Warp/Wavefront Execution: Reports & Metrics

Reports and metrics

Reports & Metrics for Warp/Wavefront Execution centers on eligible warps per cycle, issue stall cycles, and replay events. The objective is to connect profiler evidence to root-cause mechanism and release-safe action.

Reports must turn eligible warps per cycle, issue stall cycles, and replay events into a release decision. Single counter improvements are insufficient without workload context and traceability.

Trend snapshot

BEFORE / AFTER — Warp/Wavefront Execution

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.

Roofline interpretation

BANDWIDTH ROOFLINE — Warp/Wavefront Execution

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

Interpretation: identify compute vs memory bound

• Track eligible warps per cycle, issue stall cycles, and replay events across representative workloads, not one microbenchmark.

• Include counter captures with matching compiler, driver, and firmware tags.

• Correlate scheduler stalls with memory and interconnect pressure before optimization.

• Report frame or kernel tail behavior, not only average throughput.

GPU deep dive

SIMT abstractions are productive only when launch geometry and divergence behavior align with hardware.

Concept diagram

PROGRAMMING MODEL STACK

host API -> kernel launch -> grid -> block -> warp -> lane

Metric graph

KERNEL EFFICIENCY TREND

warp execution efficiency  ██████████
memory replay ratio        █████
idle issue slots           ███

Reports and artifacts

• occupancy report

• warp efficiency summary

• kernel launch audit

• replay counter snapshot

Mini case study

A block-size bump improved theoretical occupancy but increased replay and reduced achieved throughput by 22%.

Debug branches

• Map launch geometry to active warps per SM

• Correlate branch masks with divergence hotspots

• Validate occupancy against achieved IPC

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.

Report interpretation

Warps (or wavefronts) are the scheduling unit; their readiness, dependency state, and memory scoreboard status determine front-end issue throughput. 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 eligible warps per cycle, issue stall cycles, and replay events 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 warp-state histogram, issue scoreboard dump, and replay counter log.

The programming model is a contract between algorithm intent and SIMT execution reality. 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.

For Warp/Wavefront Execution, reports should explain why eligible warps per cycle, issue stall cycles, and replay events changed, not merely that it changed. Ask whether the movement came from useful work, reduced waste, different scheduling, changed memory traffic, or hidden throttling.

A strong report includes counter consistency checks: the story told by occupancy should agree with issue activity; the memory story should agree with cache and transaction behavior; the silicon story should agree with clocks, voltage, thermals, and power telemetry.