epiphany-graph-rs/TUNING_DOCTRINE.md

Tuning Doctrine

This file records tuning hypotheses, sweep results, and working defaults for the realtime 3D layout solver.

The goal is not "fast" in the abstract. The goal is to preserve the visible large-scale structure of the graph while spending as little frame time as we can get away with. Fast nonsense is still nonsense, just with better posture.

Current Measurement Harness

Run:

cargo run --release --example tuning_sweep

The sweep compares approximate repulsion against exact all-pairs repulsion on fixed positions. It reports:

• elapsed time
• mean absolute error
• max absolute error
• mean relative error
• RMS relative error
• max relative error

The exact solver is the ground truth for repulsion force at the same positions. This does not yet evaluate full layout trajectory divergence, structural-force error, edge readability, or user-facing visual quality.

Force Scale Doctrine

Different force categories should not share one approximation strategy.

• Near-field forces should be local and precise: collision, node spacing, short springs, local cluster cohesion.

• Far-field forces should preserve large-scale structure: broad repulsion, weak component separation, community/body relationships, core/periphery pressure.

• Hierarchy forces should remain explicit: rank pressure, ordering pressure, edge direction constraints.

• Fold/body forces should operate on analyzed structure: SCCs, communities, biconnected components, core shells, and fold groups.

The practical split:

local precision     -> exact/local neighborhoods/grid
global field        -> Barnes-Hut/coarse bodies
semantic structure  -> analysis-derived fold/body constraints
hierarchy           -> Sugiyama-derived constraints

Do not tune this as one force soup. That is how the machine becomes fast and wrong in a way that looks expensive.

Preliminary Sweep

Date: 2026-05-07

Command:

cargo run --release --example tuning_sweep

Datasets:

• layered_dag_256: shallow hierarchy with rank/order-like spread.
• clustered_fold_512: dense local folded bodies.
• uniform_cloud_1024: broad 3D distribution.
• uniform_cloud_4096: larger broad 3D distribution.

Candidate set:

• exact
• Barnes-Hut theta: 0.35, 0.50, 0.65, 0.80, 1.00, 1.20
• spatial grid: (cell=120, radius=1), (cell=180, radius=1), (cell=240, radius=1), (cell=180, radius=2)

Representative results:

Dataset	Candidate	Time	Mean Rel Error	RMS Rel Error	Note
layered_dag_256	exact	275 us	0.000	0.000	Exact is cheap here.
layered_dag_256	grid 180/r2	557 us	0.419	0.492	Best approximate error, slower than exact.
layered_dag_256	Barnes-Hut theta 1.0	533 us	0.579	0.699	No win at this size.
clustered_fold_512	exact	897 us	0.000	0.000	Exact still cheap.
clustered_fold_512	grid 180/r2	5814 us	0.062	0.081	Accurate but slow from dense local neighborhoods.
clustered_fold_512	grid 240/r1	691 us	0.087	0.110	Best practical approximate candidate.
clustered_fold_512	Barnes-Hut theta 1.2	1546 us	0.856	1.012	Bad fit for dense folded local bodies.
uniform_cloud_1024	exact	5079 us	0.000	0.000	Exact still competitive.
uniform_cloud_1024	Barnes-Hut theta 1.2	3235 us	0.444	0.632	First speed win, high error.
uniform_cloud_1024	grid 180/r2	4010 us	0.770	0.848	Faster than exact, worse global error.
uniform_cloud_4096	exact	60155 us	0.000	0.000	Too slow for per-frame use.
uniform_cloud_4096	Barnes-Hut theta 1.2	18804 us	0.309	0.451	Best large-scale candidate in this sweep.
uniform_cloud_4096	Barnes-Hut theta 0.8	31793 us	0.328	0.460	More cost, little accuracy gain.
uniform_cloud_4096	grid 180/r2	27519 us	0.782	0.828	Faster than exact, poor far-field preservation.

Hypotheses Tested

Hypothesis 1: exact repulsion remains viable for small graphs

Result: confirmed.

On this machine, exact all-pairs repulsion is still hard to beat at 256-512 nodes and remains competitive at 1024. The implementation is simple, cache friendly, and has no tree/grid build overhead.

Doctrine:

• Use exact repulsion for small graphs and quality baselines.
• Do not automatically pay approximation overhead below roughly 1000 nodes without measuring first.

Hypothesis 2: Barnes-Hut preserves far-field structure better than grid

Result: directionally confirmed for broad large graphs, not for dense local folds.

At uniform_cloud_4096, Barnes-Hut theta 1.2 beat exact runtime by about 3.2x while keeping mean relative error around 0.309. Spatial grid was faster than exact but had much worse mean relative error around 0.782.

Doctrine:

• Use Barnes-Hut for large broad layouts where global repulsion matters.
• Keep grid as a local/near-field approximation, not the primary large-scale structure force.

Hypothesis 3: lower Barnes-Hut theta is always better

Result: rejected for this preliminary harness.

Lower theta increased runtime sharply but did not buy enough error reduction in the tested cases. For uniform_cloud_4096, theta 1.2 was fastest and had the best mean relative error among tested Barnes-Hut candidates. That is suspicious enough to keep watching, but clear enough not to default to tiny theta.

Doctrine:

• Keep default barnes_hut_theta = 0.65 conservative for now.
• Benchmark theta 0.8, 1.0, and 1.2 on real Epiphany graphs before lowering theta out of superstition.

Hypothesis 4: dense folded clusters need local treatment

Result: confirmed.

Barnes-Hut performed poorly on clustered_fold_512, while spatial grid with larger cells handled local dense structure much better. Dense local bodies need near-field precision or body-aware forces; far-field aggregation alone does not understand folds.

Doctrine:

• For folded bodies, use local exact/grid interactions inside the body.
• Use Barnes-Hut between bodies or for broad background repulsion.
• Multilevel layout should promote fold groups into coarse particles, then solve internals locally.

Current Recommended Defaults

Current crate defaults remain:

repulsion_mode = RepulsionMode::BarnesHut
barnes_hut_theta = 0.65
barnes_hut_max_depth = 24
grid_cell_size = 180.0
grid_radius = 1

Supervised tuning recommendation:

• < 1000 nodes: try Exact first.
• 1000-4000 broad nodes: sweep Barnes-Hut theta 0.8, 1.0, 1.2.
• dense folded bodies: use grid/local exact internally.
• mixed graphs: split force categories instead of picking one global repulsion approximation.

Next Tuning Work

1. Add force-category sweeps:

   • local exact radius
   • Barnes-Hut far-field theta
   • component/body repulsion scale
   • community cohesion scale
   • hierarchy strength

2. Add multilevel measurement:

   • compare raw node Barnes-Hut against fold-group coarse repulsion plus local body solves.

3. Add trajectory metrics:

   • compare final coordinates after N ticks, not only one-step force error.
   • measure rank preservation, component separation, edge length variance, and fold group compactness.

4. Add real graph corpora:

   • small DAG
   • cyclic concept cluster
   • mixed hierarchy/community graph
   • large Epiphany memory graph

The doctrine for now is simple: exact is the truth, Barnes-Hut is the large scale field, grid is local blunt force, and fold groups are the path to making large graphs readable without making every node audition for camera time.

Organ Battleground

Date: 2026-05-07

Harness:

.\scripts\fetch_tuning_datasets.ps1
cargo run --release --example organ_battleground

Actual datasets:

• SNAP email-Eu-core: directed email network from a European research institution, 1,005 nodes and 25,571 edges.
• SNAP p2p-Gnutella08: directed Gnutella peer-to-peer network snapshot from August 8 2002, 6,301 nodes and 20,777 edges. The battleground currently limits it to the first 3,000 remapped nodes for runtime.

Synthetic fixtures:

• synthetic_layered_512: hierarchy-heavy layered graph.
• synthetic_clustered_1024: clustered ring/fold graph.

Candidate families:

• raw exact node repulsion
• raw Barnes-Hut node repulsion
• raw spatial-grid node repulsion
• structured body candidates: intra-body exact/grid plus inter-body coarse repulsion

Representative results:

Dataset	Candidate	Time	Mean Rel Error	RMS Rel Error	Note
synthetic_layered_512	exact	1557 us	0.000	0.000	Baseline.
synthetic_layered_512	Barnes-Hut theta 1.0 + body exact	3774 us	8.151	12.443	Body forces are not raw-force approximations here.
synthetic_layered_512	structured theta 0.65, body exact 0.20, body far 0.50	2852 us	5.075	7.702	Less bad, still semantic not physical.
synthetic_clustered_1024	exact	5651 us	0.000	0.000	Baseline.
synthetic_clustered_1024	spatial grid	2765 us	0.491	0.498	Best raw approximation among tested.
synthetic_clustered_1024	structured grid body	13920 us	0.962	0.973	Too much body overhead for this fixture.
snap_email_eu_core	exact	3405 us	0.000	0.000	Baseline still cheap around 1k nodes.
snap_email_eu_core	spatial grid	920 us	0.489	0.567	Fastest useful approximation.
snap_email_eu_core	Barnes-Hut theta 1.2	5978 us	0.571	0.756	Slower than exact here.
snap_p2p_gnutella08 sampled	exact	29018 us	0.000	0.000	Baseline too expensive per frame.
snap_p2p_gnutella08 sampled	Barnes-Hut theta 1.0	17847 us	0.669	0.828	Speed win, high error.
snap_p2p_gnutella08 sampled	spatial grid	2737 us	0.784	0.941	Huge speed win, poor far field.

Hypothesis 8: body organs approximate exact node repulsion cheaply

Result: rejected.

The first body-force organs do not approximate raw exact node repulsion. They add semantic pressure, so comparing them only against exact physical force makes them look bad, because they are intentionally changing the field.

Doctrine:

• Do not treat body forces as a direct approximation of exact all-node repulsion.
• Measure body forces with structural metrics: component separation, fold group compactness, rank preservation, edge length variance, and camera-readable body spacing.

Hypothesis 9: actual graph fixtures change the policy

Result: confirmed.

On real SNAP fixtures, exact remains viable around 1k nodes, spatial grid is a very strong local approximation, and Barnes-Hut begins to matter on the larger Gnutella sample. This matches the synthetic sweep direction, but the real graphs make the crossover less theoretical and more irritatingly specific.

Doctrine:

• Keep exact for small real graphs.
• Use grid when local structure dominates and frame budget is tight.
• Use Barnes-Hut for larger broad graphs where far-field structure matters.
• Do not enable body organs by default as an "accuracy" optimization. They need structure-quality metrics and likely multilevel coarse solving.

Next Organ Work

The next battleground must score structure, not only force error:

• fold group compactness before/after tick
• distance between community centers
• rank error versus Sugiyama target Y
• edge length variance
• bridge/articulation readability
• node displacement from exact trajectory after N ticks

The organs are real, but they are not proven useful yet. Good. Better a failed organ in a jar than one quietly installed in the patient.

Second Sweep: More Knobs

Date: 2026-05-07

New knobs:

• barnes_hut_near_radius: prevents Barnes-Hut aggregation inside a local radius, forcing recursion toward exact local behavior.
• near_repulsion_scale: scales exact/local pairwise repulsion.
• far_repulsion_scale: scales aggregate Barnes-Hut far-field repulsion.

New candidates added:

• Barnes-Hut theta 0.8, 1.0, 1.2
• near radius 120 or 240
• far scale 0.85 or 1.0

Representative second-pass results:

Dataset	Candidate	Time	Mean Rel Error	RMS Rel Error	Note
layered_dag_256	exact	178 us	0.000	0.000	Exact still wins small.
layered_dag_256	Barnes-Hut theta 1.0, near 180, far 1.0	556 us	0.560	0.676	Best plain Barnes-Hut balance here.
layered_dag_256	Barnes-Hut theta 1.0, near 240, far 1.0	644 us	0.555	0.673	Tiny accuracy gain, more cost.
layered_dag_256	grid 180/r2	452 us	0.419	0.492	Still better local approximation than Barnes-Hut.
clustered_fold_512	exact	821 us	0.000	0.000	Exact still cheap.
clustered_fold_512	Barnes-Hut theta 1.2, near 120, far 0.85	1910 us	0.916	0.946	Damping far field helps a little, still poor.
clustered_fold_512	grid 240/r1	568 us	0.087	0.110	Best practical local folded-body candidate.
uniform_cloud_1024	exact	3433 us	0.000	0.000	Exact remains viable.
uniform_cloud_1024	Barnes-Hut theta 1.2, near 240, far 1.0	2633 us	0.476	0.658	Speed win, high error.
uniform_cloud_1024	Barnes-Hut theta 1.2, near 120, far 0.85	3270 us	0.475	0.686	Far damping did not clearly help.
uniform_cloud_4096	exact	70970 us	0.000	0.000	Too slow per frame.
uniform_cloud_4096	Barnes-Hut theta 1.2, near 180, far 1.0	18528 us	0.343	0.486	Best broad large graph speed/accuracy point here.
uniform_cloud_4096	Barnes-Hut theta 1.0, near 180, far 1.0	28078 us	0.336	0.472	More time, modest accuracy gain.
uniform_cloud_4096	Barnes-Hut theta 1.0, near 120, far 0.85	20274 us	0.366	0.517	Far damping worsened force accuracy.
uniform_cloud_4096	Barnes-Hut theta 1.0, near 240, far 1.0	36039 us	0.350	0.483	Larger near radius cost more without enough gain.

Hypothesis 5: larger local exact radius improves Barnes-Hut enough to pay

Result: mostly rejected.

Increasing barnes_hut_near_radius from 180 to 240 often increased runtime with only tiny accuracy improvement, and sometimes worsened mean relative error. The current implementation already recurses for target-containing octants, so a large near radius is not a free lunch. It is lunch with a service charge and slightly cold fries.

Doctrine:

• Keep barnes_hut_near_radius = 180.0 as the default.
• Sweep near radius on real dense graphs, but do not raise it globally.

Hypothesis 6: reducing far-field scale preserves local accuracy

Result: rejected for force-ground-truth accuracy.

far_repulsion_scale = 0.85 sometimes reduced absolute force magnitude, but it did not improve relative error against exact all-pairs ground truth. On broad 4096-node clouds it worsened mean/RMS relative error compared to far scale 1.0.

Doctrine:

• Keep far_repulsion_scale = 1.0 for physical force approximation.
• Use far-field scale as an artistic/readability control later, not as an accuracy knob.

Hypothesis 7: exact/grid/Barnes-Hut should be selected per force category

Result: strengthened.

The second sweep made the split more obvious:

• exact wins small graphs.
• grid wins dense local folded bodies.
• Barnes-Hut wins large broad far-field layouts.
• far-field damping and larger near radius do not replace category separation.

Doctrine:

• Add a hybrid repulsion architecture next: local exact/grid inside fold groups and near neighborhoods; Barnes-Hut between distant bodies; coarse fold-group repulsion for global structure.