One possibility which seems not so well-known is that clocks with per-core state might not be perfectly synchronized. If your initial measurement is from core0,

One possibility which seems not so well-known is that clocks with per-core state might not be perfectly synchronized. If your initial measurement is from core0, then we migrate to core1, the end measurement could even be 'before' the initial. Then there are manufacturing differences between cores that affect e.g. their leakage current and thus the (turbo) frequency at which they can run. So the measurement noise is indeed not one-sided, that is to say: measurements are not always overestimates. Thus a trimmed mean on both sides is a good idea, and pinning threads to a core when measuring is also helpful.