add test for queue efficiency

specs/core/0_beacon-chain.md

@@ -1311,7 +1311,7 @@ def process_registry_updates(state: BeaconState) -> None:
         if is_active_validator(validator, get_current_epoch(state)) and validator.effective_balance <= EJECTION_BALANCE:
             initiate_validator_exit(state, ValidatorIndex(index))
 
-    # Queue validators eligible for activation and not yet dequeued for activation prior
+    # Queue validators eligible for activation and not yet dequeued for activation
     activation_queue = sorted([
         index for index, validator in enumerate(state.validators)
         if validator.activation_eligibility_epoch != FAR_FUTURE_EPOCH

test_libs/pyspec/eth2spec/test/phase_0/epoch_processing/test_process_registry_updates.py

@@ -63,6 +63,34 @@ def test_activation_queue_sorting(spec, state):
     assert state.validators[churn_limit - 2].activation_epoch != spec.FAR_FUTURE_EPOCH
 
 
+@with_all_phases
+@spec_state_test
+def test_activation_queue_efficiency(spec, state):
+    churn_limit = spec.get_validator_churn_limit(state)
+    mock_activations = churn_limit * 2
+
+    epoch = spec.get_current_epoch(state)
+    for i in range(mock_activations):
+        mock_deposit(spec, state, i)
+        state.validators[i].activation_eligibility_epoch = epoch + 1
+
+    # Run first registry update. Do not yield test vectors
+    for _ in run_process_registry_updates(spec, state):
+        pass
+
+    # Half should churn in first run of registry update
+    for i in range(mock_activations):
+        if i < mock_activations // 2:
+            assert state.validators[i].activation_epoch < spec.FAR_FUTURE_EPOCH
+        else:
+            assert state.validators[i].activation_epoch == spec.FAR_FUTURE_EPOCH
+
+    # Second half should churn in second run of registry update
+    yield from run_process_registry_updates(spec, state)
+    for i in range(mock_activations):
+        assert state.validators[i].activation_epoch < spec.FAR_FUTURE_EPOCH
+
+
 @with_all_phases
 @spec_state_test
 def test_ejection(spec, state):