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

[Stephan Knauss]

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