Fix item_per_sec metric in ThroughputMonitor (#19080)

Author: awaelchli

src/lightning/fabric/utilities/throughput.py

@@ -185,10 +185,11 @@ def compute(self) -> _THROUGHPUT_METRICS:
 
             if len(self._lengths) == self._lengths.maxlen:
                 elapsed_lengths = self._lengths[-1] - self._lengths[0]
-                avg_length = elapsed_lengths / elapsed_batches
+                dev_items_per_sec = elapsed_lengths / elapsed_time
+                metrics[f"device{self.separator}items_per_sec"] = dev_items_per_sec
                 if add_global_metrics:
-                    metrics["items_per_sec"] = samples_per_sec * avg_length
-                metrics[f"device{self.separator}items_per_sec"] = dev_samples_per_sec * avg_length
+                    items_per_sec = dev_items_per_sec * self.world_size
+                    metrics["items_per_sec"] = items_per_sec
 
         if len(self._flops) == self._flops.maxlen:
             elapsed_flops = sum(self._flops) - self._flops[0]

tests/tests_fabric/utilities/test_throughput.py

@@ -132,7 +132,7 @@ def test_throughput():
         "lengths": 8,
         "device/batches_per_sec": 2.0,
         "device/samples_per_sec": 4.0,
-        "device/items_per_sec": 16.0,
+        "device/items_per_sec": 8.0,
     }
 
     with pytest.raises(ValueError, match="Expected the value to increase"):
@@ -149,7 +149,7 @@ def test_throughput():
         "lengths": 20,
         "device/batches_per_sec": 1.0,
         "device/flops_per_sec": 10.0,
-        "device/items_per_sec": 20.0,
+        "device/items_per_sec": 10.0,
         "device/mfu": 0.2,
         "device/samples_per_sec": 2.0,
     }
@@ -166,7 +166,7 @@ def test_throughput():
         "lengths": 20,
         "device/batches_per_sec": 1.0,
         "device/flops_per_sec": 10.0,
-        "device/items_per_sec": 20.0,
+        "device/items_per_sec": 10.0,
         "device/samples_per_sec": 2.0,
     }
 
@@ -215,7 +215,7 @@ def test_throughput_monitor():
                 "lengths": 24,
                 "device|batches_per_sec": 1.0,
                 "device|samples_per_sec": 3.0,
-                "device|items_per_sec": 18.0,
+                "device|items_per_sec": 6.0,
                 "device|flops_per_sec": 10.0,
                 "device|mfu": 0.1,
             },
@@ -229,7 +229,7 @@ def test_throughput_monitor():
                 "lengths": 30,
                 "device|batches_per_sec": 1.0,
                 "device|samples_per_sec": 3.0,
-                "device|items_per_sec": 18.0,
+                "device|items_per_sec": 6.0,
                 "device|flops_per_sec": 10.0,
                 "device|mfu": 0.1,
             },
@@ -285,7 +285,7 @@ def test_throughput_monitor_world_size():
                 "batches_per_sec": 2.0,
                 "samples_per_sec": 6.0,
                 "items_per_sec": 12.0,
-                "device/items_per_sec": 18.0,
+                "device/items_per_sec": 6.0,
                 "flops_per_sec": 20.0,
                 "device/flops_per_sec": 10.0,
                 "device/mfu": 0.1,
@@ -303,7 +303,7 @@ def test_throughput_monitor_world_size():
                 "batches_per_sec": 2.0,
                 "samples_per_sec": 6.0,
                 "items_per_sec": 12.0,
-                "device/items_per_sec": 18.0,
+                "device/items_per_sec": 6.0,
                 "flops_per_sec": 20.0,
                 "device/flops_per_sec": 10.0,
                 "device/mfu": 0.1,

tests/tests_pytorch/callbacks/test_throughput_monitor.py

@@ -57,7 +57,7 @@ def test_throughput_monitor_fit(tmp_path):
     expected = {
         "train|device|batches_per_sec": 1.0,
         "train|device|samples_per_sec": 3.0,
-        "train|device|items_per_sec": 18.0,
+        "train|device|items_per_sec": 6.0,
         "train|device|flops_per_sec": 10.0,
         "train|device|mfu": 0.1,
         "epoch": 0,
@@ -207,7 +207,7 @@ def test_throughput_monitor_fit_gradient_accumulation(tmp_path):
     expected = {
         "train|device|batches_per_sec": 1.0,
         "train|device|samples_per_sec": 3.0,
-        "train|device|items_per_sec": 18.0,
+        "train|device|items_per_sec": 6.0,
         "train|device|flops_per_sec": 10.0,
         "train|device|mfu": 0.1,
     }