Re: Less than ideal error reporting in pg_stat_statements

On Wed, Sep 23, 2015 at 3:01 AM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> I think that the real problem here is that garbage collection needs to
> deal with OOM more appropriately.

+1

I've also been seeing lots of log messages saying "LOG: out of
memory" on a server that's hosting development databases. I put off
debugging this until now because it didn't seem to have any adverse
effects on the system.

The file on my system is currently 5.1GB (!). I don't know how it got
there -- under normal circumstances we don't have any enormous
queries, but perhaps our application bugs during development triggered
that.

The configuration on this system is pg_stat_statements.max = 10000 and
pg_stat_statements.track = all.

----
The comment near gc_qtexts says:
* This won't be called often in the typical case, since it's likely that
* there won't be too much churn, and besides, a similar compaction process
* occurs when serializing to disk at shutdown or as part of resetting.
* Despite this, it seems prudent to plan for the edge case where the file
* becomes unreasonably large, with no other method of compaction likely to
* occur in the foreseeable future.
[...]
* Load the old texts file. If we fail (out of memory, for instance) just
* skip the garbage collection.

So, as I understand it: if the system runs low on memory for an
extended period, and/or the file grows beyond 1GB (MaxAlloc), garbage
collection stops entirely, meaning it starts leaking disk space until
a manual intervention.

It's very frustrating when debugging aides cause further problems on a
system. If the in-line compaction doesn't materialize (or it's decided
not to backport it), I would propose instead to add a test to
pgss_store() to avoid growing the file beyond MaxAlloc (or perhaps
even a lower limit). Surely dropping some statistics is better than
this pathology.

Regards,
Marti