Friday, February 10, 2017

Stingray Renderer Walkthrough #3: Render Contexts

Stingray Renderer Walkthrough #3: Render Contexts

Render Contexts Overview

In the last post we covered how to create and destroy various GPU resources. In this post we will go through the system we have for recording a stream of rendering commands/packages that later gets consumed by the render backend (RenderDevice) where they are translated into actual graphics API calls. We call this interface RenderContext and similar to RenderResourceContext we can have multiple RenderContexts in flight at the same time to achieve data parallelism.

Let’s back up and reiterate a bit what was said in the Overview post. Typically in a frame we take the result of the view frustum culling, split it up into a number of chunks, allocate one RenderContext per chunk and then kick one worker thread per chunk. Each worker thread then sequentially iterates over its range of renderable objects and calls their render() function. The render() function takes the chunk’s RenderContext as one of its argument and is responsible for populating it with commands. When all worker threads are done the resulting RenderContexts gets “dispatched” to the RenderDevice.

So essentially the RenderContext is the output data structure for the second stage Render as discussed in the Overview post.

The RenderContext is very similar to the RenderResourceContext in the sense that it’s a fairly simple helper class for populating a command buffer. There is one significant difference though; the RenderContext also has a mechanics for reasoning about the ordering of the commands in the buffer before they get translated into graphics API calls by the RenderDevice.

Ordering & Buffers

We need a way to reorder commands in one or many RenderContexts to make sure triangles end up on the screen in the right order, or more generally speaking; to schedule our GPU work.

There are many ways of dealing with this but my favorite approach is to just associate one or many commands with a 64 bit sort key and when all commands have been recorded simply sort them on this key before translating them into actual graphics API calls. The approach we are using in Stingray is heavily inspired by Christer Ericsson’s blog post “Order your graphics draw calls around!”. I will be covering our sorting system in more details in my next post, for now the only thing important to grasp is that while the RenderContext records commands it does so by populating two buffers. One is a simple array of a POD struct called Command:

struct Command
{
    uint64_t sort_key;
    void *head;
    uint32_t command_flags;
};
  • sort_key - 64 bit sort key used for reordering commands before being consumed by the RenderDevice, more on this later.
  • head - Pointer to the actual data for this command.
  • command_flags - A bit flag encoding some hinting about what kind of command head is actually pointing to. This is simply an optimization to reduce pointer chasing in the RenderDevice, it will be covered in more detail in a later post.

Render Package Stream

The other buffer is what we call a RenderPackageStream and is what holds the actual command data. The RenderPackageStream class is essentially just a few helper functions to put arbitrary length commands into memory. The memory backing system for RenderPackageStreams is somewhat more complex than a simple array though, this is because we need a way to keep its memory footprint under control. For efficiency, we want to recycle the memory instead of reallocating it every frame, but depending on workload we are likely to get some RenderContexts becoming much larger than others. This creates a problem when using simple arrays to store the commands as the workload will shift slightly over time causing all arrays having to grow to fit the worst case scenario, resulting in lots of wasted memory.

To combat this we allocate and return fixed size blocks of memory from a pool. As we know the size of each command before writing them to the buffer we can make sure that a command doesn’t end up spanning multiple blocks; if we detect that we are about to run out of memory in the active block we simply allocate a new block and move on. If we detect that a single command will span multiple blocks we make sure to allocate them sequentially in memory. We return a block to the pool when we are certain that the consumer of the data (in this case the RenderDevice) is done with it. (This memory allocation approach is well described in Christian Gyrling’s excellent GDC 2015 presentation Parallelizing the Naughty Dog Engine Using Fibers)

You might be wondering why we put the sort_key in a separate array instead of putting it directly into the header data of the packages written to the RenderPackageStream, there are a number of reasons for that:

  1. The actual package data can become fairly large even for regular draw calls. Since we want to make the packages self contained we have to put all data needed to translate the command into an graphics API call inside the package. This includes handles to all resources, constant buffer reflections and similar. I don’t know of any way to efficiently sort an array with elements of varying sizes.

  2. Since we allocate the memory in blocks, as described above, we would need to introduce some form of “jump label” and insert that into the buffer to know how and when to jump into the next memory block. This would further complicate the sorting and traversal of the buffers.

  3. It allows us to recycle the actual package data from one draw call to another when rendering multi-pass shaders as we simply can inject multiple Commands pointing to the same package data. (Which shader pass to use when translating the package into graphic API calls can later be extracted from the sort_key.)

  4. We can reduce pointer chasing by encoding hints in the Command about the contents of the package data. This is what we do in command_flags mentioned earlier.

Render Context interface

With the low-level concepts of the RenderContext covered let’s move on and look at how it is used from a users perspective.

If we break down the API there are essentially three different types of commands that populates a RenderContext:

  1. State commands - Commands affecting the state of the rendering pipeline (e.g render target bindings, viewports, scissoring, etc) + some miscellaneous commands.
  2. Rendering commands - Commands used to trigger draw calls and compute work on the GPU.
  3. Resource update commands - Commands for updating GPU resources.

1. State Commands

“State commands” are a series of commands getting executed in sequence for a specific sort_key. The interface for starting/stopping the recording looks like this:

class RenderContext
{
    void begin_state_command(uint64_t sort_key, uint32_t gpu_affinity_mask = GPU_DEFAULT);
    void end_state_command();
};
  • sort_key - the 64 bit sort key.
  • gpu_affinity_mask - I will cover this towards the end of this post but, for now just think of it as a bit mask for addressing one or many GPUs.

Here’s a small example showing what the recording of a few state commands might look like:

rc.begin_state_command(sort_key);
for (uint32_t i=0; i!=MAX_RENDER_TARGETS; ++i)
    rc.set_render_target(i, nullptr);
rc.set_depth_stencil_target(depth_shadow_map);
rc.clear(RenderContext::CLEAR_DEPTH);
rc.set_viewports(1, &viewport);
rc.set_scissor_rects(1, &scissor_rect);
rc.end_state_command();

While state commands primarily are used for doing bigger graphics pipeline state changes (like e.g. changing render targets) they are also used for some miscellaneous things like clearing of bound render targets, pushing/poping timer markers, and some other stuff. There is no obvious reasoning for grouping these things together under the name “state commands”, it’s just something that has happened over time. Keep that in mind as we go through the list of commands below.

Common commands

  • set_render_target(uint32_t slot, RenderTarget *target, const SurfaceInfo& surface_info);

    • slot - Which index of the “Multiple Render Target” (MRT) chain to bind
    • target - What RenderTarget to bind
    • surface_info - SurfaceInfo is a struct describing which surface of the RenderTarget to bind.
    struct SurfaceInfo {
        uint32_t array_index; // 0 in all cases except if binding a texture array
        uint32_t slice;       // 0 for 2D textures, 0-5 for cube maps, 0-n for volume textures
        uint32_t mip_level;   // 0-n depending on wanted mip level
    };
    
  • set_depth_stencil_target(RenderTarget *target, const SurfaceInfo& surface_info); - Same as above but for depth stencil.

  • clear(RenderContext::ClearFlags flags); - Clears currently bound render targets.

    • flags - enum bit flag describing what parts of the bound render targets to clear.
    enum ClearFlags {
        CLEAR_SURFACE   = 0x1,
        CLEAR_DEPTH     = 0x2,
        CLEAR_STENCIL   = 0x4
    };
    
  • set_viewports(uint32_t n_viewports, const Viewport *viewports);

    • n_viewports - Number of viewports to bind.
    • viewports - Pointer to first Viewport to bind. Viewport is a struct describing the dimensions of the viewport:
    struct Viewport {
        float x, y, width, height;
        float min_depth, max_depth;
    };
    

    Note that x, y, width and height are in unsigned normalized [0-1] coordinates to decouple render target resolution from the viewport.

  • set_scissor_rects(uint32_t n_scissor_rects, const ScissorRect *scissor_rects);

    • n_scissor_rects - Number of scissor rectangles to bind
    • scissor_rects - Pointer to the first ScissorRect to bind.
    struct ScissorRect {
        float x, y, width, height;
    };
    

    Note that x, y, width and height are in unsigned normalized [0-1] coordinates to decouple render target resolution from the scissor rectangle.

A bit more exotic commands

  • set_stream_out_target(uint32_t slot, RenderResource *resource, uint32_t offset);
    • slot - Which index of the stream out buffers to bind
    • resource - Which RenderResource to bind to that slot (has to point to a VertexStream)
    • offset - A byte offset describing where to begin writing in the buffer pointed to by resource.
  • set_instance_multiplier(uint32_t multiplier);
    Allows the user to scale the number instances to render for each render() call (described below). This is a convenience function to make it easier to implement things like Instanced Stereo Rendering.

Markers

  • push_marker(const char *name)
    Starts a new marker scope named name. Marker scopes are both used for gathering RenderDevice statistics (number of draw calls, state switches and similar) as well as for creating GPU timing events. The user is free to nestle markers if they want to better group statistics. More on this in a later post.
  • pop_marker(const char *name)
    Stops an existing marker scope named name.

2. Rendering

With most state commands covered let’s move on and look at how to record commands for triggering draw calls and compute work to a RenderContext.

For that we have a single function called render():

class RenderContext
{
    RenderJobPackage *render(const RenderJobPackage* job,
        const ShaderTemplate::Context& shader_context, uint64_t interleave_sort_key = 0,
        uint64_t shader_pass_branch_key = 0, float job_sort_depth = 0.f,
        uint32_t gpu_affinity_mask = GPU_DEFAULT);
};

job

First argument piped to render() is a pointer to a RenderJobPackage, and as you can see the function also returns a pointer to a RenderJobPackage. What is going on here is that the RenderJobPackage piped as argument to render() gets copied to the RenderPackageStream, the copy gets patched up a bit and then a pointer to the modified copy is returned to allow the caller to do further tweaks to it. Ok, this probably needs some further explanation…

The RenderJobPackage is basically a header followed by an arbitrary length of data that together contains everything needed to make it possible for the RenderDevice to later translate it into either a draw call or a compute shader dispatch. In practice this means that after the RenderJobPackage header we also pack RenderResource::render_resource_handle for all resources to bind to all different shader stages as well as full representations of all non-global shader constant buffers.

Since we are building multiple RenderContexts in parallel and might be visiting the same renderable object (mesh, particle system, etc) simultaneously from multiple worker threads, we cannot mutate any state of the renderable when calling its render() function.

Typically all renderable objects have static prototypes of all RenderJobPackages they need to be drawn correctly (e.g. a mesh with three materials might have three RenderJobPackages - one per material). Naturally though, the renderable objects don’t know anything about in which context they will be drawn (e.g. from what camera or in what kind of lighting environment) up until the point where their render() function gets called and the information is provided. At that point their static RenderJobPackages prototypes somehow needs to be patched up with this information (which typically is in the form of shader constants and/or resources).

One way to handle that would be to create a copy of the prototype RenderJobPackage on the stack, patch up the stack copy and then pipe that as argument to RenderContext::render(). That is a fully valid approach and would work just fine, but since RenderContext::render() needs to create a copy of the RenderJobPackage anyway it is more efficient to patch up that copy directly instead. This is the reason for RenderContext::render() returning a pointer to the RenderJobPackage on the RenderPackageStream.

Before diving into the RenderJobPackage struct let’s go through the other arguments of RenderContext::render():

shader_context

We will go through this in more detail in the post about our shader system but essentially we have an engine representation called ShaderTemplate, each ShaderTemplate has a number of Contexts.

A Context is basically a description of any rendering passes that needs to run for the RenderJobPackage to be drawn correctly when rendered in a certain “context”. E.g. a simple shader might declare two contexts: “default” and “shadow”. The “default” context would be used for regular rendering from a player camera, while the “shadow” context would be used when rendering into a shadow map.

What I call a “rendering pass” in this scenario is basically all shader stages (vertex, pixel, etc) together with any state blocks (rasterizer, depth stencil, blend, etc) needed to issue a draw call / dispatch a compute shader in the RenderDevice.

interleave_sort_key

RenderContext::render() automatically figures out what sort keys / Commands it needs to create on it’s command array. Simple shaders usually only render into one layer in a single pass. In those scenarios RenderContext::render() will create a single Command on the command array. When using a more complex shader that renders into multiple layers and/or needs to render in multiple passes; more than one Command will be created, each command referencing the same RenderJobPackage in its Command::head pointer.

This can feel a bit abstract and is hard to explain without giving you the full picture of how the shader system works together with the data-driven rendering system which in turn dictates the bit allocation patterns of the sort keys, for now it’s enough to understand that the shader system somehow knows what Commands to create on the command array.

The shader author can also decide to bypass the data-driven rendering system and put the scheduling responsibility entirely in the hands of the caller of RenderContext::render(), in this case the sort key of all Commands created will simply become 0. This is where the interleave_sort_key comes into play, this variable will be bitwise ORed with the sort key before being stored in the Command.

shader_pass_branch_key

The shader system has a feature for allowing users to dynamically turn on/off certain rendering passes. Again this becomes somewhat abstract without providing the full picture but basically this system works by letting the shader author flag certain passes with a “tag”. A tag is simply a string that gets mapped to a bit within a 64 bit bit-mask. By bitwise ORing together multiple of these tags and piping the result in shader_pass_branch_key the user can control what passes to activate/deactivate when rendering the RenderJobPackage.

job_sort_depth

A signed normalized [0-1] floating point value used for controlling depth sorting between RenderJobPackages. As you will see in the next post this value simply gets mapped into a bit range of the sort key, removing the need for doing any kind of special trickery to manage things like back-to-front / front-to-back sorting of RenderJobPackages.

gpu_affinity_mask

Same as the gpu_affinity_mask parameter piped to begin_state_command().

RenderJobPackage

Let’s take a look at the actual RenderJobPackage struct:

struct RenderJobPackage
{
    BatchInfo batch_info;
    #if defined(COMPUTE_SUPPORTED)
        ComputeInfo compute_info;
    #endif

    uint32_t size;                          // size of entire package including extra data

    uint32_t n_resources;                   // number of resources assigned to job.
    uint32_t resource_offset;               // offset from start of RenderJobPackage to first RenderResource.

    uint32_t shader_resource_data_offset;   // offset to shader resource data
    RenderResource::Handle shader;          // shader used to execute job

    uint64_t instance_hash;                 // unique hash used for instance merging

    #if defined(DEVELOPMENT)
        ResourceID resource_tag;            // debug tag associating job to a resource on disc
        IdString32 object_tag;              // debug tag associating job to an object
        IdString32 batch_tag;               // debug tag associating job to a sub-batch of an object
    #endif
};

batch_info & compute_info

First two members are two nestled POD structs mainly containing the parameters needed for doing any kind of drawing or dispatching of compute work in the RenderDevice:

struct BatchInfo
{
    enum PrimitiveType {
        TRIANGLE_LIST,
        LINE_LIST
        // ...
    };
    enum FrontFace {
        COUNTER_CLOCK_WISE = 0,
        CLOCK_WISE = 1
    };

    PrimitiveType primitive_type;
    uint32_t vertex_offset;         // Offset to first vertex to read from vertex buffer.
    uint32_t primitives;            // Number of primitives to draw
    uint32_t index_offset;          // Offset to the first index to read from the index buffer
    uint32_t vertices;              // Number of vertices in batch (used if batch isn't indexed)
    uint32_t instances;             // Number of instances of this batch to draw
    FrontFace front_face;           // Defines which triangle winding order
};

Most of these are self explanatory, I think the only thing worth pointing out is the front_face enum. This is here to dynamically handle flipping of the primitive winding order when dealing with objects that are negatively scaled on an uneven number of axes. For typical game content it’s rare that we see content creators using mesh mirroring when modeling, for other industries however it is a normal workflow.

struct ComputeInfo
{
    uint32_t thread_count[3];
    bool async;
};

So while BatchInfo mostly holds the parameters needed to render something, ComputeInfo hold the parameters to dispatch a compute shader. The three element array thread_count containing the thread group count for x, y, z. If async is true the graphics API’s “compute queue” will be used instead of the “graphics queue”.

resource_offset

Byte offset from start of RenderJobPackage to an array of n_resources with RenderResource::Handle. Resources found in this array can be of the type VertexStream, IndexStream or VertexDeclaration. Based on the their type and order in the array they get bound to the input assembler stage in the RenderDevice.

shader_resource_data_offset

Byte offset from start of RenderJobPackage to a data block holding handles to all RenderResources as well as all constant buffer data needed by all the shader stages. The layout of this data blob will be covered in the post about the shader system.

instance_hash

We have a system for doing what we call “instance merging”, this system figures out if two RenderJobPackages only differ on certain shader constants and if so merges them into the same draw call. The shader author is responsible but not required to implement support for this feature. If the shader supports “instance merging” the system will use the instance_hash to figure out if two RenderJobPackages can be merged or not. Typically the instance_hash is simply a hash of all RenderResource::Handle that the shader takes as input.

resource_tag & object_tag & batch_tag

Three levels of debug information to make it easier to back track errors/warning inside the RenderDevice to the offending content.

3. Resource updates

The last type of commands are for dynamically updating various RenderResources (Vertex/Index/Raw buffers, Textures, etc).

The interface for updating a buffer with new data looks like this:

class RenderContext
{
    void *map_write(RenderResource *resource, render_sorting::SortKey sort_key,
        const ShaderTemplate::Context* shader_context = 0,
        shader_pass_branching::Flags shader_pass_branch_key = 0,
        uint32_t gpu_affinity_mask = GPU_DEFAULT);
};

resource

This function basically returns a pointer to the first byte of the buffer that will replace the contents of the resource. map_write() figures out the size of the buffer by casting the resource to the correct type (using the type information encoded in the RenderResource::render_resource_handle). It then allocates memory for the buffer and a small header on the RenderPackageStream and returns a pointer to the buffer.

sort_key & shader_context & shader_pass_branch_key

In some rare situations you might need to update the same buffer with different data multiple times within a frame. A typical example could be the vertex buffer of a particle system implementing some kind of level-of-detail system causing the buffers to change depending on e.g camera position. To support that the user can provide a bunch of extra parameters to make sure the contents of the GPU representation of the buffer is updated right before the graphics API draw calls are triggered for the different rendering passes. This works in a similar way how RenderContext::render() can create multiple Commands on the command array referencing the same data.

Unless you need to update the buffer multiple times within the frame it is safe to just set all of the above mentioned parameters to 0, making it very simple to update a buffer:

void *buf = rc.map_write(resource, 0);
// .. fill bits in buffer ..

Note: To shorten the length of this post I’ve left out a few other flavors of updating resources, but map_write is the most important one to grasp.

GPU Queues, Fences & Explicit MGPU programming

Before wrapping up I’d like to touch on a few recent additions to the Stingray renderer, namely how we’ve exposed control for dealing with different GPU Queues, how to synchronize between them and how to control, communicate and synchronize between multiple GPUs.

New graphics APIs such as DX12 and Vulkan exposes three different types of command queues: Graphics, Compute and Copy. There’s plenty of information on the web about this so I won’t cover it here, the only thing important to understand is that these queues can execute asynchronously on the GPU; hence we need to have a way to synchronize between them.

To handle that we have exposed a simple fence API that looks like this:

class RenderContext
{
    struct FenceMessage
    {
        enum Operation { SIGNAL, WAIT };
        Operation operation;
        IdString32 fence_name;
    };
    void signal_fence(IdString32 fence_name, render_sorting::SortKey sort_key,
        uint32_t queue = GRAPHICS_QUEUE, uint32_t gpu_affinity_mask = GPU_DEFAULT);
    void wait_fence(IdString32 fence_name, render_sorting::SortKey sort_key,
        uint32_t queue = GRAPHICS_QUEUE, uint32_t gpu_affinity_mask = GPU_DEFAULT);
};

Here’s a pseudo code snippet showing how to synchronize between the graphics queue and the compute queue:

uint64_t sort_key = 0;

// record a draw call
rc.render(graphics_job, graphics_shader, sort_key++);

// record an asynchronous compute job
// (ComputeInfo::async bool in async_compute_job is set to true to target the graphics APIs compute queue)
rc.render(async_compute_job, compute_shader, sort_key++);

// now lets assume the graphics queue wants to use the result of the async_compute_job,
// for that we need to make sure that the compute shader is done running
rc.wait_fence(IdString32("compute_done"), sort_key++, GRAPHICS_QUEUE);
rc.signal_fence(IdString32("compute_done"), sort_key++, COMPUTE_QUEUE);

rc.render(graphics_job_using_result_from_compute, graphics_shader2, sort_key++);

As you might have noticed all methods for populating a RenderContext described in this post also takes an extra parameter called gpu_affinity_mask. This is a bit-mask used for directing commands to one or many GPUs. The idea is simple, when we boot up the renderer we enumerate all GPUs present in the system and decide which one to use as our default GPU (GPU_DEFAULT) and assign that to bit 1. We also let the user decide if there are other GPUs present in the system that should be available to Stingray and if so assign them bit 2, 3, 4, and so on. By doing so we can explicitly direct control of all commands put on the RenderContext to one or many GPUs in a simple way.

As you can see that is also true for the fence API described above, on top of that there’s also a need for a copy interface to copying resources between GPUs:

class RenderContext
{
    void copy(RenderResource *dst_resource, RenderResource *src_resource,
        render_sorting::SortKey sort_key, Box *src_box = 0, uint32_t dst_offsets[3] = 0,
        uint32_t queue = GRAPHICS_QUEUE, uint32_t gpu_affinity_mask = GPU_DEFAULT,
        uint32_t gpu_source = GPU_DEFAULT, uint32_t gpu_destination = GPU_DEFAULT);
};

Even though this work isn’t fully completed I still wanted to share the high-level idea of what we are working towards for exposing explicit MGPU control to the Stingray renderer. We are actively working on this right now and with some luck I might be able to revisit this with more concrete examples when getting to the post about the render_config & data-driven rendering.

Next up

With that I think I’ve covered the most important aspects of the RenderContext. Next post will dive a bit deeper into bit allocation ranges of the sort keys and the system for sorting in general, hopefully that post will become a bit shorter.

No comments:

Post a Comment