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From Marco de Abreu <marco.g.ab...@googlemail.com>
Subject Re: Details regarding upcoming PR for runtime TensorRT integration
Date Mon, 11 Jun 2018 17:53:58 GMT
Hello Marek,

this sounds great! Definitely looking forward to it.

It seems like our mailing list destroyed your formatting. You might want to
consider putting it into a Google Docs document or uploading it to
confluence.

Best regards,
Marco

On Mon, Jun 11, 2018 at 10:50 AM Marek Kolodziej <mkolod@gmail.com> wrote:

> *Hi everyone,This is a quick summary of NVIDIA’s plans for open-sourcing an
> initial integration of TensorRT as a runtime accelerator of MxNet (PR for
> discussion coming in the next few days, ETA of the first draft of the PR is
> this Friday or even earlier). Feedback is appreciated.Best,Marek
> KolodziejNeed for runtime MxNet-TensorRT integration 1. TensorRT provides
> significant acceleration of model inference on NVIDIA GPUs compared to
> running the full graph in MxNet using unfused GPU operators. In addition to
> faster fp32 inference, TensorRT optimizes fp16 inference, and is capable of
> int8 inference (provided the quantization steps are performed). Besides
> increasing throughput, TensorRT significantly reduces inference latency,
> especially for small batches. See more here
> <https://developer.nvidia.com/tensorrt>.2. Despite its benefits, using
> pre-trained models with TensorRT typically requires some effort - either
> re-writing the model using TensorRT’s graph building APIs, or exporting a
> model to ONNX, followed by an import step. Even if the import is simplified
> using ONNX, the TensorRT user still needs to provide their own data
> pipeline, which used to exist in the framework, but no longer does in a
> stand-alone TensorRT deployment with a client application.3. TensorRT is
> very performant, but does not have the full set of MxNet’s operators. While
> that could be addressed with TensorRT plugins, it’s much simpler to reuse
> already-exisitng MxNet operators. Also, the user shouldn’t care about
> knowing which operators are supported by TensorRT and which ones aren’t -
> runtime integration allows the graph partitioner to extract subgraphs
> capable of running inside of TensorRT, place the subgraph in a TensorRT
> operator in MxNet, execute that operator as part of MxNet’s graph
> execusion, and handle non-TensorRT-compatible nodes as regular MxNet
> operators remaining after the TensorRT subgraph extraction and node
> substitution. The goal is to accelerate inference without changing user
> experience.Design considerations 1. Since TensorRT can only determine all
> possible optimizations once the tensor shapes are known, it is imperative
> that all the shape information be provided. This means that the best time
> to construct the TensorRT graph is bind time. The coming PR can selectively
> apply the TensorRT optimization for inference-only graphs at symbol bind
> time. This is in fact consistent with the assumptions about TensorRT made
> on the MxNet Wiki here
> <
> https://cwiki.apache.org/confluence/display/MXNET/Unified+integration+with+external+acceleration+libraries
> >.
> 2. Since as mentioned in #1, TensorRT graph building needs shape
> information only available at bind time, an important goal was not to
> disrupt any existing APIs. Even though C++ permits default function
> arguments, the Python bindings for symbol-related methods (e.g. simple
> bind) are exposed via a C, not C++, API, wired on the Python side using
> Ctypes (e.g. see here
> <
> https://github.com/apache/incubator-mxnet/blob/master/python/mxnet/symbol/symbol.py#L1486:L1521
> >
> for the simple bind integration). This precludes the addition of extra
> arguments without causing breaking changes in the C API. Also, adapting the
> Python code to such changes wouldn’t be enough, since all frontend
> languages use the C (not C++) API for the FFI. Fortunately, C API changes
> could be avoided, by simply letting the user enable or disable the TensorRT
> pass using an environment variable (USE_TENSORRT=1 to enable). This also
> does not diminish the flexibility of the integration, since the graph pass
> can read the environment variable each time symbol binding is done, and
> hence permits turning the graph passes on and off, depending on need. The
> ability to enable and disable the TensorRT pass at runtime also makes unit
> testing easier.3. TensorRT requires that the workspace size is provided at
> graph construction time. This value constitutes the upper limit on the
> amount of memory that TensorRT can use, and does not determine immediate
> use. Since this amount can be hard for the user to know, its limit should
> be set to a reasonable value that the user need not concern themselves
> with. Given that TensorRT integration is applied at bind time and that
> TensorRT engines wrapped in TensorRT nodes are constructed during the graph
> pass rather than the memory allocation pass,  MxNet will only allocate the
> amount needed for the nodes remaining after the TensorRT subgraphs have
> been extracted. This means that no memory will be doubly allocated - first
> for the complete MxNet subgraph and then for TensorRT. However, the
> question remains whether the memory used per TensorRT engine should be a
> configurable parameter, either as a method argument or an environment
> variable, or whether TensorRT should be able to use the maximum available
> GPU memory and then reserve only what it needs. I would like to suggest the
> latter. Since the TensorRT subgraph will typically use less memory than the
> same subgraph in MxNet (due to more layer fusion), it’s extremely unlikely
> that a model which runs purely as an MxNet graph would fail with an ouf of
> memory error when parts or most of the graph run inside TensorRT. Fewer
> knobs (in this case, not giving the user the ability to tweak the maximum
> amount of memory availble to TensorRT would simplify use.4. TensorRT can
> accept graphs constructed using two main approaches: (a) via the TensorRT
> graph API, (b) using ONNX. Approach (a) seems simple on the surface - one
> traverses the NNVM graph, finds subgraphs that TensorRT can execute,
> converts the subgraphs to TensorRT graphs, and substitutes the subgraphs
> with TensorRT nodes, each of which contain the TensorRT engine
> corresponding to the subgraph. However, the approach taken by NVIDA was to
> use ONNX as tha IR. The reason for this is twofold. First, ONNX is a very
> well-known IR, which is supported by the entire deep learning software
> community. This ensures that the design of the IR gets as much feedback as
> possible as to whether the IR is feature complete, and what the semantics
> are. NVIDIA already maintains an ONNX-to-TensorRT converter (link
> <https://github.com/onnx/onnx-tensorrt>), and will continue to do so.
> Whatever changes that may apply to the TensorRT APIs or the internal
> features may be nicely hidden behind the well-established ONNX IR. Second,
> ONNX is growing beyond being merely an IR. As it becomes more of a
> standard, its adoption will be associated with other benefits, such as the
> ability to verify standard compliance.5. Despite the advantages of using
>  the ONNX route described in #4, there are some costs. The main one is the
> dependency on Protobuf. This is a valid criticism on the surface, however,
> since the TensorRT integration requires an opt-in during build time, adding
> one more dependency is not a problem if it is not a mandatory dependency.
> Moreover, the same Protobuf dependency already exists for the MxNet ONNX
> importer, which is now part of the MxNet source tree (link
> <
> https://github.com/apache/incubator-mxnet/blob/76417594e56a85ec0cc9412b9dd2c7e2ab581d8b/docs/api/python/contrib/onnx.md
> >),
> rather than being located in a separate repository. Just like the use of
> the ONNX importer is optional and requires ONNX (and hence also Protobuf),
> the TensorRT build is optional. 6. The optional integration of TensorRT
> will be guarded using a config.mk <http://config.mk> flag (USE_TENSORRT),
> which will function similarly to other flags, such as USE_CUDA, USE_CUDNN,
> etc. Needless to say, USE_TENSORRT will depend on CUDA and cuDNN.7. In
> order to simplify evaluation of the TensorRT build, usability and to run
> unit tests, the PR will come with a Dockerfile, which will allow anyone to
> build MxNet with TensorRT, along with its dependencies, i.e. Protobuf and
> ONNX. APIs / user experienceThere is no change in the inference APIs,
> except for the need to set the MXNET_USE_TENSORRT environment variable to
> 1. For example, in Python, we can simply
> do:os.environ["MXNET_USE_TENSORRT"] = “1”Note that for backward
> compatibility, if the environment variable is not set, it will default to
> 0. Also, unlike some other environment variables that are only checked
> during MxNet initialization, this one gets checked every time graph binding
> happens. This typically happens only once during the inference
> application’s life cycle, but since one can re-bind a symbol to say compare
> a TensorRT and a non-TensorRT run, the check will happen during each
> bind/re-bind to enable that. Since the TensorRT graph pass is enabled using
> an environment variable, no break in the C++, C or any frontend language
> API is needed. Note that there is one more change required - in calling
> simple bind. This doesn’t change the simple bind API, but how it’s called
> relative to the “usual” case, by using some of the arguments which are
> optional. This has to do with the shared_buffer parameter. Before
> explaining how the call changes, let’s consider why it’s necessary: 1. The
> TensorRT graph needs to be constructed during the simple bind call, but
> before memory gets allocated for the non-TensorRT part of the graph. 2.
> TensorRT needs the weights, not just the shapes, to be provided before the
> engine is constructed - it will store them inside the ICudaEngine object.
> The engine will then be serialized inside the NNVM TensorRT op, and
> deserialized when the graph executor takes over. This means that the
> weights need to be provided to the simple bind call to construct the
> TensorRT engine.3. The way to provide the weights is to hand them over to
> the simple bind call via the “shared buffer” argument. The shared buffer
> weights can be provided during the bind call and can be freed by the
> frontend language once binding is complete (e.g. by exiting the relevant
> scope in Python, or calling del).Since we need both arg_params (weights)
> and aux_params (e.g. BatchNorm moments), we need to merge arg_params and
> aux_params into one dictionary. Here’s a Python example:def
> merge_dicts(*dict_args):    """Merge arg_params and aux_params to populate
> shared_buffer"""    result = {}    for dictionary in dict_args:
>        result.update(dictionary)    return resultNow let’s see a use
> example:device = mx.gpu(0)sym, arg_params, aux_params =
>    mx.model.load_checkpoint(model_name, num_epochs)executor =
> sym.simple_bind(ctx=device,    data=data_shape,
>    softmax_label=(batch_size,),    shared_buffer=merge_dicts(arg_params,
> aux_params),,    grad_req='null',    force_rebind=True)Now we can simply
> update data in the executor’s arg dict and run the forward
> pass:executor.arg_dict["data"][:] =
> my_data_batchexecutor.forward(is_train=False)predictions =
> executor.outputs[0].asnumpy()Limitations of initial integration and
> suggested future work 1. Since the new accelerator API proposal (link
> <
> https://cwiki.apache.org/confluence/display/MXNET/Unified+integration+with+external+acceleration+libraries
> >)
> was only published a few days ago and the implementation is still on an
> MxNet fork, the current TensorRT integration doesn’t use that API yet, but
> could be refactored in a future commit to use it. There is nothing in the
> current design that would prevent making use of that API in the near
> future.2. Building the TensorRT engine takes a non-trivial amount of time,
> because the compiler evaluates performance and the hardware on the system
> before creating the fused layers on demand, and then needs to actually
> compile them. For ResNet-50 this may be a few seconds, but larger models
> also exist which may take longer. TensorRT comes with the ability to
> serialize the TensorRT engine for a particular hardware platform. This is
> called the serialization of a TensorRT plan, which is the engine along with
> the ahead-of-time-compiled fused kernels for a given GPU. The first PR of
> the TensorRT integration will not provide for TensorRT plan caching, so
> using TensorRT might have a small start-up cost, but for long-running
> inference processes, this shouldn’t be a problem. Caching the TensorRT plan
> will be addressed in a future commit.3. As mentioned before, the
> reproducibility of the build will be demonstrated using a Docker file that
> will provide an easy way to evaluate the build. The Docker recipe was
> tested on Linux on x86_64, but not other platforms supported by TensorRT
> (Linux on 64-bit ARM  (aarch64), Android on aarch64, QNX on aarch64).
> Supporting other platforms, e.g. Linux on aarch64 (e.g. L4T, i.e. Linux for
> Tegra, on the NVIDIA Jetson platform) is left for subsequent commits. 4.
> The current commit supports many, but not all, of TensorRT operators. For
> example, this integration can run CNNs such as VGG, or ResNet, but not
> necessarily everything that TensorRT can support. More operators will be
> covered in future commits.5. TensorRT supports plugins, which can be
> integrated into the graph pass. However, this was not a priority since the
> runtime TensorRT integration can always fall back to existing MxNet
> operators. Supporting plugins is possible, but will be added in future
> commits.6. The upcoming PR will support fp16 and fp32, but not int8. Since
> int8 support in MxNet is itself very new, figuring out calibration and
> other details is left for a future commit.7. TensorRT 4 is going to have a
> new feature called BYOM (bring your own memory). This means that instead of
> telling TensorRT how much memory it can use, the data/scratch space tensors
> can be provided by MxNet, and can be re-used by MxNet when not running the
> forward pass. The memory in permanent use will then be limited to TensorRT
> storing weights. Support for this feature will be added in a future
> commit.*
>

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