Rasterization & Early-Z: Interview Drills

Interview drills

Interview Drills 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.

PROMPT
You see raster throughput, early-Z kill rate, and overdraw reduction on Rasterization & Early-Z. Walk through root cause and release decision.

STRONG ANSWER
1. Names failing workload/scene and first broken metric.
2. Explains Rasterization maps primitives to fragments while early depth/stencil tests cull occluded work before expensive shader execution.
3. Requests raster tile occupancy map, depth-test effectiveness report, and overdraw heatmap.
4. Proposes bounded fix + owner + validation matrix.

WEAK ANSWER
Suggests generic tuning without SIMT, warp, cache, or interconnect evidence.

Whiteboard diagram

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.

Debug tree to narrate

ROOT-CAUSE TREE — Rasterization & Early-Z

raster throughput, early-Z kill rate, and overdraw reduction regressed
        |
  reproducible on replay?
      /              \
    no                yes
    |                  |
env/test noise    counter triage
                   |
             compute-bound or memory-bound?
                /                  \
             compute            memory/interconnect
             issue stalls       cache/NoC/DRAM stalls

Stop at first failing mechanism, then patch.

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.

Interview answer expansion

A strong interview answer for Rasterization & Early-Z starts with the workload and metric, then states the mechanism in plain language: Rasterization maps primitives to fragments while early depth/stencil tests cull occluded work before expensive shader execution.

Then it gives a measurement plan. Good answers name lane masks, issue slots, cache/transaction counters, memory-controller state, NoC congestion, thermal/DVFS telemetry, or stage queues depending on the topic.

Finally, it proposes one bounded fix and explains regression risk. GPU interviews reward tradeoff ownership: what improves, what may regress, and how you would know before tapeout or release.