Since {HEAD ~ the number} of {TAIL ~ 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 the event queue decreases 
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Since { HEAD ~ the number } of { TAIL ~ 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 the event queue decreases
OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH