[trainer] implement support for full fp16 in evaluation/predict (#10268)

* implement --fp16_full_eval

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* style

* add test

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/trainer.py

@@ -218,6 +218,8 @@ class Trainer:
         - **place_model_on_device** -- Whether or not to automatically place the model on the device - it will be set
           to :obj:`False` if model parallel or deepspeed is used, or if the default
           ``TrainingArguments.place_model_on_device`` is overridden to return :obj:`False` .
+        - **is_in_train** -- Whether or not a model is currently running ``train`` (e.g. when ``evaluate`` is called
+          while in ``train``)
 
     """
 
@@ -243,6 +245,7 @@ class Trainer:
         set_seed(self.args.seed)
         self.hp_name = None
         self.deepspeed = None
+        self.is_in_train = False
 
         # memory metrics - must set up as early as possible
         self._memory_tracker = TrainerMemoryTracker(self.args.skip_memory_metrics)
@@ -273,7 +276,7 @@ class Trainer:
 
         # 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):
+        if self.is_model_parallel or (args.deepspeed and args.do_train) or (args.fp16_full_eval and not args.do_train):
             self.place_model_on_device = False
 
         default_collator = default_data_collator if tokenizer is None else DataCollatorWithPadding(tokenizer)
@@ -713,6 +716,10 @@ class Trainer:
         return model
 
     def _wrap_model(self, model, training=True):
+        # already initialized its own DDP and AMP
+        if self.deepspeed:
+            return model
+
         # Mixed precision training with apex (torch < 1.6)
         if self.use_apex and training:
             model, self.optimizer = amp.initialize(model, self.optimizer, opt_level=self.args.fp16_opt_level)
@@ -731,8 +738,6 @@ class Trainer:
             model = ShardedDDP(model, self.optimizer)
         elif is_sagemaker_distributed_available():
             model = DDP(model, device_ids=[dist.get_local_rank()], broadcast_buffers=False)
-        elif self.deepspeed:
-            pass  # already initialized its own DDP earlier
         elif self.args.local_rank != -1:
             if self.args.ddp_find_unused_parameters is not None:
                 find_unused_parameters = self.args.ddp_find_unused_parameters
@@ -773,6 +778,8 @@ class Trainer:
         # memory metrics - must set up as early as possible
         self._memory_tracker.start()
 
+        self.is_in_train = True
+
         if "model_path" in kwargs:
             resume_from_checkpoint = kwargs.pop("model_path")
             warnings.warn(
@@ -1088,6 +1095,12 @@ class Trainer:
             self.lr_scheduler = None
             self.model_wrapped = self.model
             gc.collect()  # force memory release
+            # to restore normal behavior outside of train replay the place_model_on_device logic w/o deepspeed
+            self.place_model_on_device = self.args.place_model_on_device
+            if self.is_model_parallel:
+                self.place_model_on_device = False
+
+        self.is_in_train = False
 
         self._memory_tracker.stop_and_update_metrics(metrics)
 
@@ -1689,6 +1702,11 @@ class Trainer:
 
         model = self._wrap_model(self.model, training=False)
 
+        # if full fp16 is wanted on eval and this ``evaluation`` or ``predict`` isn't called while
+        # ``train`` is running, half it first and then put on device
+        if not self.is_in_train and self.args.fp16_full_eval:
+            model = model.half().to(self.args.device)
+
         batch_size = dataloader.batch_size
         num_examples = self.num_examples(dataloader)
         logger.info("***** Running %s *****", description)

src/transformers/training_args.py

@@ -155,7 +155,7 @@ class TrainingArguments:
             :func:`~transformers.Trainer.model_init` function to instantiate the model if it has some randomly
             initialized parameters.
         fp16 (:obj:`bool`, `optional`, defaults to :obj:`False`):
-            Whether to use 16-bit (mixed) precision training (through NVIDIA Apex) instead of 32-bit training.
+            Whether to use 16-bit (mixed) precision training instead of 32-bit training.
         fp16_opt_level (:obj:`str`, `optional`, defaults to 'O1'):
             For :obj:`fp16` training, Apex AMP optimization level selected in ['O0', 'O1', 'O2', and 'O3']. See details
             on the `Apex documentation <https://nvidia.github.io/apex/amp.html>`__.
@@ -163,6 +163,9 @@ class TrainingArguments:
             The backend to use for mixed precision training. Must be one of :obj:`"auto"`, :obj:`"amp"` or
             :obj:`"apex"`. :obj:`"auto"` will use AMP or APEX depending on the PyTorch version detected, while the
             other choices will force the requested backend.
+        fp16_full_eval (:obj:`bool`, `optional`, defaults to :obj:`False`):
+            Whether to use full 16-bit precision evaluation instead of 32-bit. This will be faster and save memory but
+            can harm metric values.
         local_rank (:obj:`int`, `optional`, defaults to -1):
             Rank of the process during distributed training.
         tpu_num_cores (:obj:`int`, `optional`):
@@ -353,7 +356,7 @@ class TrainingArguments:
 
     fp16: bool = field(
         default=False,
-        metadata={"help": "Whether to use 16-bit (mixed) precision (through NVIDIA Apex) instead of 32-bit"},
+        metadata={"help": "Whether to use 16-bit (mixed) precision instead of 32-bit"},
     )
     fp16_opt_level: str = field(
         default="O1",
@@ -368,6 +371,10 @@ class TrainingArguments:
         default="auto",
         metadata={"help": "The backend to be used for mixed precision.", "choices": ["auto", "amp", "apex"]},
     )
+    fp16_full_eval: bool = field(
+        default=False,
+        metadata={"help": "Whether to use full 16-bit precision evaluation instead of 32-bit"},
+    )
     local_rank: int = field(default=-1, metadata={"help": "For distributed training: local_rank"})
 
     tpu_num_cores: Optional[int] = field(
@@ -488,8 +495,10 @@ class TrainingArguments:
         if self.run_name is None:
             self.run_name = self.output_dir
 
-        if is_torch_available() and self.device.type != "cuda" and self.fp16:
+        if is_torch_available() and self.device.type != "cuda" and (self.fp16 or self.fp16_full_eval):
-            raise ValueError("Mixed precision training with AMP or APEX (`--fp16`) can only be used on CUDA devices.")
+            raise ValueError(
+                "Mixed precision training with AMP or APEX (`--fp16`) and FP16 evaluation can only be used on CUDA devices."
+            )
         if self.report_to is None:
             logger.info(
                 "The default value for the training argument `--report_to` will change in v5 (from all installed "

tests/test_trainer.py

@@ -14,6 +14,7 @@
 # limitations under the License.
 
 import dataclasses
+import gc
 import os
 import tempfile
 import unittest
@@ -29,6 +30,7 @@ from transformers.testing_utils import (
     require_sentencepiece,
     require_tokenizers,
     require_torch,
+    require_torch_gpu,
     require_torch_multi_gpu,
     slow,
 )
@@ -912,6 +914,62 @@ class TrainerIntegrationTest(unittest.TestCase):
         trainer = get_regression_trainer(skip_memory_metrics=True)
         self.check_mem_metrics(trainer, self.assertNotIn)
 
+    @require_torch_gpu
+    def test_fp16_full_eval(self):
+
+        # this is a sensitive test so let's keep debugging printouts in place for quick diagnosis.
+        # it's using pretty large safety margins, but small enough to detect broken functionality.
+        debug = 0
+
+        bs = 8
+        # make the params somewhat big so that there will be enough RAM consumed to be able to
+        # measure things. We should get about 64KB for a+b in fp32
+        a = torch.ones(1000, bs) + 0.001
+        b = torch.ones(1000, bs) - 0.001
+
+        # 1. with mem metrics enabled
+        trainer = get_regression_trainer(a=a, b=b, eval_len=16)
+        metrics = trainer.evaluate()
+        del trainer
+        gc.collect()
+
+        fp32_init = metrics["init_mem_gpu_alloc_delta"]
+        fp32_eval = metrics["eval_mem_gpu_alloc_delta"]
+
+        if debug:
+            print(f"fp32_init {fp32_init}")
+            print(f"fp32_eval {fp32_eval}")
+
+        # here we expect the model to be preloaded in trainer.__init__ and consume around 64K gpu ram.
+        # perfect world: fp32_init == 64<<10
+        self.assertGreater(fp32_init, 59_000)
+        # after eval should be no extra memory allocated - with a small margin (other than the peak
+        # memory consumption for the forward calculation that gets recovered)
+        # perfect world: fp32_eval == close to zero
+        self.assertLess(fp32_eval, 5_000)
+
+        # 2. with mem metrics disabled
+        trainer = get_regression_trainer(a=a, b=b, eval_len=16, fp16_full_eval=True)
+        metrics = trainer.evaluate()
+        fp16_init = metrics["init_mem_gpu_alloc_delta"]
+        fp16_eval = metrics["eval_mem_gpu_alloc_delta"]
+
+        if debug:
+            print(f"fp16_init {fp16_init}")
+            print(f"fp16_eval {fp16_eval}")
+
+        # here we expect the model to not be preloaded in trainer.__init__, so with a small margin it should be close to 0
+        # perfect world: fp16_init == close to zero
+        self.assertLess(fp16_init, 5_000)
+        # here we put the model on device in eval and only `half()` of it, i.e. about 32K,(again we ignore the peak margin which gets returned back)
+        # perfect world: fp32_init == 32<<10
+        self.assertGreater(fp16_eval, 27_000)
+
+        # 3. relative comparison fp32 vs full fp16
+        # should be about half of fp16_init
+        # perfect world: fp32_init/2 == fp16_eval
+        self.assertAlmostEqual(fp16_eval, fp32_init / 2, delta=5_000)
+
 
 @require_torch
 @require_optuna