| From: | "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com> |
|---|---|
| To: | Josh berkus <josh(at)agliodbs(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Peter Geoghegan <pg(at)heroku(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Bruce Momjian <bruce(at)momjian(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Logic behind parallel default? WAS: Rename max_parallel_degree? |
| Date: | 2016-05-31 18:12:23 |
| Message-ID: | 574DD407.6070107@commandprompt.com |
| Views: | Raw Message | Whole Thread | Download mbox | Resend email |
| Thread: | |
| Lists: | pgsql-hackers |
On 05/31/2016 11:05 AM, Josh berkus wrote:
> On 05/31/2016 11:00 AM, Tom Lane wrote:
>> ! If this occurs, the plan will run with fewer workers than expected,
>> ! which may be inefficient. The default value is 2. Setting this
>> ! value to 0 disables parallel query execution.
>
> Is there a thread on how we determined this default of 2? I can't find
> one under likely search terms.
>
> I'm concerned about the effect of overallocating parallel workers on
> systems which are already running out of cores (e.g. AWS instances), and
> run with default settings. Possibly max_parallel_workers takes care of
> this, which is why I want to understand the logic here.
>
I don't remember the thread but I seem to recall that the default of 2
is explicitly during Beta, RC etc... so that we can see what happens.
Robert could speak better to that.
JD
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