computer units independence

hello,

I have a question about independence of CUs (compute units). Assuming I have 4 CUs each to run 6 work-groups (total 24). Are CUs fully independent, or they are synchronized? by synchronized I mean all CUs start and finish running one work-group together with something like a work-group synchronization point, and after that, all CUs go to run next round of work-groups together?