Re: Parallel Seq Scan

On 2 January 2015 at 11:13, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:

>
> On Fri, Jan 2, 2015 at 4:09 PM, Thom Brown <thom(at)linux(dot)com> wrote:
> >
> > On 1 January 2015 at 10:34, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >>
> >> > 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.
> >
> >
> > Yes, I'm fairly sure it is. I guess what I'm confused about is that 8
> parallel sequential scans in separate sessions (1 per session) don't cause
> the server to crash, but in a single session (8 in 1 session), they do.
> >
>
> It could be possible that master backend retains some memory
> for longer period which causes it to hit OOM error, by the way
> in your test does always master backend hits OOM or is it
> random (either master or worker)
>

Just ran a few tests, and it appears to always be the master that hits OOM,
or at least I don't seem to be able to get an example of the worker hitting
it.

>
> >
> > Will there be a GUC to influence parallel scan cost? Or does it take
> into account effective_io_concurrency in the costs?
> >
> > And will the planner be able to decide whether or not it'll choose to
> use background workers or not? For example:
> >
>
> Yes, we are planing to introduce cost model for parallel
> communication (there is some discussion about the same
> upthread), but it's still not there and that's why you
> are seeing it to choose parallel plan when it shouldn't.
> Currently in patch, if you set parallel_seqscan_degree, it
> will most probably choose parallel plan only.
>

Ah, okay. Great.

Thanks.

Thom