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PerformanceGuide.md

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Shader Resource Variables

Diligent Engine uses three variable types:

  • Static variables can be set only once in each pipeline state object or pipeline resource signature. Once bound, the resource can't be changed.
  • Mutable variables can be set only once in each shader resource binding instance. Once bound, the resource can't be changed.
  • Dynamic variables can be bound any number of times.

Internally, static and mutable variables are implemented in the same way. Static variable bindings are copied from PSO or Signature to the SRB either when the SRB is created or when IPipelineState::InitializeStaticSRBResources() method is called.

Dynamic variables introduce some overhead every time an SRB is committed (even if no bindings have changed). Prefer static/mutable variables over dynamic ones whenever possible.

Dynamic Buffers

USAGE_DYNAMIC buffers introduce some overhead in every draw or dispatch command that uses an SRB with dynamic buffers. Note that extra work is performed even if none of the dynamic buffers have been mapped between the commands. If this is the case, an application should use DRAW_FLAG_DYNAMIC_RESOURCE_BUFFERS_INTACT flag to tell the engine that none of the dynamic buffers have been updated between the commands. Note that the first time an SRB is bound, dynamic buffers are properly bound regardless of the DRAW_FLAG_DYNAMIC_RESOURCE_BUFFERS_INTACT flag.

Constant and structured buffers set with IShaderResourceBinding::SetBufferRange method count as dynamic buffers when the specified range does not cover the entire buffer regardless of the buffer usage, unless the variable was created with NO_DYNAMIC_BUFFERS flag. Note that in the latter case, IShaderResourceBinding::SetBufferOffset method can't be used to set dynamic offset. Similar to USAGE_DYNAMIC buffers, if an applications knows that none of the dynamic offsets have changed between the draw calls, it may use DRAW_FLAG_DYNAMIC_RESOURCE_BUFFERS_INTACT flag.

An application should try to use as few dynamic buffers as possible. On some implementations, the number of dynamic buffers that can be used by a PSO may be limited by as few as 8 buffers. If an application knows that no dynamic buffers will be bound to a shader resource variable, it should use the SHADER_VARIABLE_FLAG_NO_DYNAMIC_BUFFERS flag when defining the variables through the PSO layout or PIPELINE_RESOURCE_FLAG_NO_DYNAMIC_BUFFERS when defining the variable through pipeline resource signature. It is an error to bind USAGE_DYNAMIC buffer to a variable that was created with NO_DYNAMIC_BUFFERS flag. Likewise, it is an error to set dynamic offset for such variable.

Try to optimize dynamic buffers usage, but don't strive to avoid them as they are usually the fastest way to upload frequently changing data to the GPU.

Render pass

Use IRenderPass for better performance on mobile GPUs, including ARM-based laptops and desktops.

Vulkan backend natively supports render pass subpasses. The subpasses allow rendering multiple passes in on-chip memory without the need to load and store data to the global memory. This is faster, saves memory bandwidth and reduces power consumption.

Metal backend and Vulkan on top of Metal don't natively support subpasses. To leverage the on-chip memory, use Imageblocks, Tile shader, and Raster order groups.

Profilers

References