| # Pitfalls — each of these has actually cost real time |
|
|
| ## Measurement lies |
|
|
| - **Timing the host, not the GPU.** An eager reference measured 185 ms wall vs **2.06 ms GPU-busy**. |
| Use CUDA events around the region, and check the two agree. |
| - **Timing under ncu.** It serializes and replays; the number is meaningless. |
| - **Warm cache.** Re-running on the same buffer measures L2, not HBM. Use fresh inputs per rep. |
| - **Autotune cache keyed on a stale shape.** The first call compiles and tunes; time the steady state. |
| - **Clock throttling.** Long benchmarks drift. Lock clocks or take min-of-N, not mean. |
| - **First-call outliers.** cuBLAS/SDPA pick an algorithm per shape on first sight; warm up per shape. |
|
|
| ## Correctness traps |
|
|
| - **A fast wrong kernel is the worst outcome.** Correctness gate before optimization, always. |
| - **Guessed tolerances.** Measure `E` (independent correct implementation) and `D` (feature dropped); |
| the gate goes between them. A gate that rejects a *more precise* kernel is a real and common bug. |
| - **Top-k / argmax gates.** Near-ties flip on ordinary noise; two correct implementations disagree. |
| - **Integer outputs compared as floats.** `.float()` is lossy above 2²⁴, so distinct large ids compare |
| equal and a wrong kernel passes. |
| - **The more precise implementation can be further from the reference.** An fp64 RoPE differed 3.6e-4 |
| from a reference that a merely-rearranged fp32 version matched to 1.6e-5. |
| - **fp8 rounding is coarse.** e4m3 eps is 0.125 — a 1% perturbation is *below* one ULP, so error is set |
| by which elements flip a code, not by the perturbation size. |
| - **`tensor / python_float` is a reciprocal-multiply** and flips ~0.05% of fp8 codes. Use a 0-dim tensor. |
| |
| ## Implementation traps |
| |
| - **Hardcoded SM count** or a grid sized from one. Query `multi_processor_count`. |
| - **Persistent kernel with a grid larger than fits.** The spinning blocks deadlock. Check co-residency. |
| - **Assuming a dimension is a multiple of the tile.** Ragged shapes are graded deliberately. |
| - **Register spills** masquerading as a memory problem. Check `LDL/STL` in the SASS. |
| - **Padding shared memory to avoid bank conflicts** — breaks 128-bit alignment for 2-byte types. |
| Swizzle instead. |
| - **bf16 accumulation.** Accumulate in fp32; a bf16 running sum drifts past tolerance on its own. |
| |
| ## Reading a profile wrong |
| |
| Counters are hints, not verdicts. A measured example from this machine: cuBLAS's own bf16 GEMM |
| (`nvjet_tst_256x128_64x4...`, 4096³) profiles at **SM 90.5%, DRAM 16.5%, occupancy 14.6%, 168 |
| registers/thread, 136k shared-memory bank conflicts** — and it is essentially the fastest thing that |
| runs on the device. Three of those numbers look like defects. |
|
|
| - **Low occupancy is not a bug.** A GEMM wants big register-resident accumulator tiles; that costs |
| registers, which caps occupancy by construction. Occupancy only matters when you are *latency*-bound |
| and have nothing else hiding the latency. Check the SM/DRAM throughputs first: if one is already |
| high, occupancy is not your problem. |
| - **High register count is not automatically a spill.** 168 regs/thread is a deliberate trade. Confirm |
| a spill by looking for `LDL`/`STL` in the SASS (`tools/dump_ir.sh sass`), not by reading the count. |
| - **Bank conflicts reported on a highly-tuned kernel** are often on a path that is already overlapped. |
| Fix them when a *swizzle* is missing, not because the counter is nonzero. |
|
|
| The order that works: establish which resource is saturated (SpeedOfLight), then look for a *specific* |
| mechanism explaining the gap. Never chase a counter that merely looks unhealthy. |
|
|
| ## Process traps |
|
|
| - **Abandoning a good optimization because it currently has a bug.** The most expensive habit there is. |
| Record *why* a variant was dropped: slower, or merely broken. |
| - **Losing the last-known-good.** Always keep a runnable fallback. |
| - **Optimizing before profiling.** And profiling with ncu before nsys has told you the time is in the |
| kernel at all. |
| - **Stopping because a percentage looks good.** The leaderboard is uncapped and the roofline is an |
| attribution formula, not a wall — a kernel that moves fewer bytes than counted can beat it. |
|
|
