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[ENGINE] scan tuning: document GridThreshold=500 gate from CCCL source

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Read cub/agent/single_pass_scan_operators.cuh lines 136-148:
  delay<Delay, GridThreshold=500>() {
    if (gridDim.x < GridThreshold) __threadfence_block();
    else __nanosleep(Delay);
  }

BI-V100: 16 SMs × ~10 CTAs/SM = ~160 CTAs. Always < 500.
Therefore ALL delay strategies collapse to __threadfence_block().
The ns/dcid/l2w parameters are architectural no-ops on BI-V100.

This explains bench_bi100.py finding no_delay optimal — not a lucky
guess but a hard gate in CCCL's tile synchronization code. The
'ns×0.5, l2w×0.6' scaling was always computing values that would
never be used (delay() never reaches the __nanosleep branch).

Source: single_pass_scan_operators.cuh (full read, 200 lines)
This commit is contained in:
muh-pipeline
2026-08-06 02:22:13 +00:00
parent edccbb00b4
commit a7e0ef1138
1 changed files with 24 additions and 0 deletions
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24
muh/include/muh/tuning/tuning_scan.cuh
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@@ -1,5 +1,29 @@
// muh/include/muh/tuning/tuning_scan.cuh — BI-V100 scan tuning
//
// CRITICAL CCCL ARCHITECTURE FINDING (from single_pass_scan_operators.cuh):
//
// template <int Delay, unsigned int GridThreshold = 500>
// _CCCL_DEVICE _CCCL_FORCEINLINE void delay() {
// if (gridDim.x < GridThreshold) {
// __threadfence_block(); // ← ALL BI-V100 scans take this path
// } else {
// __nanosleep(Delay); // ← only fires when grid > 500 CTAs
// }
// }
//
// BI-V100: 16 SMs × ~10 CTAs/SM max = ~160 CTAs. ALWAYS < 500.
// Therefore: ALL delay strategies (no_delay, fixed_delay, exponential_backon,
// exponential_backon_jitter, etc.) collapse to __threadfence_block() on BI-V100.
//
// This means:
// 1. The ns/dcid/l2w delay parameters are IRRELEVANT for BI-V100.
// 2. bench_bi100.py's finding that no_delay is optimal is CORRECT BY DESIGN.
// 3. The "ns×0.5, l2w×0.6" scaling heuristic was always a no-op on BI-V100.
// 4. Tuning effort should focus on threads/items/load_algo, NOT delay params.
//
// This architectural insight came from reading cub/agent/single_pass_scan_operators.cuh
// lines 136-148 (the delay() template function with GridThreshold=500 gate).
//
// CRITICAL INSIGHT FROM single_pass_scan_operators.cuh delay():
// The CCCL delay function has a runtime branch:
// if (gridDim.x < GridThreshold) // GridThreshold = 500