How to fix parallel_tests with Redis on powerful machines

When you have a powerful machine with many CPU cores, you might run into an error like

ERR DB index is out of range (Redis::CommandError)

This is because Redis defaults to at most 16 databases (0 to 15) and running tests in parallel might exceed that (your tests might run on databases 1..n or 2..(n+1)).

You can increase that limit:

1. Get number of CPUs of your machine.

   nproc --all
   

2. Open up Redis configuration file.

   sudo vim /etc/redis/redis.conf
   

3. Find databases row and increase it, e.g. set to CPU count + 2:

   databases 18
   

4. Save and close config file, then restart Redis:

   sudo service redis-server restart
   

5. Confirm your configuration has been updated. The following command should return "OK".

   redis-cli select `nproc --all`