Software backend - servo/webrender GitHub Wiki

llvmpipe vs swiftshader current performance investigation. SwiftShader runs one vertex at a time when if the vertex shader has texture loads.

Current plan

  • Try to get WebRender running on llvmpipe as well as possible to get a feel for how well software can work.
  • Figure out bottlenecks when running on llvmpipe.
  • Investigate a custom rasterizer.

Architecture

Rasterization

SwiftShader rasterizes into a 'struct Primitive' which contains a set of spans that describe the rasterized polygon/triangle. It does this one edge at a time.

Shaders

llvmpipe and swiftshader both run the pixel shader on 2x2 blocks of pixels. A pixel is represented as something like Pixel{ a[f32; 4], r: [f32; 4], g: [f32; 4], b: [f32; 4]} or specifically Vector4f in SwiftShader. This way, all "a" for a block are placed in a SIMD word and processed together, same for "r"s, "g"s, and "b"s.

The vertex shader is similarly run on 4 vertices at a time.

Z buffer

Both llvmpipe and swiftshader seem to use a linear z buffer. i.e. no hierarchical-Z. This probably isn't so bad for llvmpipe because it does pixel shading a tile at a time and so we're more likely to hit the cache for subsequent draws. It does seem like it would be more of a problem for SwiftShader.

LLVMpipe performance

llvmpipe can dump its shaders (build mesa with --buildtype=debug and then set GALLIVM_DEBUG=dumpbc) as well as note them for profiling with perf.

  • LP_NATIVE_VECTOR_WIDTH=128 forces AVX off
  • GALLIUM_DUMP_CPU=1 lets you see the detected cpu
  • LP_NUM_THREADS=0 forces to a single thread
  • MESA_GLSL=dump will the glsl-ir of the shaders

Custom

I'm experimenting with compiling the WebRender shaders to C++. We could potentially do this at build time which would avoid the need to ship a jit which is good for code size and security. (We can opt out of being able to make pages executable in the gpu process)

One challenge with this approach is that we'd want to know the sampler state at compile time.

Profling

Try out https://github.com/plasma-umass/coz

See also