threads option is overridden before passing it to number-processes arg

[david-mart]

Hello, first of all I want to thank you for this amazing library.

When running ps aux during the import stage on a docker container

python3 /usr/local/bin/nominatim import --osm-file /nominatim/data.pbf --threads 24 --no-updates
/usr/local/lib/nominatim/osm2pgsql --hstore --latlon --slim --log-progress true --number-processes 1 --cache 0 --style /nominatim/style.lua --output flex --create --flat-nodes /nominatim/flatnode/flatnode.file /nominatim/data.pbf

I noticed that --number-processes is set to 1. After briefly looking through the code, I see that this value is overridden in the import script.

I was wondering if this is intentional, and if it's possible to bump it up?

[mtmail]

Quoting from #1408 (comment) (@lonvia is one of the maintainers of osm2psql and maintainer of Nominatim)

The number of processes was set to 1 intentionally. The osm2pgsql import itself is not multithreaded (or rather it is but not in any way that is influenced by this parameter). The only phase that is parallel is indexing. In this phase a high number of threads tends to be counter-productive. In my experience it is better to increase the maintenance memory to make creating a single index faster than increasing the number of parallel threads.