Re: max_parallel_degree > 0 for 9.6 beta

On 23/04/16 00:56, Robert Haas wrote:
> On Thu, Apr 21, 2016 at 7:20 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> On Thu, Apr 21, 2016 at 4:01 PM, Gavin Flower
>>> <GavinFlower(at)archidevsys(dot)co(dot)nz> wrote:
>>>> Why not 4? As most processors now have at least 4 physical cores, & surely
>>>> it be more likely to flush out race conditions.
>>> Because if we did that, then it's extremely likely that people would
>>> end up writing queries that are faster only if workers are present,
>>> and then not get any workers.
>> Is that because max_worker_processes is only 8 by default? Maybe we
>> need to raise that, at least for beta purposes?
> I'm not really in favor of that. I mean, almost all of our default
> settings are optimized for running PostgreSQL on, for example, a
> Raspberry Pi 2, so it would seem odd to suddenly swing the other
> direction and assume that there are more than 8 unused CPU cores. It
> doesn't make sense to me to roll out settings in beta that we wouldn't
> be willing to release with if they work out. That's why, honestly, I
> would prefer max_parallel_degree=1, which I think would be practical
> for many real-world deployments. max_parallel_degree=2 is OK. Beyond
> that, we're just setting people up to fail, I think. Higher settings
> should probably only be used on substantial hardware, and not
> everybody has that.
>
If Java can find out how many processors there are available to it,
since JDK1.4, then surely PostgreSQL can do the same?

So how about the default being half the available processors rounded up
to the nearest integer?

Perhaps the GUC for workers should be a percentage of the available
processors, with the minimum & maximum workers optionally specified - or
something of that nature?

Cheers,
Gavin