Re: Logging parallel worker draught

Hi,

I see the thread went a bit quiet, but I think it'd be very useful (and
desirable) to have this information in log. So let me share my thoughts
about the patch / how it should work.

The patch is pretty straightforward, I don't have any comments about the
code as is. Obviously, some of the following comments might require
changes to the patch, etc.

I see there was discussion about logging vs. adding this to the pgstats
system. I agree with Alvaro it should not be one or the other - these
are complementary approaches, used by different tools. The logging is
needed for tools like pgbadger etc. for example.

I do see a lot of value in adding this to the statistics collector, and
to things like pg_stat_statements, but that's being discussed in a
separate thread, so I'll comment on that there.

As for whether to have a GUC for this, I certainly think we should have
one. We have GUC for logging stuff that we generally expect to happen,
like lock waits, temp files, etc.

True, there are similar cases that we just log every time, like when we
can't fork a process ("could not fork autovacuum launcher process"), but
I'd say those are unexpected to happen in general / seem more like an
error in the environment. While we may exhaust parallel workers pretty
easily / often, especially on busy systems.


There's a couple things I'm not quite sure about:


1) name of the GUC

I find the "log_parallel_worker_draught" to be rather unclear :-( Maybe
it's just me and everyone else just immediately understands what this
does / what will happen after it's set to "on", but I find it rather
non-intuitive.


2) logging just the failures provides an incomplete view

As a DBA trying to evaluate if I need to bump up the number of workers,
I'd be interested what fraction of parallel workers fails to start. If
it's 1%, that's probably a short spike and I don't need to do anything.
If it's 50%, well, that might have unpredictable impact on user queries,
and thus something I may need to look into. But if we only log the
failures, that's not possible.

I may be able to approximate this somehow by correlating this with the
query/transaction rate, or something, but ideally I'd like to know how
many parallel workers we planned to use, and how many actually started.

Obviously, logging this for every Gather [Merge] node, even when all the
workers start, that may be a lot of data. Perhaps the GUC should not be
on/off, but something like

log_parallel_workers = {none | failures | all}

where "failures" only logs when at least one worker fails to start, and
"all" logs everything.

AFAIK Sami made the same observation/argument in his last message [1].


3) node-level or query-level?

There's a brief discussion about whether this should be a node-level or
query-level thing in [2]:

> I wonder if it will be better to accumulate the total # of workers
> planned and # of workers launched and logging this information at the
> end of execution?

And there's a reference to log_temp_files, but it's not clear to me if
that's meant to be an argument for doing it the same way (node-level).

I think we should not do node-level logging just because that's what
log_temp_files=on dies. I personally think log_temp_files was
implemented like this mostly because it was simple.

There's no value in having information about every individual temp file,
because we don't even know which node produced it. It'd be perfectly
sufficient to log some summary statistics (number of files, total space,
...), either for the query as a whole, or perhaps per node (because
that's what matters for work_mem). So I don't think we should mimic this
just because log_temp_files does this.

I don't know how difficult would it be to track/collect this information
for the whole query.


regards


[1]
https://www.postgresql.org/message-id/D04977E3-9F54-452C-A4C4-CDA67F392BD1%40amazon.com

[2]
https://www.postgresql.org/message-id/11e34b80-b0a6-e2e4-1606-1f5077379a34%40dalibo.com
-- 
Tomas Vondra
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