A scheduling processor assigns {TAIL ~ the work items} to the execution engines for generation of {HEAD ~ the gather entries} 
A scheduling processor assigns { TAIL ~ the work items } to the execution engines for generation of { HEAD ~ the gather entries }
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD