Since the number of {HEAD ~ event execution units} decreases, the number of times an event execution unit is registered in an event queue and a registered event is extracted from {TAIL ~ the event queue} decreases 
Since the number of { HEAD ~ event execution units } decreases , the number of times an event execution unit is registered in an event queue and a registered event is extracted from { TAIL ~ the event queue } decreases
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH