[OSM-dev] Carto-style rendering and PostgreSQL parallel Worker

[Darafei "Komяpa" Praliaskouski]

Parallel workers should not kick in on the rendering setups as parallelism
there is driven by having metatiles rendered in parallel. Postgres
parallelism is for long running analytic queries, not fast "get me
everything in a small box" ones.

On Wed, May 5, 2021 at 9:59 PM Stephan Knauss <osm at stephans-server.de>
wrote:

> Hello,
>
> what is your experience with PostgreSQL parallel workers on a map
> rendering setup?
>
> On the psql mailing list there was a recent discussion regarding whether
> the default value for the planner to decide for parallel workers is too
> low.
>
> I decided to give it a try and disabled parallel workers completely by
> setting
>
> max_parallel_workers_per_gather = 0
>
> I do monitor the rendering throughput on Z10-20 and load value.
> I see no larger change in rendering throughput, but a significant
> reduction on the load value, as there is no longer a second process to
> be scheduled.
>
> What is your experience with parallel workers on a rendering setup?
> Does it bring benefit for you? Did you change the cost factors?
>
> I am rendering an extract and assume that a large part of the database
> is cached in RAM, other parts are on fast nvme storage. So the overhead
> in cost to set up a parallel worker might be a point to consider.
>
> I marked on the chart the time at which I disabled parallel workers:
>
> https://i.postimg.cc/xdLTHXrY/parallel-worker-load-rendering-rate.png
>
> Stephan
>
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>


-- 
Darafei "Komяpa" Praliaskouski
OSM BY Team - http://openstreetmap.by/
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