Merge pull request #173 from NVIDIA/api_refactor

Unified mixed precision API + backend performance improvements

docs/source/amp.rst

@@ -4,8 +4,231 @@
 apex.amp
 ===================================
 
-Amp (Automatic Mixed Precision) is a tool designed for ease of use and maximum safety in FP16 training. All potentially unsafe ops are performed in FP32 under the hood, while safe ops are performed using faster, Tensor Core-friendly FP16 math. Amp also automatically implements dynamic loss scaling.
+This page documents the updated API for Amp (Automatic Mixed Precision),
+a tool to enable Tensor Core-accelerated training in only 3 lines of Python.
 
-The intention of Amp is to be the "on-ramp" to easy FP16 training: achieve all the numerical stability of full FP32 training, with most of the performance benefits of full FP16 training.
+Users **should not** manually cast their model or data to ``.half()``, regardless of what ``opt_level``
+or properties are chosen.  Amp intends that users start with an existing default (FP32) script,
+add the three lines corresponding to the Amp API, and begin training with mixed precision.
+Amp can also be disabled, in which case the original script will behave exactly as it used to.
+In this way, there's no risk adhering to the Amp API, and a lot of potential performance benefit.
 
-Currently, complete API documentation resides on the Github page: https://github.com/NVIDIA/apex/tree/master/apex/amp.
+Example::
+
+        # Declare model and optimizer as usual
+        model = torch.nn.Linear(D_in, D_out).cuda().half()
+        optimizer = torch.optim.SGD(model.parameters(), lr=1e-3)
+
+        # Allow Amp to perform casts as required by the opt_level
+        model, optimizer = amp.initialize(model, optimizer, opt_level="O1")
+        ...
+        # loss.backward() becomes:
+        with amp.scale_loss(loss, optimizer) as scaled_loss:
+            scaled_loss.backward()
+        ...
+
+A `runnable, comprehensive Imagenet example`_ demonstrating good practices can be found
+on the Github page.
+
+GANs are a tricky case that many people have requested.  A `comprehensive DCGAN example`_
+is under construction.
+
+.. _`runnable, comprehensive Imagenet example`:
+    https://github.com/NVIDIA/apex/tree/master/examples/imagenet
+
+.. _`comprehensive DCGAN example`:
+    https://github.com/NVIDIA/apex/tree/master/examples/dcgan
+
+``opt_level``\ s and Properties
+-------------------------------
+
+Amp allows users to easily experiment with different pure and mixed precision modes, including
+pure FP16 training and pure FP32 training.  Commonly-used default modes are chosen by
+selecting an "optimization level" or ``opt_level``; each ``opt_level`` establishes a set of
+properties that govern Amp's implementation of pure or mixed precision training.
+Finer-grained control of how a given ``opt_level`` behaves can be achieved by passing values for
+particular properties directly to ``amp.initialize``.  These manually specified values will
+override the defaults established by the ``opt_level``.
+
+Properties
+**********
+
+Currently, the under-the-hood properties that govern pure or mixed precision training are the following:
+
+- ``cast_model_type``:  Casts your model's parameters and buffers to the desired type.
+- ``patch_torch_functions``: Patch all Torch functions and Tensor methods to perform Tensor Core-friendly ops like GEMMs and convolutions in FP16, and any ops that benefit from FP32 precision in FP32.
+- ``keep_batchnorm_fp32``:  To enhance precision and enable cudnn batchnorm (which improves performance), it's often beneficial to keep batchnorms in particular in FP32 even if the rest of the model is FP16.
+- ``master_weights``:  Maintain FP32 master weights to accompany any FP16 model weights.  FP32 master weights are stepped by the optimizer to enhance precision and capture small gradients.
+- ``loss_scale``:  If ``loss_scale`` is a float value, use this value as the static (fixed) loss scale.  If ``loss_scale`` is the string ``"dynamic"``, adapatively adjust the loss scale over time.  Dynamic loss scale adjustments are performed by Amp automatically.
+
+Again, you often don't need to specify these properties by hand.  Instead, select an ``opt_level``,
+which will set them up for you.  After selecting an ``opt_level``, you can optionally pass property
+kwargs as manual overrides.
+
+If you attempt to override a property that does not make sense for the selected ``opt_level``,
+Amp will raise an error with an explanation.  For example, selecting ``opt_level="O1"`` combined with
+the override ``master_weights=True`` does not make sense.  ``O1`` inserts casts
+around Torch functions rather than model weights.  Data, activations, and weights are recast
+out-of-place on the fly as they flow through patched functions.  Therefore, the model weights themselves
+can (and should) remain FP32, and there is no need to maintain separate FP32 master weights.
+
+``opt_level``\ s
+****************
+
+``O0`` and ``O3`` are not true mixed precision, but they are useful for establishing baselines.
+
+``O0``:  FP32 training
+^^^^^^^^^^^^^^^^^^^^^^
+Your incoming model should be FP32 already, so this is likely a no-op.
+``O0`` can be useful to establish an accuracy baseline.
+
+| Default properties set by ``O0``:
+| ``cast_model_type=torch.float32``
+| ``patch_torch_functions=False``
+| ``keep_batchnorm_fp32=None`` (effectively, "not applicable")
+| ``master_weights=False``
+| ``loss_scale=1.0``
+|
+|
+
+``O3``:  FP16 training
+^^^^^^^^^^^^^^^^^^^^^^
+``O3`` may not achieve the stability of the true mixed precision options ``O1`` and ``O2``.
+However, it can be useful to establish a speed baseline for your model, against which
+the performance of ``O1`` and ``O2`` can be compared.  If your model uses batch normalization,
+to establish "speed of light" you can try ``O3`` with the additional property override
+``keep_batchnorm_fp32=True`` (which enables cudnn batchnorm, as stated earlier).
+
+| Default properties set by ``O3``:
+| ``cast_model_type=torch.float16``
+| ``patch_torch_functions=False``
+| ``keep_batchnorm_fp32=False``
+| ``master_weights=False``
+| ``loss_scale=1.0``
+|
+|
+
+``O1`` and ``O2`` are different implementations of mixed precision.  Try both, and see
+what gives the best speedup and accuracy for your model.
+
+``O1``:  Conservative Mixed Precision
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Patch all Torch functions and Tensor methods to cast their inputs according to a whitelist-blacklist
+model.  Whitelist ops (for example, Tensor Core-friendly ops like GEMMs and convolutions) are performed
+in FP16.  Blacklist ops that benefit from FP32 precision (for example, batchnorm and softmax)
+are performed in FP32.  ``O1`` also uses dynamic loss scaling, unless overridden.
+
+| Default properties set by ``O1``:
+| ``cast_model_type=None`` (not applicable)
+| ``patch_torch_functions=True``
+| ``keep_batchnorm_fp32=None`` (not necessary to specify True, batchnorm inputs are cast to FP32)
+| ``master_weights=None`` (not applicable, model weights remain FP32)
+| ``loss_scale="dynamic"``
+|
+|
+
+``O2``:  Fast Mixed Precision
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+``O2`` casts the model to FP16, keeps batchnorms in FP32, maintains master weights in FP32,
+and implements dynamic loss scaling (unless overridden).
+Unlike ``O1``, ``O2`` does not patch Torch functions or Tensor methods.
+
+| Default properties set by ``O2``:
+| ``cast_model_type=torch.float16``
+| ``patch_torch_functions=False``
+| ``keep_batchnorm_fp32=True``
+| ``master_weights=True``
+| ``loss_scale="dynamic"``
+|
+|
+
+
+Unified API
+-----------
+
+.. automodule:: apex.amp
+.. currentmodule:: apex.amp
+
+.. autofunction:: initialize
+
+.. autofunction:: scale_loss
+
+.. autofunction:: master_params
+
+Advanced use cases
+------------------
+
+The unified Amp API supports gradient accumulation across iterations,
+multiple backward passes per iteration, multiple models/optimizers,
+and custom/user-defined autograd functions.  Gradient clipping and GANs also
+require special treatment, but this treatment does not need to change
+for different ``opt_level``\ s.  Further details can be found here:
+
+.. toctree::
+   :maxdepth: 1
+
+   advanced
+
+Transition guide for old API users
+----------------------------------
+
+We strongly encourage moving to the new Amp API, because it's more versatile, easier to use, and future proof.  The original :class:`FP16_Optimizer` and the old "Amp" API are deprecated, and subject to removal at at any time.
+
+For users of the old "Amp" API
+******************************
+
+In the new API, ``opt-level O1`` performs the same patching of the Torch namespace as the old thing
+called "Amp."
+However, the new API allows static or dynamic loss scaling, while the old API only allowed dynamic loss scaling.
+
+In the new API, the old call to ``amp_handle = amp.init()``, and the returned ``amp_handle``, are no
+longer exposed or necessary.  The new ``amp.initialize()`` does the duty of ``amp.init()`` (and more).
+Therefore, any existing calls to ``amp_handle = amp.init()`` should be deleted.
+
+The functions formerly exposed through ``amp_handle`` are now free
+functions accessible through the ``amp`` module.
+
+The backward context manager must be changed accordingly::
+
+    # old API
+    with amp_handle.scale_loss(loss, optimizer) as scaled_loss:
+        scaled_loss.backward()
+    ->
+    # new API
+    with amp.scale_loss(loss, optimizer) as scaled_loss:
+        scaled_loss.backward()
+
+For now, the deprecated "Amp" API documentation can still be found on the Github README:  https://github.com/NVIDIA/apex/tree/master/apex/amp.  The old API calls that `annotate user functions`_ to run
+with a particular precision are still honored by the new API.
+
+.. _`annotate user functions`:
+    https://github.com/NVIDIA/apex/tree/master/apex/amp#annotating-user-functions
+
+
+For users of the old FP16_Optimizer
+***********************************
+
+``opt-level O2`` is equivalent to :class:`FP16_Optimizer` with ``dynamic_loss_scale=True``.
+Once again, the backward pass must be changed to the unified version::
+
+    optimizer.backward(loss)
+    ->
+    with amp.scale_loss(loss, optimizer) as scaled_loss:
+        scaled_loss.backward()
+
+One annoying aspect of FP16_Optimizer was that the user had to manually convert their model to half
+(either by calling ``.half()`` on it, or using a function or module wrapper from
+``apex.fp16_utils``), and also manually call ``.half()`` on input data.  **Neither of these are
+necessary in the new API.  No matter what --opt-level
+you choose, you can and should simply build your model in the default FP32 format.**  The new Amp
+API will perform the right conversions during
+``model, optimizer = amp.initialize(model, optimizer, opt_level=....)`` based on the ``--opt-level``
+and any overridden flags.  Floating point input data may be FP32 or FP16, but you may as well just
+let it be FP16, because the ``model`` returned by ``amp.initialize`` will have its ``forward``
+method patched to cast the input data appropriately.
+
+.. note::
+    Aside from the call to ``amp.initialize`` itself, it's never necessary to manually cast
+    your model or data with the new API.  Therefore, a script that adheres to the new API
+    can switch between different ``opt-level``\ s without having to make any other changes.

docs/source/index.rst

@@ -11,14 +11,7 @@ Apex (A PyTorch Extension)
 This site contains the API documentation for Apex (https://github.com/nvidia/apex),
 a Pytorch extension with NVIDIA-maintained utilities to streamline mixed precision and distributed training.  Some of the code here will be included in upstream Pytorch eventually. The intention of Apex is to make up-to-date utilities available to users as quickly as possible.
 
-Installation requires CUDA 9 or later, PyTorch 0.4 or later, and Python 3. Install by running
-
-::
-
-   git clone https://www.github.com/nvidia/apex
-   cd apex
-   python setup.py install [--cuda_ext] [--cpp_ext]
-
+Installation instructions can be found here:  https://github.com/NVIDIA/apex#quick-start.
 
 .. toctree::
    :maxdepth: 1
@@ -26,12 +19,6 @@ Installation requires CUDA 9 or later, PyTorch 0.4 or later, and Python 3. Insta
 
    amp
 
-.. toctree::
-   :maxdepth: 1
-   :caption: FP16/Mixed Precision Utilities
-
-   fp16_utils
-
 .. toctree::
    :maxdepth: 1
    :caption: Distributed Training
@@ -50,6 +37,11 @@ Installation requires CUDA 9 or later, PyTorch 0.4 or later, and Python 3. Insta
 
    layernorm
 
+..   .. toctree::
+     :maxdepth: 1
+     :caption: Deprecated mixed precision API
+     fp16_util
+
 ..   reparameterization
 ..   RNN
    

examples/FP16_Optimizer_simple/distributed_apex_legacy_launcher/README.md

-**distributed_data_parallel.py** and **run.sh** show an example using `FP16_Optimizer` with
-`apex.parallel.DistributedDataParallel` in conjuction with the legacy Apex
-launcher script, `apex.parallel.multiproc`.  See 
-[FP16_Optimizer_simple/distributed_apex](https://github.com/NVIDIA/apex/tree/torch_launcher/examples/FP16_Optimizer_simple/distributed_apex) for a more up-to-date example that uses the Pytorch launcher
-script, `torch.distributed.launch`.
-The usage of `FP16_Optimizer` with distributed does not need to change from ordinary 
-single-process usage.  Test via
-```bash
-bash run.sh
-```

examples/FP16_Optimizer_simple/distributed_apex_legacy_launcher/distributed_data_parallel.py

-import torch
-import argparse
-from apex.parallel import DistributedDataParallel as DDP
-from apex.fp16_utils import FP16_Optimizer
-
-parser = argparse.ArgumentParser()
-parser.add_argument('--dist-url', default='tcp://224.66.41.62:23456', type=str,
-                    help='url used to set up distributed training')
-parser.add_argument('--world-size', default=2, type=int,
-                    help='Number of distributed processes.')
-parser.add_argument("--rank", type=int,
-                    help='Rank of this process')
-
-args = parser.parse_args()
-
-torch.cuda.set_device(args.rank)
-torch.distributed.init_process_group(backend='nccl',
-                                     init_method=args.dist_url,
-                                     world_size=args.world_size,
-                                     rank=args.rank)
-
-torch.backends.cudnn.benchmark = True
-
-N, D_in, D_out = 64, 1024, 16
-
-x = torch.randn(N, D_in, device='cuda', dtype=torch.half)
-y = torch.randn(N, D_out, device='cuda', dtype=torch.half)
-
-model = torch.nn.Linear(D_in, D_out).cuda().half()
-model = DDP(model)
-
-optimizer = torch.optim.SGD(model.parameters(), lr=1e-3)
-### Construct FP16_Optimizer ###
-optimizer = FP16_Optimizer(optimizer)
-###
-
-loss_fn = torch.nn.MSELoss()
-
-for t in range(500):
-    optimizer.zero_grad()
-    y_pred = model(x)
-    loss = loss_fn(y_pred.float(), y.float())
-    ### Change loss.backward() to: ###
-    optimizer.backward(loss)
-    ###
-    optimizer.step()
-
-print("final loss = ", loss)
-

examples/FP16_Optimizer_simple/distributed_apex_legacy_launcher/run.sh

-#!/bin/bash
-# By default, apex.parallel.multiproc will attempt to use all available GPUs on the system.  
-# The number of GPUs to use can be limited by setting CUDA_VISIBLE_DEVICES:
-export CUDA_VISIBLE_DEVICES=0,1
-python -m apex.parallel.multiproc distributed_data_parallel.py

examples/README.md

-## Contents:
+This directory contains examples illustrating Apex mixed precision and distributed tools.
 
-**distributed**:  Walkthrough of apex distributed data parallel utilities.
-
-**FP16_Optimizer_simple**:  Simple examples demonstrating various use cases of `FP16_Optimizer` to automatically manage master parameters and static or dynamic loss scaling.
-
-**imagenet**:  Example based on [https://github.com/pytorch/examples/tree/master/imagenet](https://github.com/pytorch/examples/tree/master/imagenet) showing the use of `FP16_Optimizer`, as well as manual management of master parameters and loss scaling for illustration/comparison.
-
-**word_language_model**:  Example based on [https://github.com/pytorch/examples/tree/master/word_language_model](https://github.com/pytorch/examples/tree/master/word_language_model) showing the use of `FP16_Optimizer`, as well as manual management of master parameters and loss scaling for illustration/comparison.
-
-**docker**:  Example of a minimal Dockerfile that installs Apex on top of an existing container.
+**Note for users of the pre-unification API**:
+`deprecated_api` contains examples illustrating the old (pre-unified) APIs.  These APIs will be removed soon, and users are strongly encouraged to switch.  The separate mixed precision tools called `Amp` and `FP16_Optimizer` in the old API are exposed via different flags/optimization levels in the new API.

examples/dcgan/README.md

+Under construction...

examples/deprecated_api/README.md

+## Contents:
+
+**distributed**:  Walkthrough of apex distributed data parallel utilities.
+
+**FP16_Optimizer_simple**:  Simple examples demonstrating various use cases of `FP16_Optimizer` to automatically manage master parameters and static or dynamic loss scaling.
+
+**imagenet**:  Example based on [https://github.com/pytorch/examples/tree/master/imagenet](https://github.com/pytorch/examples/tree/master/imagenet) showing the use of `FP16_Optimizer`, as well as manual management of master parameters and loss scaling for illustration/comparison.
+
+**word_language_model**:  Example based on [https://github.com/pytorch/examples/tree/master/word_language_model](https://github.com/pytorch/examples/tree/master/word_language_model) showing the use of `FP16_Optimizer`, as well as manual management of master parameters and loss scaling for illustration/comparison.
+
+**docker**:  Example of a minimal Dockerfile that installs Apex on top of an existing container.