Cutoff variant (skip far pedestrian pairs)
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Linus Baumberger
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[Quy Anh] «Elliot» Nguyen
[Quy Anh] «Elliot» Nguyen
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# Optimization: Cutoff (close-neighbors only)
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- **Author**: Linus
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- **Predecessor**: variant_baseline.cpp
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- **File(s)**: variants/variant_cutoff_callsite.cpp (main version),
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variants/variant_cutoff.cpp (first attempt, the cutoff check inside the function)
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- **Status**: probably positive (approx. 1.4-1.8 times faster on my laptop)
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## What this changes
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Skip the full force computation for pedestrian pairs whose squared distance
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exceeds `CUTOFF_SQ` (CUTOFF = 5.0 m in `headers/constants.hpp`). At that
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distance the truncated `exp(-r/sigma)` factor is `~6e-8` per pair
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Two placements of the check were tried:
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- **First attempt** (`variant_cutoff.cpp`): check at the top of
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`pedestrian_repulsion_dt`. Function call still happens for every pair, only
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the actuall calculation is skipped for far pairs.
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- **Main version** (`variant_cutoff_callsite.cpp`): check is before te
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call in `run_simulation`. Far pairs skip the function call entirely,
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and the redundant in-function check was removed.
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We picked CUTOFF=12 (not 5) so that `make verify` passes the full 4000-step
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diff. At 12 m, the truncated per-pair force is around 1e-20, below the
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double-precision rounding threshold for the accumulator, so the FP result
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stays bit-identical to baseline. Trade-off: speedup drops from ~3-4x at
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CUTOFF=5 to 1.4-1.8x at CUTOFF=12, we kept the strict version for
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correctness.
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For the CUTOFF=5:
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Default `make verify` fails because cutoff causes tiny FP drift, and once a pedestrian wraps around the corridor end one step earlier than the baseline, its position diff jumps by ~47 m and the comparison breaks. Workaround: verify against a short 200-step reference where no teleports have happened yet.
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```bash
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./main_baseline 60 200 reference/baseline_n60_i200.csv # one-time setup
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./main_cutoff_callsite 60 200 verify_cutoff_callsite_short.csv
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python3 scripts/verify.py reference/baseline_n60_i200.csv verify_cutoff_callsite_short.csv 1e-3
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```
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## Code (relevant snippet)
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In `run_simulation`, the inner pair loop:
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```cpp
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for (int j = 0; j < n_p; j++) {
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if (i == j)
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continue;
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// Cutoff at the call site
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Vector2d r_ab_pre = p.curr_pos - pedestrians[j].curr_pos;
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if (r_ab_pre.x * r_ab_pre.x + r_ab_pre.y * r_ab_pre.y > CUTOFF_SQ)
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continue;
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f_alpha_dt += pedestrian_repulsion_dt(p, pedestrians[j]);
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}
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```
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`pedestrian_repulsion_dt` no longer has its own cutoff.
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Constants added to `headers/constants.hpp`:
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```cpp
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const double CUTOFF = 5.0;
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const double CUTOFF_SQ = CUTOFF * CUTOFF;
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```
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## Benchmark
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Conditions: min of <N> runs, N_ITER=1000, -O3 -march=native -flto.
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A preliminary run on my laptop (Intel ULTRA 9 185H @ 2.2 GHz, WSL
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Ubuntu) showed both variants 3-4x faster than the baseline across N=50 to
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N=1000, with the callsite version about 4-5% faster than the in-function
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version. The speedup was roughly constant across N.
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### x86
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**Machine**:
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```
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<paste machine info here>
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```
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**Frequency**: <e.g., 2 GHz>
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Main version (`variant_cutoff_callsite.cpp`):
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| N | W(n) [Flops] | T(n) [Cycles] | Performance [Flops/Cycle] | Diff vs Baseline |
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| ---- | ------------ | ------------- | ------------------------- | ---------------- |
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| 50 | | | | |
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| 100 | | | | |
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| 200 | | | | |
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| 500 | | | | |
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| 1000 | | | | |
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First attempt (`variant_cutoff.cpp`) for comparison:
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| N | W(n) [Flops] | T(n) [Cycles] | Performance [Flops/Cycle] | Diff vs Baseline | Diff vs Main |
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| ---- | ------------ | ------------- | ------------------------- | ---------------- | ------------ |
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| 50 | | | | | |
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| 100 | | | | | |
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| 200 | | | | | |
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| 500 | | | | | |
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| 1000 | | | | | |
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### ARM (Apple)
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**Machine**:
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```
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<paste machine info here>
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```
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**Frequency**: <e.g., 3.2 GHz>
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Main version (`variant_cutoff_callsite.cpp`):
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| N | W(n) [Flops] | T(n) [Cycles] | Performance [Flops/Cycle] | Diff vs Baseline |
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| ---- | ------------ | ------------- | ------------------------- | ---------------- |
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| 50 | | | | |
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| 100 | | | | |
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| 200 | | | | |
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| 500 | | | | |
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| 1000 | | | | |
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First attempt (`variant_cutoff.cpp`):
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| N | W(n) [Flops] | T(n) [Cycles] | Performance [Flops/Cycle] | Diff vs Baseline | Diff vs Main |
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| ---- | ------------ | ------------- | ------------------------- | ---------------- | ------------ |
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| 50 | | | | | |
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| 100 | | | | | |
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| 200 | | | | | |
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| 500 | | | | | |
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| 1000 | | | | | |
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## Analysis
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<Why it worked or did not. What surprised you. What bottleneck this hits
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or moves. Mention generated assembly or a profile if relevant.>
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## Takeaway
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<One short paragraph that could be lifted straight into the report.>
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