's no hope
> of advancing relfrozenxid but that we can still accomplish one of the
> other goals. In that case, the vacuuming is not useless. I think the
> place to put logic around this would be in the triggering logic for
> autovacuum. If we're going to force a relation to be vacuumed because
> of (M)XID wraparound danger, we could first check whether there seems
> to be any hope of advancing relfrozenxid(minmxid). If not, we discount
> that as a trigger for vacuum, but may still decide to vacuum if some
> other trigger warrants it. In most cases, if there's no hope of
> advancing relfrozenxid, there won't be any bloat to remove either, but
> aborted transactions are a counterexample. And the XID and MXID
> horizons can advance at completely different rates.

I think that you'd agree that the arguments in favor of skipping are
strongest for an aggressive anti-wraparound autovacuum (as opposed to
any other kind of aggressive VACUUM, including aggressive autovacuum).
Aside from the big benefit I pointed out already (avoiding blocking
useful anti-wraparound vacuums that starts a little later by not
starting a conflicting useless anti-wraparound vacuum now), there is
also more certainty about downsides. We can know the following things
for sure:

* We only launch an (aggressive) anti-wraparound autovacuum because we
need to advance relfrozenxid. In other words, if we didn't need to
advance relfrozenxid then (for better or worse) we definitely wouldn't
be launching anything.

* Our would-be OldestXmin exactly matches the preexisting
pg_class.relfrozenxid (and pg_class.relminmxid). And so it follows
that we're definitely not going to be able to do the thing that is
ostensibly the whole point of anti-wraparound vacuum (advance
relfrozenxid/relminmxid).

> One reason I haven't pursued this kind of optimization is that it
> doesn't really feel like it's fixing the whole problem. It would be a
> little bit sad if we did a perfect job preventing useless vacuuming
> but still allowed almost-useless vacuuming. Suppose we have a 1TB
> relation and we trigger autovacuum. It cleans up a few things but
> relfrozenxid is still old. On the next pass, we see that the
> system-wide xmin has not advanced, so we don't trigger autovacuum
> again. Then on the pass after that we see that the system-wide xmin
> has advanced by 1. Shall we trigger an autovacuum of the whole
> relation now, to be able to do relfrozenxid++? Seems dubious.

I can see what you mean, but just fixing the most extreme case can be
a useful goal. It's often enough to stop the system from going into a
tailspin, which is the real underlying goal here. Things that approach
the most extreme case (but don't quite hit it) don't have that
quality.

An anti-wraparound vacuum is supposed to be a mechanism that the
system escalates to when nothing else triggers an autovacuum worker to
run (which is aggressive but not anti-wraparound). That's not really
true in practice, of course; anti-wraparound av often becomes a
routine thing. But I think that it's a good ideal to strive for -- it
should be rare.

The draft patch series now adds opportunistic freezing -- I should be
able to post a new version in a few days time, once I've tied up some
loose ends. My testing shows an interesting effect, when opportunistic
freezing is applied on top of the relfrozenxid thing: every autovacuum
manages to advance relfrozenxid, and so we'll never have to run an
aggressive autovacuum (much less an aggressive anti-wraparound
autovacuum) in practice. And so (for example) when autovacuum runs
against the pgbench_history table, it always sets its relfrozenxid to
a value very close to the OldestXmin -- usually the exact OldestXmin.

Opportunistic freezing makes us avoid setting the all-visible bit for
a heap page without also setting the all-frozen bit -- when we're
about to do that, we go freeze the heap tuples and then set the entire
page all-frozen (so we freeze anything <= OldestXmin, not <=
FreezeLimit). We also freeze based on this more aggressive <=
OldestXmin cutoff when pruning had to delete some tuples.

The patch still needs more polishing, but I think that we can make
anti-wraparound vacuums truly exceptional with this design -- which
would make autovacuum a lot easier to deal with operationally. This
seems like a feasible goal for Postgres 15, even (though still quite
ambitious). The opportunistic freezing stuff isn't free (the WAL
records aren't tiny), but it's still not all that expensive. Plus I
think that the cost can be further reduced, with a little more work.

> Part of the problem here, for both vacuuming-for-bloat and
> vacuuming-for-relfrozenxid-advancement, we would really like to know
> the distribution of old XIDs in the table.

What I see with the draft patch series is that the oldest XID just
isn't that old anymore, consistently -- we literally never fail to
advance relfrozenxid, in any autovacuum, for any table. And the value
that we end up with is consistently quite recent. This is something
that I see both with BenchmarkSQL, and pgbench. There is a kind of
virtuous circle, which prevents us from ever getting anywhere near
having any table age in the tens of millions of XIDs.

I guess that that makes avoiding useless vacuuming seem like less of a
priority. ISTM that it should be something that is squarely aimed at
keeping things stable in truly pathological cases.

> So I'm not certain of the way forward here. Just because we can't
> prevent almost-useless vacuuming is not a sufficient reason to
> continue allowing entirely-useless vacuuming that we can prevent. And
> it seems like we need a bunch of new bookkeeping to do any better than
> that, which seems like a lot of work. So maybe it's the most practical
> path forward for the time being, but it feels like more of a
> special-purpose kludge than a truly high-quality solution.

I'm sure that either one of us will be able to poke holes in any
definition of "useless" that is continuous (rather than discrete) --
which, on reflection, pretty much means any definition that is
concerned with bloat. I think that you're right about that: the
question there must be "why are we even launching these
bloat-orientated autovacuums that actually find no bloat?".

--
Peter Geoghegan