Rasterization & Early-Z: Worked Example

Worked example

Worked Example for Rasterization & Early-Z centers on raster throughput, early-Z kill rate, and overdraw reduction. The objective is to connect profiler evidence to root-cause mechanism and release-safe action.

A regression flags raster throughput, early-Z kill rate, and overdraw reduction. Correct triage freezes revisions, validates mechanism with counters/traces, then applies one reversible fix before full rollout.

Execution snapshot

SIMT EXECUTION — Rasterization & Early-Z

warp 0 lanes:  0 1 2 3 4 5 6 7 ... 31
active mask :  1 1 1 1 0 0 1 1 ...  1
instruction :  IF branch taken on active lanes

cycle 10: issue warp 0
cycle 11: issue warp 3
cycle 12: warp 0 reconverges

Focus: lane masking and warp progress
Metric tracked: raster throughput, early-Z kill rate, and overdraw reduction

Raster and early depth culling

RASTER + EARLY-Z FLOW

triangles -> setup -> raster tiles -> early-Z/depth-stencil -> fragment queue
                                      | pass | fail |
                                      | shade| cull |

Higher early-Z kill rate reduces fragment ALU pressure and memory traffic.

1. Capture baseline and regressed workload traces.

2. Tag launch geometry, build revisions, and runtime environment.

3. Compare expected vs observed warp and memory behavior.

4. Collect raster tile occupancy map, depth-test effectiveness report, and overdraw heatmap.

5. Apply one bounded fix and predefine rollback conditions.

Did the fix hold?

BEFORE / AFTER — Rasterization & Early-Z

metric quality
  ^
  |                        o target region
  |                 o post-fix validation
  |            o
  |      o baseline (failing)
  +------------------------------------------> iteration
      evidence capture  mechanism fix  closure

Use this to prove improvement is causal, not incidental.

GPU deep dive

Frame-time stability depends on balancing fixed-function stages with programmable shader pressure.

Concept diagram

GRAPHICS PIPELINE

vertex -> tessellation -> raster -> fragment -> ROP/blend

Metric graph

FRAME-TIME PRESSURE

fragment shading load  ████████
raster backpressure    █████
ROP/blend stalls       ████

Reports and artifacts

• stage occupancy timeline

• early-Z efficiency report

• ROP queue depth

• overdraw heatmap

Mini case study

Async compute overlapped with heavy fragment scenes and triggered ROP queue buildup, causing p99 frame spikes.

Debug branches

• Correlate frame spikes with stage-level queues

• Validate early-Z effectiveness under real content

• Isolate graphics-compute arbitration conflicts

Senior review question

Ask: which metric and benchmark pairing proves this topic is truly closed in production context?

Key takeaways

• Always pair micro-kernel metrics with end-to-end workload impact.

• Lock toolchain, driver, and launch metadata before comparing performance results.

Common pitfalls

• Optimizing occupancy without checking memory-system saturation.

• Comparing profiler captures from different driver or compiler builds.

• Declaring wins without reproducible accuracy and performance gates.

Worked-example reasoning

Suppose raster throughput, early-Z kill rate, and overdraw reduction regresses on one product workload. The shallow answer is to tune launch shape or widen a buffer. The deeper answer is to first compare baseline and regressed traces, then explain which part of Rasterization maps primitives to fragments while early depth/stencil tests cull occluded work before expensive shader execution. changed.

If the first failing evidence is lane-mask loss, investigate divergence and reconvergence. If it is transaction inflation, inspect coalescing and memory layout. If it is eligible-warp starvation, inspect dependencies, barriers, and scoreboard waits. If it is stable until temperature rises, pull in power and physical-design evidence.

Only after that classification should the team choose a fix. The fix might be a kernel rewrite, compiler scheduling change, cache policy, arbitration adjustment, RTL change, floorplan change, or product workload guardrail.