Re: Perform streaming logical transactions by background workers and parallel apply

On Fri, Dec 16, 2022 at 2:47 PM houzj(dot)fnst(at)fujitsu(dot)com
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> > ---
> > + active_workers = list_copy(ParallelApplyWorkerPool);
> > +
> > + foreach(lc, active_workers)
> > + {
> > + int slot_no;
> > + uint16 generation;
> > + ParallelApplyWorkerInfo *winfo =
> > (ParallelApplyWorkerInfo *) lfirst(lc);
> > +
> > + LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
> > + napplyworkers =
> > logicalrep_pa_worker_count(MyLogicalRepWorker->subid);
> > + LWLockRelease(LogicalRepWorkerLock);
> > +
> > + if (napplyworkers <=
> > max_parallel_apply_workers_per_subscription / 2)
> > + return;
> > +
> >
> > Calling logicalrep_pa_worker_count() with lwlock for each worker seems
> > not efficient to me. I think we can get the number of workers once at
> > the top of this function and return if it's already lower than the
> > maximum pool size. Otherwise, we attempt to stop extra workers.
>
> How about we directly check the length of worker pool list here which
> seems simpler and don't need to lock ?
>

I don't see any problem with that. Also, if such a check is safe then
can't we use the same in pa_free_worker() as well? BTW, shouldn't
pa_stop_idle_workers() try to free/stop workers unless the active
number reaches below max_parallel_apply_workers_per_subscription?

--
With Regards,
Amit Kapila.