You may specify start:stop:step sizes for thread or cache tiling. If only level1 is specified, it can be either thread or cache (examples below). If both level1 and level2 are specified, level1 (outer level) should be given as thread and level2 should be given as cache (inner) tiling. Examples:

thread 2:12:1
tests 2 to 12 threads with step size 1. Start and stop sizes refer to number of elements to attempt to fit into cache. For example,
cache 64:512:8
tests 64 up to 512 elements, increasing by 8 elements each trial.