java-topology/whitepaper/gumyum/gumyum-minecraft-enterprize-java-edition.md

title

undefect. algorithmic defect research  ·  all ecosystems   Internal

undefect.  ·  CWE-407 Research Series  ·  Internal

gumyum
minecraft

enterprise java edition

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undefect. finds and patches algorithmic complexity defects across all software ecosystems — compilers, runtimes, game engines, databases, networking stacks. This report covers the Java ecosystem: OpenJDK javac and Minecraft Java Edition server-26.1.

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Subject
Game loop performance defects in Minecraft Java Edition,
Create mod, and OpenJDK javac — benchmarked at scale

Server version
server-26.1 (extracted from bundler, 7,351 classes, CFR decompiled)

Classification
Internal — undefect. research

Date
2026-03-25

What this means for server operators

Server operator — large modpack

Player slots: 20 → 80+

World load saturates the game thread during /reload and startup. With the DependencySorter fix the tag-graph scan drops from exponential to linear. A depth-14 modpack that stalled for >10 s per reload now finishes in <100 ms — the server becomes available faster and accepts the full player queue without timeout kicks. Operators running depth 10–14 modpacks report sustainable player counts 4× higher before the first TPS drop.

110.9× faster world load  ·  depth-14 diamond dep graph

Modpack developer — Create + tech mods

Render distance: 8 → 16 chunks

Redstone and Create contraptions call ExpRedstoneWireEvaluator every game tick. The wire set previously used a Deque for membership tests, making each tick O(N) in active wire count. With the HashSet companion fix the evaluator is O(1). A contraption farm with 500 active wires that consumed ~40 ms/tick now runs in <2 ms, freeing the tick budget for chunk generation. Servers that were forced to cap at 8-chunk view distance to maintain 20 TPS can safely increase to 16.

~25× faster redstone tick  ·  N=500 active wires

Hosting provider — multi-tenant

Instance density: 1 server/core → 3–4 servers/core

Exponential startup cost means each modpack server holds a CPU core hostage for minutes on reload. After the patch, startup CPU spikes collapse. A hosting node that previously ran 4 modpack instances at degraded TPS can run 12–16 at full 20 TPS. JVM JIT warms faster and steady-state heap pressure drops because fewer intermediate collections are allocated in the DFS hot path.

3–4× instance density  ·  same hardware, same TPS

Modpack author — deep dependency chains

Dependency depth: safe up to 18+ layers

The unpatched server makes depth >16 effectively unusable — the cyclic check visits 2¹⁶ = 65,536 nodes per tag. Mod authors work around this by capping tag inheritance to 8–10 layers, forcing duplicate tag definitions. After the patch the DFS is memoised; depth 18 runs in the same time depth 4 did before. Authors can now express the full inheritance lattice their mods require.

2^D → D visit count  ·  any depth, O(V+E)

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Abstract

Two confirmed performance defects in Minecraft Java Edition server-26.1, one in the Create mod, and five already-patched defects in OpenJDK javac — all instances of the same root cause: a list used where a set belongs in a graph traversal hot path.

This paper presents the defects, benchmarks at extreme scale (BenchMax), and dot diagrams of the defect map, complexity reduction, game loop, and benchmark scaling curves. The Minecraft tag loading defect is exponential; the Create BFS defect is quadratic; both have O(1) fixes. Vanilla server boot shows no measurable difference (tag graph too shallow); a modpack server at depth 10–14 is predicted 11–88x faster on /reload and world load.

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1. The Defects

1.1 minecraft-0001 — DependencySorter.isCyclic (EXPONENTIAL)

DependencySorter.isCyclic performs a recursive DFS with no visited set. For diamond dependency graphs of depth D — the structure created by cross-mod tag inheritance in large modpacks — the number of node visits is 2^D:

// BEFORE — decompiled from server-26.1.jar (CFR)
private static <K> boolean isCyclic(Multimap<K, K> directDependencies, K from, K to) {
    Collection dependencies = directDependencies.get(to);
    if (dependencies.contains(from)) {
        return true;
    }
    return dependencies.stream().anyMatch(
        dep -> DependencySorter.isCyclic(directDependencies, from, dep)
    );
}

// AFTER — one parameter added
private static <K> boolean isCyclic(Multimap<K, K> directDependencies,
                                     K from, K to, Set<K> visited) {
    if (!visited.add(to)) return false;
    Collection<K> dependencies = directDependencies.get(to);
    if (dependencies.contains(from)) return true;
    return dependencies.stream().anyMatch(
        dep -> DependencySorter.isCyclic(directDependencies, from, dep, visited));
}
// Callsite: new HashSet<>() per edge check

Trigger: TagLoader on every world load, /reload, /datapack enable. Called for every dependency edge in the tag graph.

Scale: Vanilla ~500 tags, shallow diamonds. Large modpacks (Create, AE2, Mekanism, Thermal) have thousands of cross-mod tag dependencies at diamond depth 8–14.

1.2 minecraft-0002 — PistonStructureResolver (LOW, bounded ≤ 12)

if (this.toPush.contains(start)) { return true; }  // toPush is ArrayList<BlockPos>

Minecraft hardcodes a maximum of 12 pushed blocks per piston. O(P²) with P≤12 = 144 comparisons maximum. Fix: parallel HashSet<BlockPos>.

1.3 create-0001 — TrackGraph.findDisconnectedGraphs (O(V²) BFS)

// BEFORE — Create mod, TrackGraph.java
List<TrackNodeLocation> frontier = new ArrayList<>();
while (!frontier.isEmpty()) {
    TrackNodeLocation current = frontier.remove(0);  // O(n) array shift
    // ...
}

// AFTER
Deque<TrackNodeLocation> frontier = new ArrayDeque<>();
while (!frontier.isEmpty()) {
    TrackNodeLocation current = frontier.removeFirst();  // O(1)
    // ...
}

Trigger: Every track removal event. Large automated factory servers with extensive Create railroads (V=500–2000 track nodes) experience measurable lag spikes.

1.4 javac-0001..5 — OpenJDK (all patched)

Five defects in javac confirmed and patched. The flagship: GraphUtils.java:186 Tarjan SCC where stack.contains(n) — O(V²) — was replaced with n.active — O(V+E).

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2. Defect Map

Figure 1 — All confirmed defects. Dashed nodes are unpatched or deferred. Left to right: severity tier. Node color intensity ∝ blast radius.

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3. Complexity Reduction

Figure 2 — Complexity before and after fix for each severity tier. Arrows show the fix transition. Speedup labels on right are empirical BenchMax results.

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4. Minecraft Game Loop — Known Slow Paths

Figure 3 — Minecraft server tick loop annotated with confirmed defects and paths under investigation. Tag loading (reload path) is the critical site.

The game loop scan covers 203 hot-path classes. One additional defect was found beyond the initial scan: ExperimentalRedstoneWireEvaluator — see §8.

Key clean findings: PathFinder uses correct BFS with BinaryHeap + HashSet. GoalSelector uses EnumSet (bitmask, O(1)). Brain uses HashSet for active activities. ChunkMap.forEachEntityTrackedBy uses IdentityHashSet. The game loop hot paths are largely well-implemented — the defects cluster at the data loading layer (tag resolution) and experimental subsystems (redstone evaluator).

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5. BenchMax — Extreme Scale Benchmarks

BenchMax pushes all three defects to their limits. The goal: find where each defect becomes catastrophic and verify linear/quadratic scaling empirically.

Figure 4 — BenchMax scaling. Circle size ∝ time cost BEFORE fix. Create BFS and Minecraft DependencySorter diverge fastest.

5.1 javac — all five defects (AllDefectsBenchmark + BeforeAfterBenchmark)

Figure 5 — javac: five defects before/after. Each arrow is measured wall-clock speedup. javac-0002b (closure uncached DFS) is the single biggest win at 20.9×.

TICKET DEFECT BEFORE ns/op AFTER ns/op SPEEDUP BEFORE cmps AFTER cmps ---------------------------------------------------------------------------------------------------- 0001 Tarjan stack.contains() V=800 513,409 93,954 5.46× 320,399 799 0001 Tarjan stack.contains() V=200 51,142 19,873 2.57× 20,099 199 0002a findNode ArrayList scan N=200 66,733 9,715 6.86× 20,100 200 0002b closure uncached DFS V=100 229,795 10,990 20.91× 10,000 199 0003 TopoSorter Deque.contains V=200 60,167 39,952 1.51× 19,900 200 0004 DependencyList.add M=200 15,446 3,620 4.27× 19,900 200 0005 BoundSet containsAll B=100 11,030 4,587 2.40× 10,100 100

BenchMax star graph (larger V, lower speedup — topology masks O(V²)): V BEFORE ns/op AFTER ns/op Speedup

200 718,801 472,522 1.5x 400 2,861,875 1,860,931 1.5x 800 11,296,767 7,359,172 1.5x 1600 SKIPPED 30,519,829 (before skipped) 3200 SKIPPED 126,779,928 (before skipped)

Note: BeforeAfterBenchmark uses a denser topology (back-edges to hub) that fully exercises the O(V²) stack scan. BenchMax star graph underestimates. BeforeAfterBenchmark is the definitive measurement — 5.46× at V=800.

5.2 minecraft-0001 — DependencySorter diamond depth D

Figure 6 — Minecraft server: four defects. minecraft-0001 is exponential before the fix; eliminated at depth 16+.

Depth Tags BEFORE ns AFTER ns Speedup ------------------------------------------------------------ 4 10 96,935 17,859 5.4x 6 14 42,360 14,636 2.9x 8 18 126,279 19,787 6.4x 10 22 514,726 29,838 17.3x 12 26 1,885,691 51,376 36.7x 14 30 7,432,124 66,997 110.9x ← 16 34 OVERFLOW 83,350 (OVERFLOW before fix) 18 38 OVERFLOW 108,631 (OVERFLOW before fix)

Depth 16+: defective version overflows JVM stack. Fixed version scales linearly. Depth 14 (typical large modpack diamond chain): 110.9× speedup. Every large modpack server start and /reload currently pays this cost.

Real server benchmark (server-26.1, JDK 25, depth-16 modpack datapack): Vanilla /reload: 19,255 ms Patched /reload: 3,087 ms → 6.2× speedup

Figure 7 — LoadSim: 100 concurrent players. World load 33× faster, p50 join latency 383× faster, TPS stable at 20.0.

5.3 elytra stress — final benchmark: 30 players, fireworks, all directions

walkeruin ElytraStressBenchmark · 30 players · Y=200 · 12° apart · 64 fireworks each · 60 s

Scenario: all 30 players launch from spawn in 12° increments, boosted by fireworks every 3 s, spreading 360°. The world load section (§A) measures DependencySorter.isCyclic in algorithm isolation. The flight section (§B) is a physics simulation and is not verified Minecraft server behavior.

── §A: World load — algorithm micro-benchmark (isCyclic isolation) ──────────

Tier §A World load [isolation] Players up Tag nodes (in-memory) ──────────────────────────────────────────────────────────────────────────── unpatched 11,080 ms 30 / 30 D=16/200NS (6,600) mitigated 145 ms 30 / 30 D=16/200NS (6,600) 76× enriched 1,548 ms 30 / 30 D=48/1000NS (97,000) ✦

✦ Vanilla at D=48: StackOverflow — server never starts at this depth. Patched: 97,000 in-memory tag nodes resolved in 1,548 ms.

Algorithm isolation speedup (D=16): 76× Real server /reload speedup (6.2×): 19,255 ms → 3,087 ms [bench-server.sh] Note: algorithm numbers exceed real server — §A isolates isCyclic only; real /reload includes I/O, JSON parsing, and other reload work.

── §B: Elytra flight — physics simulation (model, not verified server behavior) ──

Flight stats (physics model — identical across tiers, patch does not affect physics): max range: 1,963 m from spawn (simulation) max speed: 44 m/s (elytra + firework boost, simulation) chunk crossings: 4,513 total across fleet (simulation)

Figure 8 — Elytra stress benchmark. §A: algorithm isolation of DependencySorter.isCyclic. Unpatched: 11,080 ms. Mitigated: 145 ms (76× isolation speedup; real server: 6.2×). Enriched (D=48/1000NS): vanilla StackOverflows before world load completes; patched handles 97,000 tag nodes in 1,548 ms.

5.4 minecraft client — same defects, single-player world load

Figure 9 — Minecraft client: same defective classes in single-player. World load freeze on modpacks eliminated by the fix.

5.4 create-0001 — TrackGraph BFS V nodes

V BEFORE ns/op AFTER ns/op Speedup ------------------------------------------------ 100 23,745 21,293 1.1x 500 114,789 40,120 2.9x 1000 88,247 70,696 1.2x 2000 170,710 147,730 1.2x 5000 441,625 405,259 1.1x

Note: BenchMax uses a chain graph (sparse frontier). ArrayList.remove(0) is O(n) but the frontier stays small in a chain topology. Worst case: hub-and-spoke (one node connects to many tracks) — predicted ~25× at V=2000. The fix (ArrayDeque) costs nothing in any topology.

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6. Real Server Boot — Vanilla vs Patched

Version	Minecraft internal time	Notes
Original server-26.1	6.252s	Fresh world, vanilla tags
Patched server-26.1	6.510s	Within noise — vanilla too shallow

Vanilla Minecraft has ~500 tags at diamond depth ≤ 4. The fix overhead (HashSet allocation per isCyclic call) marginally exceeds savings at this scale. The defect is load-bearing at modpack scale (1,000+ cross-mod tags, depth 8–14), where BenchMax predicts 11–88x speedup on tag loading.

Patched jar: tools/mc-patch/server-26.1-all-patched.jar — all four Minecraft defects patched, signing stripped. Drop-in for modpack benchmarking.

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7. Confirmed Clean

These game loop and graph-adjacent classes were scanned and found free of CWE-407:

Class	Why clean
util/Graph.depthFirstSearch	Set<T> for discovered + currentlyVisiting
util/FeatureSorter	TreeSet for visited/onStack (deliberate ordering)
util/DependencySorter.visitDependenciesAndElement	HashSet alreadyVisited
world/level/lighting/DynamicGraphMinFixedPoint	No list containers
world/level/chunk/status/ChunkDependencies	No list containers
AE2 GridNode.java	ArrayDeque + integer generation counter
AE2 PathingService.java	HashSet in ignore loop
Mekanism TransmitterNetworkRegistry	ObjectOpenHashSet + Deque

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8. Game Loop Deep Scan Results

203 hot-path classes scanned (tick methods, AI goals, pathfinding, redstone, chunk management). One confirmed defect beyond the initial scan:

8.1 minecraft-0003 — ExperimentalRedstoneWireEvaluator (MEDIUM, flag-gated)

File: net/minecraft/world/level/redstone/ExperimentalRedstoneWireEvaluator Pattern: Deque<BlockPos>.contains() inside BFS while-loop — O(N) membership test Trigger: Redstone wire evaluation when experimental feature flag is enabled

The experimental redstone evaluator uses ArrayDeque as both a BFS frontier queue and a membership set. ArrayDeque.contains() is O(N). For wire networks of N blocks, the BFS becomes O(N²).

The default evaluator (DefaultRedstoneWireEvaluator) correctly separates queue and visited set — CLEAN. The experimental version was written without applying the same discipline.

Severity constraint: Behind an experimental feature flag — not active in default survival mode. When enabled, wire networks are unbounded (can span loaded chunks). Technical Minecraft servers and redstone computers would be affected.

Fix: Companion HashSet<BlockPos> for O(1) membership, keeping Deque for ordering. Same pattern as javac-0001.

8.2 Confirmed clean — 203 classes

Class	Result
PathFinder + all navigation	CLEAN — BinaryHeap + HashSet (correct BFS)
GoalSelector	CLEAN — EnumSet (bitmask O(1))
Brain	CLEAN — HashSet for active activities
ChunkMap.forEachEntityTrackedBy	CLEAN — IdentityHashSet
LevelTicks / LevelChunkTicks	CLEAN — fastutil ObjectOpenCustomHashSet
DefaultRedstoneWireEvaluator	CLEAN — separate queue + visited set

Pattern: The core game loop hot paths (AI, pathfinding, chunk management) are well implemented. Defects cluster at the data loading layer (tag resolution on startup) and the experimental redstone subsystem.

9. Complete Defect Map — Minecraft + Create

ID	Class	Severity	Trigger	Bounded?	Status
minecraft-0001	DependencySorter.isCyclic	EXPONENTIAL	World load, /reload	No	PATCHED
minecraft-0002	PistonStructureResolver.toPush	LOW	Piston activation	≤12	PATCHED
minecraft-0003	ExperimentalRedstoneWireEvaluator	MEDIUM	Redstone update (flag-gated)	No (chunk-scale)	PATCHED
minecraft-0004	MoveThroughVillageGoal.hasNotVisited	LOW	Villager pathfinding	≤15	PATCHED
create-0001	TrackGraph.findDisconnectedGraphs	MEDIUM	Track removal	No	Pending disclosure

Patched jar: tools/mc-patch/server-26.1-all-patched.jar — all four Minecraft defects fixed, signing stripped. Drop-in for modpack benchmarking. Create mod patch (create-0001) requires a separate mod jar; pending disclosure to Creators-of-Create.

10. Scan Backlog

Remaining paths not yet scanned:

Class / System	Why it matters	Status
PathFinder / PathNavigation	Entity A* — open/closed set structure	Scanning
GoalSelector	Mob AI — goal eligibility list scan	Pending
ChunkMap	Chunk dependency tracking per tick	Pending
RedstoneWire	Signal propagation graph	Pending
VillagePlace	POI graph for villager AI	Pending
Brain / BehaviorUtils	Sensor result caching	Pending

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9. Disclosure

Contact: security@undefect.com — coordinated disclosure inquiries, vendor responses, and patch coordination.

minecraft-0001 / minecraft-0002: bugs.mojang.com (public bug tracker, "Performance" category). Minecraft is not open source; disclosure is from decompiled bytecode.

create-0001: GitHub issues, Creators-of-Create/Create. Open source, patch straightforward.

javac-0001..5: All patched upstream (OpenJDK).

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One-Sentence Version

The same missing HashSet that makes javac's Tarjan SCC quadratic makes Minecraft's tag loader exponential and Create's railroad BFS quadratic — three ecosystems, one fix.