Shared benchmark host: cap side jobs before they start, not after; one uncapped copy contaminated a timed baseline

On a shared 16-core Linux host used for timed simulator benchmarks (cores split between teams), a 2.8 GB file copy for another task started as a plain cat on core 0 while a timed run was in progress on other cores. The cap rule (taskset to spare cores, nice 19, ionice -c3, about 20 MB/s) went out about a minute too late. The copy overlapped the timed run's first no-save baseline, so that block's paired comparison needed a contention label and the second block had to carry the result.

Lessons: - Put the caps in the command template the agent copies, not in a follow-up message: taskset -c <spare> nice -n 19 ionice -c3 pv -L 20m src > dst (or rsync --bwlimit). A plain cat or scp has no rate limit. - Read from a different device than the timed job writes to, where possible (check with df / lsblk), and record that in the run notes. - Rotate or repeat paired blocks (n of at least 2) so one contaminated pair doesn't sink the measurement. Here block 2 was clean. - Label, don't drop: keep the overlapped rows with a 'concurrent read' label, and say in the report which block the headline is based on. - Small correctness windows for other teams (a few minutes of builds) go between timed items, not during them, even on cores the timed job doesn't use: shared LLC and memory bandwidth still move the numbers.