[tune] Hyperband Max Iter Fix (#1620)

* nits

* cumul r

* docs

* min

python/ray/tune/hyperband.py

@@ -61,9 +61,9 @@ class HyperBandScheduler(FIFOScheduler):
             procedures will use this attribute.
         max_t (int): max time units per trial. Trials will be stopped after
             max_t time units (determined by time_attr) have passed.
-            The HyperBand scheduler automatically tries to determine a
-            reasonable number of brackets based on this. The scheduler will
-            terminate trials after this time has passed.
+            The scheduler will terminate trials after this time has passed.
+            Note that this is different from the semantics of `max_t` as
+            mentioned in the original HyperBand paper.
     """
 
     def __init__(
@@ -73,6 +73,7 @@ class HyperBandScheduler(FIFOScheduler):
         FIFOScheduler.__init__(self)
         self._eta = 3
         self._s_max_1 = 5
+        self._max_t_attr = max_t
         # bracket max trials
         self._get_n0 = lambda s: int(
             np.ceil(self._s_max_1/(s+1) * self._eta**s))
@@ -117,7 +118,7 @@ class HyperBandScheduler(FIFOScheduler):
                     retry = False
                     cur_bracket = Bracket(
                         self._time_attr, self._get_n0(s), self._get_r0(s),
-                        self._eta, s)
+                        self._max_t_attr, self._eta, s)
                 cur_band.append(cur_bracket)
                 self._state["bracket"] = cur_bracket
 
@@ -257,13 +258,14 @@ class Bracket():
 
     Also keeps track of progress to ensure good scheduling.
     """
-    def __init__(self, time_attr, max_trials, init_t_attr, eta, s):
+    def __init__(self, time_attr, max_trials, init_t_attr, max_t_attr, eta, s):
         self._live_trials = {}  # maps trial -> current result
         self._all_trials = []
         self._time_attr = time_attr  # attribute to
 
         self._n = self._n0 = max_trials
         self._r = self._r0 = init_t_attr
+        self._max_t_attr = max_t_attr
         self._cumul_r = self._r0
 
         self._eta = eta
@@ -314,8 +316,9 @@ class Bracket():
         self._halves -= 1
         self._n /= self._eta
         self._n = int(np.ceil(self._n))
+
         self._r *= self._eta
-        self._r = int((self._r))
+        self._r = int(min(self._r, self._max_t_attr - self._cumul_r))
         self._cumul_r += self._r
         sorted_trials = sorted(
             self._live_trials,
@@ -364,6 +367,8 @@ class Bracket():
 
         This will not be always finish with 100 since dead trials
         are dropped."""
+        if self.finished():
+            return 1.0
         return self._completed_progress / self._total_work
 
     def _get_result_time(self, result):
@@ -373,18 +378,19 @@ class Bracket():
 
     def _calculate_total_work(self, n, r, s):
         work = 0
+        cumulative_r = r
         for i in range(s+1):
             work += int(n) * int(r)
             n /= self._eta
             n = int(np.ceil(n))
             r *= self._eta
-            r = int(r)
+            r = int(min(r, self._max_t_attr - cumulative_r))
         return work
 
     def __repr__(self):
         status = ", ".join([
             "Max Size (n)={}".format(self._n),
-            "Milestone (r)={}".format(self._r),
+            "Milestone (r)={}".format(self._cumul_r),
             "completed={:.1%}".format(self.completion_percentage())
             ])
         counts = collections.Counter([t.status for t in self._all_trials])

python/ray/tune/test/trial_scheduler_test.py

@@ -191,10 +191,10 @@ class HyperbandSuite(unittest.TestCase):
 
         Bracketing is placed as follows:
         (5, 81);
-        (8, 27) -> (3, 81);
-        (15, 9) -> (5, 27) -> (2, 81);
-        (34, 3) -> (12, 9) -> (4, 27) -> (2, 81);
-        (81, 1) -> (27, 3) -> (9, 9) -> (3, 27) -> (1, 81);"""
+        (8, 27) -> (3, 54);
+        (15, 9) -> (5, 27) -> (2, 45);
+        (34, 3) -> (12, 9) -> (4, 27) -> (2, 42);
+        (81, 1) -> (27, 3) -> (9, 9) -> (3, 27) -> (1, 41);"""
         sched = HyperBandScheduler()
         for i in range(num_trials):
             t = Trial("__fake")