An old lesson gets learned yet again <https://arstechnica.com/gadgets/2021/05/apples-m1-is-a-fast-cpu-but-m1-macs-feel-even-faster-due-to-qos/>: efficiency and responsiveness in a computer system are often at odds, and improving one can hurt the other. We can also see a negative example—one in which the fastest throughput corresponded to distinctly unhappy users—with the circa-2006 introduction of the Completely Fair Queue (cfq) I/O scheduler in the Linux kernel. ... Unfortunately, while cfq did in fact measurably improve maximum throughput, it did so at the increase of task latency—which meant that a moderately loaded system felt sluggish and unresponsive to its users, leading to a large groundswell of complaints.