--jobs 10auto would set a value on each job: If the job fails and the current --jobs value is lower, then do not lower --jobs. This is to avoid having lowering too fast if --jobs starts waaay too high:

If the optimal value is 12, and we start at 100, then 6 failures will lower --jobs to 11.7 (100*0.7*0.7*0.7*0.7*0.7*0.7). But if the first 10 jobs fail, then we will hit 2.8 and will take much longer to ramp up to 12.

So instead when a job fails, only lower --jobs if the current --jobs is the same as when the job started.

Maybe:

    --jobs 8:3:1

8 jobslots, until first error. Then retry with 3, until next error. Then retry with 1.

    --jobs 100%:50%:30%:1

100% jobslots, until first error. Then retry with 50%, until next error. Then retry with 30%, until next error. Then retry with 1.

What do we do about already running jobs? If these fail, they will trigger the step down, too. Probably easiest to kill all running jobs when the failure happens (similar to --memfree).

Restart of a killed job should not affect --retries (similar to --memfree's new behaviour).

If -j > 1, when any of the jobs fail, option to auto-switch to -j1, before or after fulfilling the retries.