Why work item number in a work group better be multip of 64?

Discussion created by zhuzxy on Aug 14, 2011
Latest reply on Aug 14, 2011 by maximmoroz

for a8-3850, it has total 400 simd engines and 5 CUs. I guess the best number for work item/work group is 80* N (400/5). But the profiler suggest to be N*64. Can anyone help me on why I understand wrong?