Re: Parallel Seq Scan

On Wed, Dec 31, 2014 at 9:46 PM, Thom Brown <thom(at)linux(dot)com> wrote:
>
> Another issue (FYI, pgbench2 initialised with: pgbench -i -s 100 -F 10
pgbench2):
>
>
> ➤ psql://thom(at)[local]:5488/pgbench2
>
> # explain (analyse, buffers, verbose) select distinct bid from
pgbench_accounts;
> server closed the connection unexpectedly
> This probably means the server terminated abnormally
> before or while processing the request.
> The connection to the server was lost. Attempting reset: Failed.
> Time: 14897.991 ms
>
> 2014-12-31 15:21:57 GMT [9164]: [260-1] user=,db=,client= LOG: server
process (PID 10869) was terminated by signal 9: Killed
> 2014-12-31 15:21:57 GMT [9164]: [261-1] user=,db=,client= DETAIL: Failed
process was running: explain (analyse, buffers, verbose) select distinct
bid from pgbench_accounts;
> 2014-12-31 15:21:57 GMT [9164]: [262-1] user=,db=,client= LOG:
terminating any other active server processes
>
> Running it again, I get the same issue. This is with
parallel_seqscan_degree set to 8, and the crash occurs with 4 and 2 too.
>
> This doesn't happen if I set the pgbench scale to 50. I suspect this is
a OOM issue. My laptop has 16GB RAM, the table is around 13GB at scale
100, and I don't have swap enabled. But I'm concerned it crashes the whole
instance.
>

Isn't this a backend crash due to OOM?
And after that server will restart automatically.

> I also notice that requesting BUFFERS in a parallel EXPLAIN output yields
no such information.
> --

Yeah and the reason for same is that all the work done related
to BUFFERS is done by backend workers, master backend
doesn't read any pages, so it is not able to accumulate this
information.

> Is that not possible to report?

It is not impossible to report such information, we can develop some
way to share such information between master backend and workers.
I think we can do this if required once the patch is more stablized.

Thanks for looking into patch and reporting the issues.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com