Re: Add parameter jit_warn_above_fraction

On Wed, 30 Mar 2022 at 02:38, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> I think WARNING is fine. After all, the parameter is called
> "jit_warn_above_fraction".

I had a think about this patch. I guess it's a little similar to
checkpoint_warning. The good thing about the checkpoint_warning is
that in the LOG message we give instructions about how the DBA can fix
the issue, i.e increase max_wal_size.

With the proposed patch I see there is no hint about what might be
done to remove/reduce the warnings. I imagine that's because it's not
all that clear which GUC should be changed. In my view, likely
jit_above_cost is the most relevant but there is also
jit_inline_above_cost, jit_optimize_above_cost, jit_tuple_deforming
and jit_expressions which are relevant too.

If we go with this patch, the problem I see here is that the amount
of work the JIT compiler must do for a given query depends mostly on
the number of expressions that must be compiled in the query (also to
a lesser extent jit_inline_above_cost, jit_optimize_above_cost,
jit_tuple_deforming and jit_expressions). The DBA does not really have
much control over the number of expressions in the query. All he or
she can do to get rid of the warning is something like increase
jit_above_cost. After a few iterations of that, the end result is
that jit_above_cost is now high enough that JIT no longer triggers
for, say, that query to that table with 1000 partitions where no
plan-time pruning takes place. Is that really a good thing? It likely
means that we just rarely JIT anything at all!

I really believe that the main problem here is that JIT only enables
when the *total* plan cost reaches a certain threshold. The number of
expressions to be compiled is not a factor in the decision at all.
That means that even if the total execution time of a plan was a true
reflection of the total estimated plan cost, then the fraction of time
spent (as is measured by jit_warn_above_fraction) doing JIT would
entirely depend on the number of expressions to compile. Of course,
the planner's not that good, but does that not indicate that the JIT
costing should really account for the number of expressions and not
just the total plan cost?

Anyway, what I'm trying to indicate here is that JIT is pretty much
impossible to tune properly and I don't really see why adding a
warning about it not being tuned correctly would help anyone. I think
it would be better to focus on making improvements to how the JIT
costing works.

I did propose a patch to address this in [1]. It does need more work
and I do plan to come back to it for v16.

I'd much rather see us address the costing problem before adding some
warning, especially a warning where it's not clear how to make go
away.

David

[1] https://www.postgresql.org/message-id/CAApHDvpQJqLrNOSi8P1JLM8YE2C+ksKFpSdZg=q6sTbtQ-v=aw@mail.gmail.com