Improve speed of the test harness

[amotl]

Is your feature request related to a problem?

The speed of the test harness can be tremendously improved.

Describe the solution you'd like

By following @andersy005 on behalf of pangeo-data/climpred#515, I just gave pytest-xdist a shot. The outcome is promising.

Before

time pytest

real	2m33.294s
user	1m3.104s
sys	0m12.530s

After

time pytest -n auto

real	0m44.155s
user	1m2.047s
sys	0m14.962s

cc @kmuehlbauer

[kmuehlbauer]

Yes, this works quite nice on a capable machine. On CI I've had different results (most CI have only 2 virtual CPU), where the gain wasn't that obvious. One thing to keep in mind, if unspecific errors are occurring, this might be due to race conditions between different parallel tests.

Thanks for sharing, @amotl .

[amotl]

Two updates on this.

• As outlined at Improve speed of the test harness by using "pytest-xdist" #254 (comment), we will have to get rid of the dbmfile-based cache in order to mitigate concurrency issues, see also Get rid of dbmfile-based dogpile cache #243.
• As outlined at Run radar tests including wradlib on GHA #241 (comment), we can also improve the speed of the test harness by bringing actions/cache into the mix in order to cache dependencies, effectively not having to go to the network each time for downloading them over and over again.

[gutzbenj]

I think this is more or less fixed. Other work has to be done probably on GH CI side to get this working in our pipelines.