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@archidevsys.co.nz <mailto:GavinFlower@archidevsys.co.nz>> 
> wrote:
>
>     On 22/04/16 06:07, Robert Haas wrote:
>
>         On Thu, Apr 21, 2016 at 1:48 PM, Tom Lane <tgl@sss.pgh.pa.us
>         <mailto:tgl@sss.pgh.pa.us>> wrote:
>
>             Robert Haas <robertmhaas@gmail.com
>             <mailto:robertmhaas@gmail.com>> writes:
>
>                 On Wed, Apr 20, 2016 at 2:28 PM, Tom Lane
>                 <tgl@sss.pgh.pa.us <mailto:tgl@sss.pgh.pa.us>> wrote:
>
>                     Andres Freund <andres@anarazel.de
>                     <mailto:andres@anarazel.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