Introduction to DirectML
Direct Machine Learning (DirectML) is a low-level API for machine learning (ML). Hardware-accelerated machine learning primitives (called operators) are the building blocks of DirectML. From those building blocks, you can develop such machine learning techniques as upscaling, anti-aliasing, and style transfer, to name but a few. Denoising and super-resolution, for example, allow you to achieve impressive raytraced effects with fewer rays per pixel.
You can integrate machine learning inferencing workloads into your game, engine, middleware, backend, or other application. DirectML has a familiar (native C++, nano-COM) DirectX 12-style programming interface and workflow, and it's supported by all DirectX 12-compatible hardware. For DirectML sample applications, including a sample of a minimal DirectML application, see DirectML sample applications.
DirectML is introduced in Windows 10, version 1903, and in the corresponding version of the Windows SDK.
Is DirectML appropriate for my project?
DirectML is a component under the Windows Machine Learning umbrella. The higher-level WinML API is primarily model-focused, with its load-bind-evaluate workflow. But domains such as games and engines typically need a lower level of abstraction, and a higher degree of developer control, in order to take full advantage of the silicon. If you're counting milliseconds, and squeezing frame times, then DirectML will meet your machine learning needs.
For reliable real-time, high-performance, low-latency, and/or resource-constrained scenarios, use DirectML (rather than WinML). You can integrate DirectML directly into your existing engine or rendering pipeline. Or, at a higher level for custom machine learning frameworks and middleware, DirectML can provide a high-performance backend on Windows.
WinML is itself implemented using DirectML as one of its backends.
What work does DirectML do; and what work must I do as the developer?
DirectML efficiently executes the individual layers of your inference model on the GPU (or on AI-acceleration cores, if present). Each layer is an operator, and DirectML provides you with a library of low-level, hardware-accelerated machine learning primitive operators. These operators have hardware-specific and architecture-specific optimizations designed in to them (more on that in the section Why does DirectML perform so well?). At the same time, you as the developer see a single, vendor-agnostic interface for executing those operators.
The library of operators in DirectML supplies all of the usual operations that you'd expect to be able to use in a machine learning workload.
- Activation operators, such as linear, ReLU, sigmoid, tanh, and more.
- Element-wise operators, such as add, exp, log, max, min, sub, and more.
- Convolution operators, such as 2D and 3D convolution, and more.
- Reduction operators, such as argmin, average, l2, sum, and more.
- Pooling operators, such as average, lp, and max.
- Neural network (NN) operators, such as gemm, gru, lstm, and rnn.
- And many more.
For maximal performance, and so that you don't pay for what you don't use, DirectML puts the control into your hands as a developer over how your machine learning workload is executed on the hardware. Figuring out which operators to execute, and when, is your responsibility as the developer. Tasks that are left to your discretion include: transcribing the model; simplifying and optimizing your layers; loading weights; resource allocation, binding, memory management (just as with Direct3D 12); and execution of the graph.
You retain high-level knowledge of your graphs (you can hard-code your model directly, or you can write your own model loader). You might design an upscaling model, for example, using several layers each of upsample, convolution, normalization, and activation operators. With that familiarity, careful scheduling, and barrier management, you can extract the most parallelism and performance from the hardware. If you're developing a game, then your careful resource management and control over scheduling enables you to interleave machine learning workloads and traditional rendering work in order to saturate the GPU.
What's the high-level DirectML workflow?
Here's the high-level recipe for how we expect DirectML to be used. Within the two main phases of initialization and execution, you record work into command lists and then you execute them on a queue.
- Create your Direct3D 12 resources—the Direct3D 12 device, command queue, command list, and resources such as descriptor heaps.
- Since you're doing machine learning inferencing as well as your rendering workload, create DirectML resources—the DirectML device, and operator instances. If you have a machine learning model where you need to perform a particular type of convolution with a particular size of filter tensor with a particular data type, then those are all parameters into DirectML's convolution operator.
- DirectML records work into Direct3D 12 command lists. So, once initialization is done, you record the binding and initialization of (for example) your convolution operator into your command list. Then, close and execute your command list on your queue as usual.
- Upload your weight tensors into resources. A tensor in DirectML is represented using a regular Direct3D 12 resource. For example, if you want to upload your weight data to the GPU, then you do that the same way you would with any other Direct3D 12 resource (use an upload heap, or the copy queue).
- Next, you need to bind those Direct3D 12 resources as your input and output tensors. Record into your command list the binding and the execution of your operators.
- Close and execute your command list.
Just as with Direct3D 12, resource lifetime and synchronization are your responsibility. For example, don't release your DirectML objects at least until they've completed execution on the GPU.
Why does DirectML perform so well?
There's a good reason why you shouldn't just write your own convolution operator (for example) as HLSL in a compute shader. The advantage of using DirectML is that—apart from saving you the effort of homebrewing your own solution—it has the capability of giving you much better performance than you could achieve with a hand-written, general-purpose compute shader for something like convolution, or lstm.
DirectML achieves this in part due to the Direct3D 12 metacommands feature. Metacommands expose a black box of functionality up to DirectML, which allows hardware vendors to provide DirectML access to vendor hardware-specific and architecture-specific optimizations. Multiple operators—for example, convolution followed by activation—can be fused together into a single metacommand. Because of these factors, DirectML has the capability to exceed the performance of even a very well-written hand-tuned compute shader written to run on a breadth of hardware.
Metacommands are part of the Direct3D 12 API, although they're loosely coupled to it. A metacommand is identified by a fixed GUID, while almost everything else about it (from its behavior and semantics to its signature and name) are not strictly part of the Direct3D 12 API. Rather, a metacommand is specified between its author and the driver that implements it. In this case, the author is DirectML. Metacommands are Direct3D 12 primitives (just like Draws and Dispatches), so they can be recorded into a command list and scheduled for execution together.
DirectML accelerates your machine learning workloads using an entire suite of machine learning metacommands. Consequently, you don't need to write vendor-specific code paths to achieve hardware acceleration for your inferencing. If you happen to run on an AI-accelerated chip, then DirectML uses that hardware to greatly accelerate operations such as convolution. You can take the same code that you wrote, without modifying it, run it on a chip that's not AI-accelerated (perhaps the integrated GPU in your laptop), and still get great GPU hardware acceleration. And if no GPU is available, then DirectML falls back to the CPU.