Prefs pre-sets

A proposal for simplifying the prefs interface by providing a few "pre-sets" - named preference sets - as the default interface.

Note: another design doc proposes more sweeping changes. This doc doesn't propose changing the preferences, just the interface.

Proposed pre-sets

Max computing

Allow computing on all CPUs and GPUs, all the time. Memory limits: 90% while idle, 50% while busy

Standard

Busy: 50% CPUs, no GPU Idle: 100% of both

Energy-saver

Busy: 25% CPUs, GPU Idle: none (let the computer go into low-power mode or turn off)