Kernel Launch & Occupancy Basics: Interview Drills

Interview drills

Interview Drills for Kernel Launch & Occupancy Basics centers on theoretical vs achieved occupancy, latency hiding score, and warp starvation rate. The objective is to connect profiler evidence to root-cause mechanism and release-safe action.

PROMPT
You see theoretical vs achieved occupancy, latency hiding score, and warp starvation rate on Kernel Launch & Occupancy Basics. Walk through root cause and release decision.

STRONG ANSWER
1. Names failing workload/scene and first broken metric.
2. Explains Occupancy is bounded by register count, shared memory, warps/SM limits, and block shape; higher occupancy helps hide latency until another bottleneck dominates.
3. Requests occupancy report, register/shared-memory budget table, and profiler timeline.
4. Proposes bounded fix + owner + validation matrix.

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

Whiteboard diagram

Occupancy effect on issue availability

WARP SCHEDULER VIEW — Kernel Launch & Occupancy Basics

cycle ->      0    1    2    3    4
eligible   [W1,W2,W5] [W2] [W2,W7] [W7] [W3,W7]
issued         W1      W2    W7      W7    W3
stall reason    -    dep wait  -   mem wait  -

Scheduler objective: keep issue slots non-empty.
Focus: connect occupancy limits to scheduler starvation windows

Debug tree to narrate

ROOT-CAUSE TREE — Kernel Launch & Occupancy Basics

theoretical vs achieved occupancy, latency hiding score, and warp starvation rate 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

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.

Interview answer expansion

A strong interview answer for Kernel Launch & Occupancy Basics starts with the workload and metric, then states the mechanism in plain language: Occupancy is bounded by register count, shared memory, warps/SM limits, and block shape; higher occupancy helps hide latency until another bottleneck dominates.

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.