speedometer

speedometer n.  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.