Rasterization & Early-Z: Expanded Case Study

Extended case study

Performance signoff review: raster throughput, early-Z kill rate, and overdraw reduction regressed after a kernel, compiler, or microarchitecture change tied to Rasterization & Early-Z.

Background

Prior build met target on baseline workloads. New regressions cluster in one workload family with similar access or control-flow behavior.

Why this case is realistic

GPU regressions rarely announce themselves as one clean unit failure. They usually appear as a product symptom: a frame-time spike, a kernel slowdown, a power-limit excursion, an unexpected memory cliff, or a benchmark delta that only reproduces under a specific scene or launch shape.

The purpose of this case is to practice connecting Rasterization & Early-Z to a full evidence chain: workload, counters, trace, root-cause mechanism, owner, fix, and regression matrix.

Symptoms observed

• raster throughput, early-Z kill rate, and overdraw reduction regression

• Tail-latency growth in selected kernels

• Mismatch between expected and observed issue efficiency

Investigation timeline

1. Hour 0: freeze workload inputs, binaries, and profiler versions

2. Hour 1: isolate failing kernels or draw calls and classify by pattern

3. Hour 2: compare warp, cache, and memory counters against golden run

4. Hour 3: replay with focused microbenchmarks

5. Hour 4: assign root cause to software mapping, hardware policy, or both

6. Hour 5: apply minimal fix with rollback guardrails

7. Hour 6: run full perf matrix and update release recommendation

Root cause

Root cause traced to Rasterization & Early-Z: Rasterization maps primitives to fragments while early depth/stencil tests cull occluded work before expensive shader execution.

Fix and validation

• Policy or code-level change with explicit owner

• Re-run raster tile occupancy map, depth-test effectiveness report, and overdraw heatmap

• Perf, power, and correctness regressions on the release matrix

Lessons learned

• Counter triage must precede broad tuning

• Mixed graphics+compute traces reveal hidden contention

• Waivers need bounded impact and explicit revisit criteria

CASE STUDY - Rasterization & Early-Z
throughput / latency / utilization before-after

Bottleneck lens

BANDWIDTH ROOFLINE — Rasterization & Early-Z

performance
   ^
   |                compute ceiling
   |               /
   |              /
   |-------------/------------------ memory ceiling
   +------------------------------------------> operational intensity
      memory-bound             compute-bound

Interpretation: identify compute vs memory bound

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.

Principal GPU review addendum

Rasterization & Early-Z is not just a definition to memorize. In a real GPU program it becomes an interaction between software shape, compiler mapping, warp execution, memory movement, interconnect policy, and physical limits. The first senior move is to name which layer is being exercised before interpreting a counter.

Rasterization maps primitives to fragments while early depth/stencil tests cull occluded work before expensive shader execution. This mechanism matters because GPUs are throughput machines: a small inefficiency repeated across lanes, warps, SMs, frames, or dispatches can dominate product performance even when a unit-level diagram looks balanced.

Use raster throughput, early-Z kill rate, and overdraw reduction as an entry point, not as the conclusion. A metric shift only becomes actionable after it is tied to a workload slice, a profiler capture, an architectural path, and a reproducible artifact such as raster tile occupancy map, depth-test effectiveness report, and overdraw heatmap.

Graphics throughput depends on balancing fixed-function stages with programmable shader pressure. The review posture is therefore evidence-first: explain what the kernel or graphics workload asked for, how the GPU mapped it onto hardware, where useful work stopped, and which owner can change the smallest boundary safely.

In review, insist on a concrete chain from workload to hardware behavior: workload shape -> compiler/runtime mapping -> warp or pipeline behavior -> memory/fabric pressure -> measured product impact. That chain prevents generic GPU tuning advice from replacing engineering evidence.