Re: tds_fdw : Severe performance degradation from postgresql 10.23 to 16.4

On 11/9/24 15:05, Achilleas Mantzios wrote:
>
> Στις 9/11/24 12:49, ο/η Tomas Vondra έγραψε:
>> On 11/8/24 20:32, Achilleas Mantzios wrote:
>>> Dear All,
>>>
>>> we have hit a serious performance regression going from 10.23 → 16.4 as
>>> far as tds_fdw (MS SQL) FDW is concerned. To cut the long story short, I
>>> recreated the good fast “old” (pgsql 10) setup on the same vm as the
>>> slow “new” (pgsql 16). Here is the bug report on github :
>>>
>>> https://github.com/tds-fdw/tds_fdw/issues/371
>>>
>>> All environment on the two pgsql clusters is shared (freetds version,
>>> tds_fdw, gcc, llvm). Only thing differs are pgsql versions. The speed on
>>> the old pgsql 10.23 is about 10 to 20 times higher than pgsql 16.4 . |
>>> Setting client_min_messages TO debug3 does yield identical output on the
>>> two systems.
>>> |
>>>
>>> The new pgsql 16.4 shows 100% CPU usage during the query execution.
>>>
>>> I know we are pretty much alone with this, but it would be very nice if
>>> anyone could help, see smth we are missing or guide us via the right
>>> path.
>>>
>> I have little experience with tds_fdw, and can't investigate that
>> locally. But it might be interesting to compare CPU profiles for the two
>> (slow and fast) cases. Chances are the difference will be an indication
>> regarding what got that slower. It might be something in PG or in the
>> FDW, hard to say.
> Thank you Tomas, do you have linux-perf in mind? How could we get the
> CPU profiles of the two ?  Just in case it was not clear, the tests were
> done on the same VM, at the same time, having the two DBs listening on
> different ports. Kinda spooky from my part to do that on the production
> system, but I wanted everything to be the same (latency to/from the ms
> sql, etc) except the pgsql version, to prove my assumption, and it seems
> pgsql version is what makes the difference (in conjunction of course
> with tds_fdw ) .
>>

Yes, I mean linux-perf. There's a wikipage with some basic instructions:

https://wiki.postgresql.org/wiki/Profiling_with_perf

But in short, I'd do this:

1) use pg_backend_pid() to get PID of the backend

SELECT pg_backend_pid();

2) start recording profile for that PID, say for 1 minute

perf record -g -p $PID -- sleep 60

3) run the query in a loop

SELECT count(*) as foo FROM mssql_bdynacom."ACDOC" \watch 1

4) once the recording stops, generate report

perf report > report.txt

Do this for both versions, share the output txt files. You may need to
install additional packages with debug symbols to get better profiles,
and stuff like that.

Also, don't forget to share the explain plans.

regard

--
Tomas Vondra