Re: pg_walinspect memory leaks

On Tue, Feb 14, 2023 at 4:07 PM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> On Tue, Feb 14, 2023 at 6:25 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> >
> > Hi,
> >
> > On 2023-02-13 15:22:02 -0800, Peter Geoghegan wrote:
> > > More concretely, it looks like GetWALRecordInfo() calls
> > > CStringGetTextDatum/cstring_to_text in a way that accumulates way too
> > > much memory in ExprContext.
> >
> > Additionally, we leak two stringinfos for each record.
> >
> >
> > > This could be avoided by using a separate memory context that is reset
> > > periodically, or something else along the same lines.
> >
> > Everything other than a per-row memory context that's reset each time seems
> > hard to manage in this case.
> >
> > Somehwat funnily, GetWALRecordsInfo() then ends up being unnecessarily
> > dilligent about cleaning up O(1) memory, after not caring about O(N) memory...
>
> Thanks for reporting. I'll get back to you on this soon.

The memory usage goes up with many WAL records in GetWALRecordsInfo().
The affected functions are pg_get_wal_records_info() and
pg_get_wal_records_info_till_end_of_wal(). I think the best way to fix
this is to use a temporary memory context (like the jsonfuncs.c),
reset it after every tuple is put into the tuple store. This fix keeps
the memory under limits. I'm attaching the patches here. For HEAD, I'd
want to be a bit defensive and use the temporary memory context for
pg_get_wal_fpi_info() too.

And, the fix also needs to be back-patched to PG15.

[1]
HEAD:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+
COMMAND
1105979 ubuntu 20 0 28.5g 28.4g 150492 R 80.7 93.0 1:47.12
postgres

PATCHED:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+
COMMAND
13149 ubuntu 20 0 173244 156872 150688 R 79.0 0.5 1:25.09
postgres

postgres=# select count(*) from
pg_get_wal_records_info_till_end_of_wal('0/1000000');
count
----------
35285649
(1 row)

postgres=# select pg_backend_pid();
pg_backend_pid
----------------
13149
(1 row)

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com