Merge branch 'lmcafee/pipe-forward-only' into 'main'

removed saving of input/output tensors for forward-only passes of pipeline schedules

See merge request ADLR/megatron-lm!300

megatron/schedules.py

@@ -194,6 +194,7 @@ def forward_backward_pipelining_with_interleaving(forward_step_func, data_iterat
         model_chunk_id = get_model_chunk_id(microbatch_id, forward=True)
         mpu.set_virtual_pipeline_model_parallel_rank(model_chunk_id)
 
+        # forward step
         if mpu.is_pipeline_first_stage():
             if len(input_tensors[model_chunk_id]) == \
                     len(output_tensors[model_chunk_id]):
@@ -205,6 +206,11 @@ def forward_backward_pipelining_with_interleaving(forward_step_func, data_iterat
                                      input_tensor, losses_reduced)
         output_tensors[model_chunk_id].append(output_tensor)
 
+        # if forward-only, no need to save tensors for a backward pass
+        if forward_only:
+            input_tensors[model_chunk_id].pop()
+            output_tensors[model_chunk_id].pop()
+
         return output_tensor
 
     def backward_step_helper(microbatch_id):
@@ -383,6 +389,10 @@ def forward_backward_pipelining_without_interleaving(forward_step_func, data_ite
     num_microbatches_remaining = \
         num_microbatches - num_warmup_microbatches
 
+    # Input, output tensors only need to be saved when doing backward passes
+    input_tensors = None
+    output_tensors = None
+    if not forward_only:
         input_tensors = []
         output_tensors = []
     losses_reduced = []
@@ -394,6 +404,7 @@ def forward_backward_pipelining_without_interleaving(forward_step_func, data_ite
                                      input_tensor, losses_reduced)
         p2p_communication.send_forward(output_tensor, timers=timers)
 
+        if not forward_only:
             input_tensors.append(input_tensor)
             output_tensors.append(output_tensor)
 
@@ -411,21 +422,23 @@ def forward_backward_pipelining_without_interleaving(forward_step_func, data_ite
                                      input_tensor, losses_reduced)
         if forward_only:
             p2p_communication.send_forward(output_tensor, timers=timers)
+
+            if not last_iteration:
+                input_tensor = p2p_communication.recv_forward(timers=timers)
+
         else:
             output_tensor_grad = \
                 p2p_communication.send_forward_recv_backward(output_tensor,
                                                              timers=timers)
 
-        # Add input_tensor and output_tensor to end of list, then pop from the
-        # start of the list for backward pass.
+            # Add input_tensor and output_tensor to end of list.
             input_tensors.append(input_tensor)
             output_tensors.append(output_tensor)
 
-        if forward_only:
-            if not last_iteration:
-                input_tensor = p2p_communication.recv_forward(timers=timers)
-        else:
-            input_tensor, output_tensor = input_tensors.pop(0), output_tensors.pop(0)
+            # Pop input_tensor and output_tensor from the start of the list for
+            # the backward pass.
+            input_tensor = input_tensors.pop(0)
+            output_tensor = output_tensors.pop(0)
 
             input_tensor_grad = \
                 backward_step(optimizer, input_tensor, output_tensor,