Re: max_parallel_degree > 0 for 9.6 beta

On 22/04/16 17:36, Amit Kapila wrote:
> On Fri, Apr 22, 2016 at 1:31 AM, Gavin Flower
> <GavinFlower(at)archidevsys(dot)co(dot)nz <mailto:GavinFlower(at)archidevsys(dot)co(dot)nz>>
> wrote:
>
> On 22/04/16 06:07, Robert Haas wrote:
>
> On Thu, Apr 21, 2016 at 1:48 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us
> <mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us>> wrote:
>
> Robert Haas <robertmhaas(at)gmail(dot)com
> <mailto:robertmhaas(at)gmail(dot)com>> writes:
>
> On Wed, Apr 20, 2016 at 2:28 PM, Tom Lane
> <tgl(at)sss(dot)pgh(dot)pa(dot)us <mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us>> wrote:
>
> Andres Freund <andres(at)anarazel(dot)de
> <mailto:andres(at)anarazel(dot)de>> writes:
>
> max_parallel_degree currently defaults to 0.
> I think we should enable
> it by default for at least the beta period.
> Otherwise we're primarily
> going to get reports back after the release.
>
> So, I suggest that the only sensible non-zero values
> here are probably
> "1" or "2", given a default pool of 8 worker processes
> system-wide.
> Andres told me yesterday he'd vote for "2". Any other
> opinions?
>
> It has to be at least 2 for beta purposes, else you are
> not testing
> situations with more than one worker process at all, which
> would be
> rather a large omission no?
>
> That's what Andres, thought, too. From my point of view, the big
> thing is to be using workers at all. It is of course possible
> that
> there could be some bugs where a single worker is not enough, but
> there's a lot of types of bug where even one worker would probably
> find the problem. But I'm OK with changing the default to 2.
>
> I'm curious.
>
> Why not 4?
>
>
> IIUC, the idea to change max_parallel_degree for beta is to catch any
> bugs in parallelism code, not to do any performance testing of same.
> So, I think either 1 or 2 should be sufficient to hit the bugs if
> there are any. Do you have any reason to think that we might miss
> some category of bugs if we don't use higher max_parallel_degree?
>
>
> With Regards,
> Amit Kapila.
> EnterpriseDB: http://www.enterprisedb.com <http://www.enterprisedb.com/>
No. Just felt that 4 would not be too great for the type of processor
chips used on servers to handle.

For complications, such as race conditions and implied logical
assumptions - I tend to think of 0, 1, 2, 3, many.

Essentially just a gut feeling that 4 might reveal more corner cases.

Cheers,
Gavin