A pattern of lights displayed on a linear set of LEDs (today)
or nixie tubes (yesterday, on ancient mainframes). The
pattern is shifted left every N times the operating system
goes through its
main loop. A swiftly moving pattern
indicates that the system is mostly idle; the speedometer
slows down as the system becomes overloaded. The speedometer
on Sun Microsystems hardware bounces back and forth like the
eyes on one of the Cylons from the wretched "Battlestar
Galactica" TV series.
Historical note: One computer, the GE 600 (later Honeywell
6000) actually had an *analog* speedometer on the front panel,
calibrated in instructions executed per second.