[Fix] Fix the calculation error of eta_min in CosineRestart (#639)

* [Fix] fix CosineRestart eta_min

* add ut case

* Enhance unit test

Enhance unit test

* remove unused code

Co-authored-by: HAOCHENYE <21724054@zju.edu.cn>

mmengine/optim/scheduler/param_scheduler.py

@@ -1224,16 +1224,11 @@ class CosineRestartParamScheduler(_ParamScheduler):
                                      self.optimizer.param_groups):
             eta_max = base_value * current_weight
             if self.eta_min_ratio is None:
-                eta_min = self.eta_min * (1 - current_weight)
+                eta_min = self.eta_min
             else:
-                eta_min = base_value * self.eta_min_ratio * (1 -
-                                                             current_weight)
+                eta_min = base_value * self.eta_min_ratio
             if step == 0:
                 values.append(eta_max)
-
-            elif (step - 1 - current_periods) % (2 * current_periods) == 0:
-                values.append(group[self.param_name] + (eta_max - eta_min) *
-                              (1 - math.cos(math.pi / current_periods)) / 2)
             else:
                 values.append(
                     (1 + math.cos(math.pi * step / current_periods)) /

tests/test_optim/test_scheduler/test_param_scheduler.py

@@ -430,6 +430,8 @@ class TestParameterScheduler(TestCase):
         targets = [
             single_targets, [t * self.layer2_mult for t in single_targets]
         ]
+
+        # Test with non-zero eta-min.
         scheduler = CosineRestartParamScheduler(
             self.optimizer,
             param_name='lr',
@@ -438,6 +440,26 @@ class TestParameterScheduler(TestCase):
             eta_min=0)
         self._test_scheduler_value(scheduler, targets, epochs=10)
 
+        epochs = 10
+        t = 10
+        eta_min = 5e-3
+        targets1 = [
+            eta_min + (0.05 - eta_min) * (1 + math.cos(math.pi * x / t)) / 2
+            for x in range(epochs)
+        ]
+        targets2 = [
+            eta_min + (0.5 - eta_min) * (1 + math.cos(math.pi * x / t)) / 2
+            for x in range(epochs)
+        ]
+        targets = [targets1, targets2]
+        scheduler = CosineRestartParamScheduler(
+            self.optimizer,
+            param_name='lr',
+            periods=[t],
+            restart_weights=[1],
+            eta_min=eta_min)
+        self._test_scheduler_value(scheduler, targets, epochs=10)
+
     def _check_scheduler_state_dict(self, construct, construct2, epochs=10):
         scheduler = construct()
         for _ in range(epochs):