From: | Josh berkus <josh(at)agliodbs(dot)com> |
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To: | 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 18:45:54 |
Message-ID: | 574DDBE2.6020609@agliodbs.com |
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On 05/31/2016 11:10 AM, Tom Lane wrote:
> Josh berkus <josh(at)agliodbs(dot)com> writes:
>> Is there a thread on how we determined this default of 2? I can't find
>> one under likely search terms.
>
> The 9.6 open-items list cites
>
> https://www.postgresql.org/message-id/flat/20160420174631(dot)3qjjhpwsvvx5bau5(at)alap3(dot)anarazel(dot)de
Looks like we didn't decide for the release, just the beta.
I can see two ways to go for the final release:
1. Ship with max_parallel_X = 2 (or similar) and a very low
max_worker_processes (e.g. 4).
2. Ship with max_parallel_X = 1 (or 0, depending), and with a generous
max_worker_processes (e.g. 16).
Argument in favor of (1): we want parallelism to work out of the gate
for users running on low-concurrency systems. These settings would let
some parallelism happen immediately, without overwhelming a 4-to-8-core
system/vm. Tuning for the user would then be fairly easy, as we could
just tell them "set max_worker_processes to half the number of cores you
have".
Argument in favor of (2): parallelism is potentially risky for .0, and
as a result we want it disabled unless users choose to enable it.
Also, defaulting to off lets users make more use of the parallel_degree
table attribute to just enable parallelism on select tables.
Thoughts?
--
--
Josh Berkus
Red Hat OSAS
(any opinions are my own)
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