| """Occupancy / register / shared-memory budget, and persistent-grid co-residency. |
| |
| Two questions this answers that cost the most time when guessed: |
| 1. which resource is limiting occupancy -- registers, shared memory, or block size |
| 2. whether a persistent kernel's grid actually fits (if it does not, a grid-wide barrier DEADLOCKS) |
| |
| python3 occupancy.py --threads 256 --regs 96 --smem 49152 [--blocks 264] |
| """ |
| import argparse |
| import torch |
|
|
|
|
| def main(): |
| ap = argparse.ArgumentParser() |
| ap.add_argument("--threads", type=int, required=True) |
| ap.add_argument("--regs", type=int, required=True, help="registers per thread (ncu: launch__registers_per_thread)") |
| ap.add_argument("--smem", type=int, default=0, help="dynamic shared memory per block, bytes") |
| ap.add_argument("--blocks", type=int, default=0, help="grid size, to check persistent co-residency") |
| a = ap.parse_args() |
|
|
| p = torch.cuda.get_device_properties(0) |
| sm = p.multi_processor_count |
| max_thr = p.max_threads_per_multi_processor |
| regs_sm = p.regs_per_multiprocessor |
| smem_sm = getattr(p, "shared_memory_per_multiprocessor", p.shared_memory_per_block_optin) |
| warps_blk = (a.threads + 31) // 32 |
|
|
| by_thread = max_thr // a.threads |
| by_regs = regs_sm // (a.regs * warps_blk * 32) if a.regs else 999 |
| by_smem = (smem_sm // a.smem) if a.smem else 999 |
| blocks_sm = max(0, min(by_thread, by_regs, by_smem)) |
| limiter = min((by_thread, "block size"), (by_regs, "registers"), (by_smem, "shared memory"))[1] |
| occ = blocks_sm * warps_blk * 32 / max_thr if max_thr else 0 |
|
|
| print(f"device {p.name} SMs={sm} max_threads/SM={max_thr} regs/SM={regs_sm} smem/SM={smem_sm//1024}KB") |
| print(f"blocks/SM: by-threads {by_thread}, by-registers {by_regs}, by-smem {by_smem}" |
| f" -> {blocks_sm} (limiter: {limiter})") |
| print(f"achieved occupancy {occ:.0%} resident blocks total {blocks_sm*sm}") |
| if a.regs >= 168: |
| print(" !! >=168 regs/thread: spills are likely; check SASS for LDL/STL") |
| if a.blocks: |
| cap = blocks_sm * sm |
| ok = a.blocks <= cap |
| print(f"\npersistent grid {a.blocks} vs co-resident capacity {cap}: {'FITS' if ok else 'DOES NOT FIT'}") |
| if not ok: |
| print(" !! a grid-wide barrier WILL DEADLOCK: blocks that never get scheduled cannot arrive.") |
|
|
|
|
| if __name__ == "__main__": |
| main() |
|
|
