You added three local variables to a working kernel and it got 30 percent slower. Explain what happened and how you would confirm it.

A few extra live values can change register allocation, residency, generated instructions or spills. Compare compiler evidence with runtime behavior before attributing a 30% slowdown to any one of them.

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Work through the numbers

Change the assumptions in a free calculator, then explain what the result establishes and what still needs measurement.

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The concepts behind this question

Use these concept explanations to review the ideas behind this question.

Advanced

⚡ Kernels & Compilers🔒 Premium

Occupancy and Register PressureOccupancy measures how many warps are resident relative to an SM’s limit. Registers, shared memory and launch shape constrain it, but higher occupancy is not always faster.

Foundational

🧩 GPU & Accelerator Architecture

GPU Memory HierarchyTrace storage scope and data reuse from registers through shared memory, caches and HBM. Worked register and traffic examples explain why a source-level load is not automatically an HBM transfer.

Foundational

🔌 Networking & Storage

Debugging a Slow All-ReduceInterpret NCCL algbw and busbw, then isolate a slow collective with controlled tests. Distinguish rank delays, GPU-memory paths and shared-fabric limits without guessing from bandwidth ratios.

Foundational

⚡ Kernels & Compilers

CUDA Programming ModelCUDA programs launch GPU kernels from host code. Thread indexing, memory access and stream ordering determine whether the result is correct.

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UP NEXT ON YOUR JOURNEY

Next in this trackA training step runs at 20 percent model FLOPs utilization. Profile it and find where the missing time goes.Next in this trackPort a hand-written CUDA kernel to MI300X. What translates mechanically, and what silently computes the wrong answer?Next in this trackDesign a kernel benchmarking setup that will not lie to you. What does it control for, and how do fake speedups get published?

FEDITOR'S NOTE

Distinguish source variables from allocated registers, register-limited residency from spills, and compiler evidence from a measured cause. State allocation and launch assumptions before using occupancy arithmetic.

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