[Bugfix] [SpecDecode] AsyncMetricsCollector: update time since last collection (#6578)

tests/spec_decode/test_metrics.py

@@ -105,6 +105,49 @@ def test_noop_until_time():
     assert metrics is not None
 
 
+def test_timer_is_reset():
+    """Verify that the internal timer inside AsyncMetricsCollector
+    is reset after collection.
+    """
+    spec_decode_sampler = MagicMock()
+    spec_decode_sampler.num_accepted_tokens = torch.tensor(0,
+                                                           dtype=torch.long,
+                                                           device='cuda')
+    spec_decode_sampler.num_emitted_tokens = torch.tensor(0,
+                                                          dtype=torch.long,
+                                                          device='cuda')
+    spec_decode_sampler.num_draft_tokens = 0
+
+    collect_interval_s = 5.0
+    timer = MagicMock()
+    timer.side_effect = [
+        0.0,
+        collect_interval_s + 0.1,
+        collect_interval_s + 0.1,
+        collect_interval_s + 0.2,
+        collect_interval_s + 0.2,
+        2 * collect_interval_s + 0.1,
+        2 * collect_interval_s + 0.1,
+    ]
+
+    collector = AsyncMetricsCollector(spec_decode_sampler=spec_decode_sampler,
+                                      timer=timer,
+                                      collect_interval_s=collect_interval_s)
+    collector.init_gpu_tensors(rank=0)
+
+    _ = collector.maybe_collect_rejsample_metrics(k=5)
+    metrics = collector.maybe_collect_rejsample_metrics(k=5)
+    assert metrics is not None
+
+    _ = collector.maybe_collect_rejsample_metrics(k=5)
+    metrics = collector.maybe_collect_rejsample_metrics(k=5)
+    assert metrics is None
+
+    _ = collector.maybe_collect_rejsample_metrics(k=5)
+    metrics = collector.maybe_collect_rejsample_metrics(k=5)
+    assert metrics is not None
+
+
 @pytest.mark.parametrize("has_data", [True, False])
 def test_initial_metrics_has_correct_values(has_data: bool):
     """Test correctness of metrics data.

vllm/spec_decode/metrics.py

@@ -145,6 +145,10 @@ class AsyncMetricsCollector:
         """
 
         ready_event.synchronize()
+
+        # update time of last collection
+        self._last_metrics_collect_time = self._timer()
+
         accepted_tokens = self._aggregate_num_accepted_tokens.item()
         emitted_tokens = self._aggregate_num_emitted_tokens.item()
         draft_tokens = self._aggregate_num_draft_tokens