[Scheduler] Extract DFlash prefill refill into a standalone MinFreeSlotsDelayer (#29089)

test/registered/scheduler/test_min_free_slots_delayer.py

@@ -0,0 +1,72 @@
+import unittest
+
+from sglang.srt.managers.min_free_slots_delayer import (
+    MinFreeSlotsDelayer,
+    resolve_min_free_slots,
+)
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=5, suite="base-a-test-cpu")
+
+
+class TestResolveMinFreeSlots(unittest.TestCase):
+    """Unit tests for resolve_min_free_slots threshold resolution."""
+
+    def test_unset_non_dflash_disables(self):
+        # Unset + not DFlash -> trigger stays disabled.
+        self.assertIsNone(resolve_min_free_slots(None, 512, is_dflash=False))
+
+    def test_unset_dflash_auto_enables(self):
+        # Unset + DFlash -> falls back to the legacy formula (full mapping).
+        self.assertEqual(resolve_min_free_slots(None, 512, is_dflash=True), 4)
+        self.assertEqual(resolve_min_free_slots(None, 8, is_dflash=True), 2)
+
+    def test_unset_dflash_small_cluster_disables(self):
+        # DFlash auto-default still respects the < 8 guard.
+        self.assertIsNone(resolve_min_free_slots(None, 7, is_dflash=True))
+        self.assertIsNone(resolve_min_free_slots(None, 0, is_dflash=True))
+
+    def test_le_one_disables(self):
+        # <= 1 can never batch, so it is a no-op.
+        self.assertIsNone(resolve_min_free_slots(1, 512))
+        self.assertIsNone(resolve_min_free_slots(0, 512))
+
+    def test_small_cluster_disables(self):
+        # max_running_requests < 8 disables, matching DFlash.
+        self.assertIsNone(resolve_min_free_slots(4, 7))
+
+    def test_caps_to_formula(self):
+        # Capped down so it never delays more aggressively than DFlash.
+        self.assertEqual(resolve_min_free_slots(10, 512), 4)
+        self.assertEqual(resolve_min_free_slots(10, 8), 2)  # (8 + 5) // 6 = 2
+
+    def test_respects_smaller_user_value(self):
+        # Below the formula cap is taken as-is.
+        self.assertEqual(resolve_min_free_slots(3, 512), 3)
+        self.assertEqual(resolve_min_free_slots(2, 8), 2)
+
+    def test_user_value_overrides_dflash_default(self):
+        # An explicit user value wins over the DFlash auto-default.
+        self.assertEqual(resolve_min_free_slots(3, 512, is_dflash=True), 3)
+
+
+class TestMinFreeSlotsDelayer(unittest.TestCase):
+    """Unit tests for the per-rank local should_delay decision."""
+
+    def test_delays_below_threshold(self):
+        delayer = MinFreeSlotsDelayer(min_free_slots=4)
+        self.assertTrue(delayer.should_delay(running_bs=100, num_allocatable_reqs=2))
+
+    def test_no_delay_at_or_above_threshold(self):
+        delayer = MinFreeSlotsDelayer(min_free_slots=4)
+        self.assertFalse(delayer.should_delay(running_bs=100, num_allocatable_reqs=4))
+        self.assertFalse(delayer.should_delay(running_bs=100, num_allocatable_reqs=8))
+
+    def test_no_delay_when_idle(self):
+        # Nothing running: no decode batch to protect, prefill at once.
+        delayer = MinFreeSlotsDelayer(min_free_slots=4)
+        self.assertFalse(delayer.should_delay(running_bs=0, num_allocatable_reqs=0))
+
+
+if __name__ == "__main__":
+    unittest.main()