Add support for ZeRO-2/3 and ZeRO-offload in fairscale

docs/source/main_classes/trainer.rst

@@ -241,6 +241,8 @@ provides support for the following features from `the ZeRO paper <https://arxiv.
 
 1. Optimizer State Sharding
 2. Gradient Sharding
+3. Model Parameters Sharding (new and very experimental)
+4. CPU offload (new and very experimental)
 
 You will need at least two GPUs to use this feature.
 
@@ -255,8 +257,9 @@ To deploy this feature:
    or find more details on `the FairScale's GitHub page
    <https://github.com/facebookresearch/fairscale/#installation>`__.
 
-2. Add ``--sharded_ddp`` to the command line arguments, and make sure you have added the distributed launcher ``-m
-   torch.distributed.launch --nproc_per_node=NUMBER_OF_GPUS_YOU_HAVE`` if you haven't been using it already.
+2. To use the first version of Sharded data-parallelism, add ``--sharded_ddp simple`` to the command line arguments,
+   and make sure you have added the distributed launcher ``-m torch.distributed.launch
+   --nproc_per_node=NUMBER_OF_GPUS_YOU_HAVE`` if you haven't been using it already.
 
 For example here is how you could use it for ``run_seq2seq.py`` with 2 GPUs:
 
@@ -268,17 +271,55 @@ For example here is how you could use it for ``run_seq2seq.py`` with 2 GPUs:
     --do_train --max_train_samples 500 --num_train_epochs 1 \
     --dataset_name wmt16 --dataset_config "ro-en" \
     --task translation_en_to_ro --source_prefix "translate English to Romanian: " \
-    --fp16 --sharded_ddp
+    --fp16 --sharded_ddp simple
 
 Notes:
 
 - This feature requires distributed training (so multiple GPUs).
 - It is not implemented for TPUs.
 - It works with ``--fp16`` too, to make things even faster.
-- One of the main benefits of enabling ``--sharded_ddp`` is that it uses a lot less GPU memory, so you should be able
-  to use significantly larger batch sizes using the same hardware (e.g. 3x and even bigger) which should lead to
+- One of the main benefits of enabling ``--sharded_ddp simple`` is that it uses a lot less GPU memory, so you should be
+  able to use significantly larger batch sizes using the same hardware (e.g. 3x and even bigger) which should lead to
   significantly shorter training time.
 
+3. To use the second version of Sharded data-parallelism, add ``--sharded_ddp zero_dp_2`` or ``--sharded_ddp zero_dp_3`
+   to the command line arguments, and make sure you have added the distributed launcher ``-m torch.distributed.launch
+   --nproc_per_node=NUMBER_OF_GPUS_YOU_HAVE`` if you haven't been using it already.
+
+For example here is how you could use it for ``run_seq2seq.py`` with 2 GPUs:
+
+.. code-block:: bash
+
+    python -m torch.distributed.launch --nproc_per_node=2 examples/seq2seq/run_seq2seq.py \
+    --model_name_or_path t5-small --per_device_train_batch_size 1   \
+    --output_dir output_dir --overwrite_output_dir \
+    --do_train --max_train_samples 500 --num_train_epochs 1 \
+    --dataset_name wmt16 --dataset_config "ro-en" \
+    --task translation_en_to_ro --source_prefix "translate English to Romanian: " \
+    --fp16 --sharded_ddp zero_dp_2
+
+:obj:`zero_dp_2` is an optimized version of the simple wrapper, while :obj:`zero_dp_3` fully shards model weights,
+gradients and optimizer states.
+
+Both are compatible with adding :obj:`cpu_offload` to enable ZeRO-offload (activate it like this: :obj:`--sharded_ddp
+"zero_dp_2 cpu_offload"`).
+
+Notes:
+
+- This feature requires distributed training (so multiple GPUs).
+- It is not implemented for TPUs.
+- It works with ``--fp16`` too, to make things even faster.
+- The ``cpu_offload`` additional option requires ``--fp16``.
+- This is an area of active development, so make sure you have a source install of fairscale to use this feature as
+  some bugs you encounter may have been fixed there already.
+
+Known caveats:
+
+- This feature is incompatible with :obj:`--predict_with_generate` in the `run_seq2seq.py` script.
+- Using :obj:`--sharded_ddp zero_dp_3` requires wrapping each layer of the model in the special container
+  :obj:`FullyShardedDataParallelism` of fairscale. This is not done automatically by any of the example scripts of the
+  :class:`~transformers.Trainer`.
+
 
 DeepSpeed
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

examples/tests/trainer/test_trainer_ext.py

@@ -64,12 +64,13 @@ def require_apex(test_case):
 
 
 class TestTrainerExt(TestCasePlus):
-    def run_seq2seq_quick(self, distributed=False, extra_args_str=None):
-        output_dir = self.run_trainer(1, "12", MBART_TINY, 1, distributed, extra_args_str)
+    def run_seq2seq_quick(self, distributed=False, extra_args_str=None, eval=True, predict_with_generate=True):
+        output_dir = self.run_trainer(1, "12", MBART_TINY, 1, distributed, extra_args_str, predict_with_generate)
         logs = TrainerState.load_from_json(os.path.join(output_dir, "trainer_state.json")).log_history
         eval_metrics = [log for log in logs if "eval_loss" in log.keys()]
         first_step_stats = eval_metrics[0]
-        assert "eval_bleu" in first_step_stats
+        if predict_with_generate:
+            assert "eval_bleu" in first_step_stats
 
     @require_torch_non_multi_gpu
     def test_run_seq2seq_no_dist(self):
@@ -88,14 +89,28 @@ def test_run_seq2seq_ddp(self):
     # test --sharded_ddp w/o --fp16
     @require_torch_multi_gpu
     @require_fairscale
-    def test_run_seq2seq_ddp_sharded_ddp(self):
-        self.run_seq2seq_quick(distributed=True, extra_args_str="--sharded_ddp")
+    def test_run_seq2seq_sharded_ddp(self):
+        self.run_seq2seq_quick(distributed=True, extra_args_str="--sharded_ddp simple")
 
     # test --sharded_ddp w/ --fp16
     @require_torch_multi_gpu
     @require_fairscale
-    def test_run_seq2seq_ddp_sharded_ddp_fp16(self):
-        self.run_seq2seq_quick(distributed=True, extra_args_str="--sharded_ddp --fp16")
+    def test_run_seq2seq_sharded_ddp_fp16(self):
+        self.run_seq2seq_quick(distributed=True, extra_args_str="--sharded_ddp simple --fp16")
+
+    # test --sharded_ddp zero2 w/o --fp16
+    @require_torch_multi_gpu
+    @require_fairscale
+    def test_run_seq2seq_fully_sharded_ddp(self):
+        self.run_seq2seq_quick(distributed=True, extra_args_str="--sharded_ddp zero2", predict_with_generate=False)
+
+    # test --sharded_ddp zero2 w/ --fp16
+    @require_torch_multi_gpu
+    @require_fairscale
+    def test_run_seq2seq_fully_sharded_ddp_fp16(self):
+        self.run_seq2seq_quick(
+            distributed=True, extra_args_str="--sharded_ddp zero2 --fp16", predict_with_generate=False
+        )
 
     @require_apex
     def test_run_seq2seq_apex(self):
@@ -131,6 +146,7 @@ def run_trainer(
         num_train_epochs: int,
         distributed: bool = False,
         extra_args_str: str = None,
+        predict_with_generate: bool = True,
     ):
         data_dir = self.examples_dir / "test_data/wmt_en_ro"
         output_dir = self.get_auto_remove_tmp_dir()
@@ -155,7 +171,6 @@ def run_trainer(
             --learning_rate 3e-3
             --warmup_steps 8
             --evaluation_strategy steps
-            --predict_with_generate
             --logging_steps 0
             --save_steps {str(eval_steps)}
             --eval_steps {str(eval_steps)}
@@ -165,7 +180,11 @@ def run_trainer(
             --task translation
             --target_lang ro_RO
             --source_lang en_XX
-        """.split()
+        """
+        if predict_with_generate:
+            args += "--predict_with_generate"
+
+        args = args.split()
 
         if extra_args_str is not None:
             args.extend(extra_args_str.split())

src/transformers/trainer.py

@@ -94,6 +94,7 @@
     EvalPrediction,
     HPSearchBackend,
     PredictionOutput,
+    ShardedDDPOption,
     TrainerMemoryTracker,
     TrainOutput,
     default_compute_objective,
@@ -132,10 +133,16 @@
     import torch_xla.distributed.parallel_loader as pl
 
 if is_fairscale_available():
+    import fairscale
     from fairscale.nn.data_parallel import ShardedDataParallel as ShardedDDP
     from fairscale.optim import OSS
     from fairscale.optim.grad_scaler import ShardedGradScaler
 
+    if version.parse(fairscale.__version__) >= version.parse("0.3"):
+        from fairscale.nn.data_parallel import FullyShardedDataParallel as FullyShardedDDP
+    else:
+        FullyShardedDDP = None
+
 if is_sagemaker_distributed_available():
     import smdistributed.dataparallel.torch.distributed as dist
     from smdistributed.dataparallel.torch.parallel.distributed import DistributedDataParallel as DDP
@@ -276,9 +283,38 @@ def __init__(
         else:
             self.is_model_parallel = False
 
+        # Setup Sharded DDP training
+        self.sharded_ddp = None
+        if len(args.sharded_ddp) > 0:
+            if args.deepspeed:
+                raise ValueError(
+                    "Using --sharded_ddp xxx together with --deepspeed is not possible, deactivate one of those flags."
+                )
+
+            if args.local_rank == -1:
+                raise ValueError("Using sharded DDP only works in distributed training.")
+            elif not is_fairscale_available():
+                raise ImportError("Sharded DDP training requires fairscale: `pip install fairscale`.")
+            elif ShardedDDPOption.SIMPLE not in args.sharded_ddp and FullyShardedDDP is None:
+                raise ImportError(
+                    "Sharded DDP in a mode other than simple training requires fairscale version >= 0.3, found "
+                    f"{fairscale.__version__}. Upgrade your fairscale library: `pip install --upgrade fairscale`."
+                )
+            elif ShardedDDPOption.SIMPLE in args.sharded_ddp:
+                self.sharded_ddp = ShardedDDPOption.SIMPLE
+            elif ShardedDDPOption.ZERO_DP_2 in args.sharded_ddp:
+                self.sharded_ddp = ShardedDDPOption.ZERO_DP_2
+            elif ShardedDDPOption.ZERO_DP_3 in args.sharded_ddp:
+                self.sharded_ddp = ShardedDDPOption.ZERO_DP_3
+
         # one place to sort out whether to place the model on device or not
         self.place_model_on_device = args.place_model_on_device
-        if self.is_model_parallel or (args.deepspeed and args.do_train) or (args.fp16_full_eval and not args.do_train):
+        if (
+            self.is_model_parallel
+            or (args.deepspeed and args.do_train)
+            or (args.fp16_full_eval and not args.do_train)
+            or (self.sharded_ddp in [ShardedDDPOption.ZERO_DP_2, ShardedDDPOption.ZERO_DP_3])
+        ):
             self.place_model_on_device = False
 
         default_collator = default_data_collator if tokenizer is None else DataCollatorWithPadding(tokenizer)
@@ -345,21 +381,6 @@ def __init__(
             if isinstance(eval_dataset, datasets.Dataset):
                 self._remove_unused_columns(self.eval_dataset, description="evaluation")
 
-        # Setup Sharded DDP training
-        self.sharded_dpp = False
-        if args.sharded_ddp:
-            if args.deepspeed:
-                raise ValueError(
-                    "Using --sharded_ddp together with --deepspeed is not possible, deactivate one of those flags."
-                )
-
-            if args.local_rank == -1:
-                raise ValueError("Using sharded DDP only works in distributed training.")
-            elif not is_fairscale_available():
-                raise ImportError("Sharded DDP training requires fairscale: `pip install fairscale`.")
-            else:
-                self.sharded_dpp = True
-
         # Mixed precision setup
         self.use_apex = False
         self.use_amp = False
@@ -375,7 +396,7 @@ def __init__(
         if args.fp16 and not args.deepspeed:  # deepspeed manages its own fp16
             if self.fp16_backend == "amp":
                 self.use_amp = True
-                self.scaler = ShardedGradScaler() if self.sharded_dpp else torch.cuda.amp.GradScaler()
+                self.scaler = ShardedGradScaler() if self.sharded_ddp is not None else torch.cuda.amp.GradScaler()
             else:
                 if not is_apex_available():
                     raise ImportError(
@@ -618,7 +639,7 @@ def create_optimizer_and_scheduler(self, num_training_steps: int):
                     "eps": self.args.adam_epsilon,
                 }
             optimizer_kwargs["lr"] = self.args.learning_rate
-            if self.sharded_dpp:
+            if self.sharded_ddp == ShardedDDPOption.SIMPLE:
                 self.optimizer = OSS(
                     params=optimizer_grouped_parameters,
                     optim=optimizer_cls,
@@ -736,8 +757,19 @@ def _wrap_model(self, model, training=True):
             return model
 
         # Distributed training (should be after apex fp16 initialization)
-        if self.sharded_dpp:
-            model = ShardedDDP(model, self.optimizer)
+        if self.sharded_ddp is not None:
+            # Sharded DDP!
+            if self.sharded_ddp == ShardedDDPOption.SIMPLE:
+                model = ShardedDDP(model, self.optimizer)
+            else:
+                mixed_precision = self.args.fp16
+                cpu_offload = ShardedDDPOption.OFFLOAD in self.args.sharded_ddp
+                zero_3 = self.sharded_ddp == ShardedDDPOption.ZERO_DP_3
+                # XXX: Breaking the self.model convention but I see no way around it for now.
+                self.model = model = FullyShardedDDP(
+                    model, mixed_precision=mixed_precision, reshard_after_forward=zero_3, cpu_offload=cpu_offload
+                ).to(self.args.device)
+
         elif is_sagemaker_distributed_available():
             model = DDP(model, device_ids=[dist.get_local_rank()], broadcast_buffers=False)
         elif self.args.local_rank != -1:
@@ -854,14 +886,15 @@ def train(
             num_train_epochs = 1
             num_update_steps_per_epoch = max_steps
 
+        delay_optimizer_creation = self.sharded_ddp is not None and self.sharded_ddp != ShardedDDPOption.SIMPLE
         if self.args.deepspeed:
             model, optimizer, lr_scheduler = init_deepspeed(self, num_training_steps=max_steps)
             self.model = model.module
             self.model_wrapped = model  # will get further wrapped in DDP
             self.deepspeed = model  # DeepSpeedEngine object
             self.optimizer = optimizer
             self.lr_scheduler = lr_scheduler
-        else:
+        elif not delay_optimizer_creation:
             self.create_optimizer_and_scheduler(num_training_steps=max_steps)
 
         self.state = TrainerState()
@@ -876,6 +909,9 @@ def train(
         if model is not self.model:
             self.model_wrapped = model
 
+        if delay_optimizer_creation:
+            self.create_optimizer_and_scheduler(num_training_steps=max_steps)
+
         # important: at this point:
         # self.model         is the Transformers Model
         # self.model_wrapped is DDP(Transformers Model), Deepspeed(Transformers Model), etc.
@@ -1025,6 +1061,9 @@ def train(
                         if hasattr(self.optimizer, "clip_grad_norm"):
                             # Some optimizers (like the sharded optimizer) have a specific way to do gradient clipping
                             self.optimizer.clip_grad_norm(self.args.max_grad_norm)
+                        elif hasattr(model, "clip_grad_norm_"):
+                            # Some models (like FullyShardedDDP) have a specific way to do gradient clipping
+                            model.clip_grad_norm_(self.args.max_grad_norm)
                         else:
                             # Revert to normal clipping otherwise, handling Apex or full precision
                             torch.nn.utils.clip_grad_norm_(
@@ -1151,8 +1190,8 @@ def _maybe_log_save_evaluate(self, tr_loss, model, trial, epoch):
 
     def _save_checkpoint(self, model, trial, metrics=None):
         # In all cases, including ddp/dp/deepspeed, self.model is always a reference to the model we
-        # want to save.
-        assert _model_unwrap(model) is self.model, "internal model should be a reference to self.model"
+        # want to save except FullyShardedDDP.
+        # assert _model_unwrap(model) is self.model, "internal model should be a reference to self.model"
 
         # Save model checkpoint
         checkpoint_folder = f"{PREFIX_CHECKPOINT_DIR}-{self.state.global_step}"
@@ -1176,7 +1215,7 @@ def _save_checkpoint(self, model, trial, metrics=None):
             self.deepspeed.save_checkpoint(output_dir)
 
         # Save optimizer and scheduler
-        if self.sharded_dpp:
+        if self.sharded_ddp == ShardedDDPOption.SIMPLE:
             self.optimizer.consolidate_state_dict()
 
         if is_torch_tpu_available():
@@ -1537,7 +1576,11 @@ def _save_tpu(self, output_dir: Optional[str] = None):
         # They can then be reloaded using `from_pretrained()`
         xm.rendezvous("saving_checkpoint")
         if not isinstance(self.model, PreTrainedModel):
-            logger.info("Trainer.model is not a `PreTrainedModel`, only saving its state dict.")
+            if isinstance(_model_unwrap(self.model), PreTrainedModel):
+                if xm.is_master_ordinal():
+                    _model_unwrap(self.model).config.save_pretrained(output_dir)
+            else:
+                logger.info("Trainer.model is not a `PreTrainedModel`, only saving its state dict.")
             state_dict = self.model.state_dict()
             xm.save(state_dict, os.path.join(output_dir, WEIGHTS_NAME))
         else:
@@ -1552,7 +1595,10 @@ def _save(self, output_dir: Optional[str] = None):
         # Save a trained model and configuration using `save_pretrained()`.
         # They can then be reloaded using `from_pretrained()`
         if not isinstance(self.model, PreTrainedModel):
-            logger.info("Trainer.model is not a `PreTrainedModel`, only saving its state dict.")
+            if isinstance(_model_unwrap(self.model), PreTrainedModel):
+                _model_unwrap(self.model).config.save_pretrained(output_dir)
+            else:
+                logger.info("Trainer.model is not a `PreTrainedModel`, only saving its state dict.")
             state_dict = self.model.state_dict()
             torch.save(state_dict, os.path.join(output_dir, WEIGHTS_NAME))
         else:

src/transformers/trainer_utils.py

@@ -421,3 +421,10 @@ def stop_and_update_metrics(self, metrics=None):
         # init doesn't have metrics to update so we just save that data for later stages to retrieve
         if metrics is not None:
             self.update_metrics(stage, metrics)
+
+
+class ShardedDDPOption(ExplicitEnum):
+    SIMPLE = "simple"
+    ZERO_DP_2 = "zero2"
+    ZERO_DP_3 = "zero3"
+    OFFLOAD = "offload"