Re: Parallel threads in query

Hi,

On 2018-11-01 19:44:54 +0100, Tomas Vondra wrote:
> On 11/01/2018 07:40 PM, Andres Freund wrote:
> > On 2018-11-01 19:33:39 +0100, Tomas Vondra wrote:
> >> In theory, simulating such global limit should be possible using a bit
> >> of shared memory for the current total, per-process counter and probably
> >> some simple abort handling (say, just like contrib/openssl does using
> >> ResourceOwner).
> >
> > Right. I don't think you even need something resowner like, given that
> > anything using threads better make it absolutely absolutely impossible
> > that an error can escape.
> >
>
> True. Still, I wonder if the process can die in a way that would fail to
> update the counter.

You'd better make that case a panic restart.

> >> A better solution might be to start a bgworker managing a connection
> >> pool and forward the requests to it using IPC (and enforce the thread
> >> count limit there).
> >
> > That doesn't really seem feasible for cases like this - after all, you'd
> > only use threads to work on individual rows if you wanted to parallelize
> > relatively granular per-row work or such. Adding cross-process IPC seems
> > like it'd make that perform badly.
> >
>
> I think that very much depends on how expensive the tasks handled by the
> threads are. It may still be cheaper than a reasonable IPC, and if you
> don't create/destroy threads, that also saves quite a bit of time.

I'm not following. How can you have a pool *and* threads? Those seem to
be contradictory in PG's architecture? You need full blown IPC with your
proposal afaict?

Greetings,

Andres Freund