Re: Rename max_parallel_degree?

On Tue, Jun 14, 2016 at 7:10 AM, Julien Rouhaud
<julien(dot)rouhaud(at)dalibo(dot)com> wrote:
>> I don't entirely like the new logic in
>> RegisterDynamicBackgroundWorker.
>
> I'm not that happy with it too. We can avoid iterating over every slots
> if the feature isn't activated though (max_parallel_workers >=
> max_worker_processes).
>
>> I wonder if we can't drive this off
>> of a couple of counters, instead of having the process registering the
>> background worker iterate over every slot. Suppose we add two
>> counters to BackgroundWorkerArray, parallel_register_count and
>> parallel_terminate_count. Whenever a backend successfully registers a
>> parallel worker, it increments parallel_register_count. Whenever the
>> postmaster marks a parallel wokrer slot as no longer in use, it
>> increments parallel_terminate_count. Then, the number of active
>> parallel workers is just parallel_register_count -
>> parallel_terminate_count. (We can't have the postmaster and the
>> backends share the same counter, because then it would need locking,
>> and the postmaster can't try to take spinlocks - can't even use
>> atomics, because those might be emulated using spinlocks.)
>>
>
> I wanted to maintain counters at first, but it seemed more invasive, and
> I thought that the max_parallel_worker would be ueful in environnements
> where there're lots of parallel workers and dynamic workers used, so
> finding a free slot would require iterating over most of the slots most
> of the time anyway. I'm of course also ok with maintaining counters.

I think we should go that way. Some day we might try to make the
process of finding a free slot more efficient than it is today; I'd
rather not double down on linear search.

Are you going to update this patch?

--
Robert Haas
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