gistfile1.clj

(let [num-workers 4
      widgets-per-worker (ceil (/ (count widgets) num-workers))
      bucketed-widgets (partition-all widgets-per-worker widgets)
      workers (doall (map (fn [widgets]
                              (thread
                                (doseq [widget widgets]
                                  (long-running-widget-processor widget))
                                true))
                           bucketed-widgets))]
  (doall (map <!! workers)))

It probably makes more sense to use future and deref instead of thread and <!!, since long-running-widget-processor isn't streaming. (I am not an expert though :) )

With futures, one way you could rewrite this is

(doall (pmap (fn [widgets]
         (doseq [widget widgets]
            (long-running-widget-processor widget))
         true)
         bucketed widgets))

Ultimately, which you should use depends mostly on the properties of long-running-widget-processor. If the task is IO-bound, then you should certainly use futures. Otherwise it is a little gray, and it's best to check by trying them out. Note that there are many options other than core.async and futures.