20070822 - New Pipeline Progress


Work on the new combined graphics, physics, and CFD pipeline is progressing quite smoothly. All the current and previous screen shots and videos on this page (except the CFD stuff) are from my 100% working older fixed function pipeline and later Shader Model 3.0 source path in which nearly all the pipeline is CPU side except for compositing. During this major rewrite and optimization effort, I keep the old development path alive to test new ideas and simplifications.

As it turns out, months ago when I had switched from development on my on-motherboard Intel GMA3000 to a NVidia 8600 GTS card, I had forgotten that I had turned off most of my aggressive hidden surface removal algorithm because the NVidia card provided a such a performance jump (both in offloading triangle setup, and fragment processing). Now with the aggressive HSR back on, I have found some substantial performance increases as I tweak and optimize the algorithm for the final pipeline.

CPU / GPU Mix
My new pipeline follows a very simple philosophy, move anything which can be parallelized which does not require double precision to the GPU, everything else is staying CPU side. The rough pipeline design is as follows. Note this is in logical order, actual order is different to hide the latency of CPU to GPU bus transfers and computation. Also I am skipping on the full Audio algorithm integration (which also features a CPU/GPU mix) until I get this 100% working.

  1. CPU: Expand/contract the world tree structure based on interaction and view.
  2. CPU: Compute next animation frame for used cell animated l-system rules (requires double precision).
  3. CPU: Do world space to eye space transform for cells (requires double precision).
  4. CPU: Transfer single precision eye space coordinates and other data to GPU via VBO transfers.
  5. GPU: Project all cells into 2D screen space, compute all data necessary for CPU hidden surface removal (HSR) algorithm, and compute all factors for final display and physics properties. This runs as a geometry shader writing out to VBOs on the GPU.
  6. GPU: Transfer back the (HSR) data and modified z sort coordinate to the cpu.
  7. CPU: Sort nodes for HSR from front to back order (radix sort). Run HSR algorithm, which is hopelessly linear, running through the cells in z order and filtering out cells which are decided to be hidden enough to prune from the final display list. HSR also computes the priority for the beginning expansion/contraction step.
  8. CPU: Send index of displayable cells in back to front order to GPU.
  9. GPU: Composite all drawable cells back to front order to 16bit framebuffer, running a shader which does emmitive and fractal environmental reflective lighting.
  10. CPU: Prep special ordered index list of cells for all the physics/CFD scattering passes. Send these index lists to the GPU via VBOs. Send parent relative cell velocity and other per cell physics/CFD properties to GPU via VBOs.
  11. GPU: Run physics/CFD scatter pass followed by gather pass. The gather pass results in new physics properties for each cell.
  12. GPU: Transfer gathered new physics properties back to CPU.
  13. CPU: Transform new physics gathered velocities back into current frame world space parent relative coordinate spaces (these require double precision).
  14. CPU: Update current cell position and properties based on gathered physics/CFD info.
  15. LOOP!

Rough Optimization Budgeting
When working on the engine pipeline design, I am always keeping track of rough estimated costs of key parts of the algorithm in terms of CPU cycles used, CPU memory bandwidth, PCIe bandwidth, latency for PCIe bus transfers, GPU SPU (scaler processing unit) cycles used, GPU texture loading/filtering bandwidth, GPU triangle setup bandwidth, and GPU ROP (raster operation processing) bandwidth. Once NVidia releases a GeForce 8 series profiling tool for Linux, I will be able to verify things like GPU cache performance and other critical factors, but for now, it is just intelligent design with lots of testing.

Maximum performance budget, I tend to try and stay way under these figures to insure a good FPS.

  1. GPU: 1450 MHz of ops * 32 SPUs = 46400 MHz of ops
  2. GPU: 1 clk for basic ops
  3. GPU: 4 clk for complex ops
  4. GPU: 32 GB/s of on card bus
  5. GPU: 725 tri/s of setup
  6. GPU: 8 TAUs at 725 Mhz = 5800 Mhz texture loads
  7. GPU: 16bit texture loads cost 2x
  8. GPU: 32bit texture loads cost 4x
  9. GPU: 8 ROPs at 725 Mhz = 5800 Mhz blends
  10. GPU: 16bit blends cost 2x
  11. GPU: 32bit blends cost 4x
  12. CPU: 2 GHz
  13. CPU: PCIe bus 4 GB/sec

My compositing engine is the most optimized and expensive part of the pipeline, using about 20% of my texture fetch and filter, 13% of my ROP, but only 6% of my SPU, and under 1% of my triangle setup budget. I also have room to move one of the two texture loads in the compositing fragment shader from a 2D to a 1D texture load, requiring some extra math (SPU time), which should help keep much more texture reads cached if texture trashing becomes a problem.