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

Registers are allocated in fixed steps out of a fixed budget per multiprocessor, so a small increase in live values can cost a whole resident block. The compiler output that shows it in two lines, the occupancy cliff arithmetic, and the two different failure modes that produce the same symptom.

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⚡ Kernels & Compilers🔒 Premium

Occupancy and Register PressureOccupancy is the fraction of an SM's 64 warp slots that are resident, and it is capped by the 65,536 registers and 228 KB of shared memory each block consumes. It decides how much memory latency the hardware can hide for free, but the fastest kernels on a GPU routinely run at 25 percent, so the interview skill is knowing when to raise it and when to stop.

Foundational

🧩 GPU & Accelerator Architecture

GPU Memory HierarchyA GPU has four places a byte can live, and they differ by a thousandfold in bandwidth: registers, shared memory on the SM, a chip-wide L2, and HBM off-chip. Almost every kernel optimization is a decision about which level a value is read from and how many times. Knowing the sizes and bandwidths for an H100 cold is what lets you say why a kernel is slow before you profile it.

Foundational

🔌 Networking & Storage

Debugging a Slow All-ReduceA training job reports its all-reduce at a third of what the fabric should deliver, every node passed its health check, and nothing is logged. This page is the isolation order that finds the cause in an hour instead of a day: measure the collective in isolation, split the job until the slow pair or rank appears, then check the specific things that make a link, a node or a placement slow. Most cases end at one NIC, one topology mismatch, or GPUDirect silently off.

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⚡ Kernels & Compilers

CUDA Programming ModelCUDA splits a program into a host that allocates, copies and enqueues work, and a device that runs thousands of identical threads organized as a grid of blocks. Getting the split right, and knowing that a launch returns before the kernel runs, decides whether your first live-coding kernel produces a correct number or a silent zero.

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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

Scored on reading the ptxas register and spill lines, on the occupancy-step arithmetic rather than a vague appeal to pressure, and on separating a lost resident block from an actual spill to local memory.

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