You could still have N threads but only want M (M<N) CPUs. For example there are situations where the single threads are mostly I/O bound, but reducing the litt

You could still have N threads but only want M (M<N) CPUs. For example there are situations where the single threads are mostly I/O bound, but reducing the little CPU work they do allows you to pack more threads (and e.g. serve more clients) on the same number of CPUs. Without restartable sequences, replacing per-thread data with per-CPU data had a trade-off: per-thread data can be accessed with less overhead, per-CPU data consumes less memory. Restartable sequences give you the best of both worlds.