perf research
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doc/store_research.md
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# Store Crate: Deep Research & Performance Analysis
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This document covers the Rust `trictrac-store` crate that backs the OpenSpiel C++ game.
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It traces the full data-flow from a C++ `get_legal_actions()` call down to individual
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board operations, documents design decisions, and identifies performance bottlenecks
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relevant to MCTS training throughput.
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---
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## 1. Module Map
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| File | Responsibility |
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|---|---|
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| `board.rs` | `Board` (`[i8;24]`), `CheckerMove`, low-level move primitives |
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| `dice.rs` | `Dice` (two `u8` values), `DiceRoller` |
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| `player.rs` | `Color`, `Player` (score / holes / bredouille flags) |
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| `game.rs` | `GameState` (the full game state machine + serialisation) |
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| `game_rules_moves.rs` | `MoveRules` — legal move generation and validation |
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| `game_rules_points.rs` | `PointsRules` — jan (scoring) computation |
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| `training_common.rs` | Action encoding/decoding (`TrictracAction`, 514-element space) |
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| `cxxengine.rs` | FFI bridge (cxx.rs) — the sole entry point from C++ |
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---
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## 2. Data Model
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### 2.1 Board
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```
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positions: [i8; 24] // index i → field i+1 (fields are 1-indexed)
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positive → white checkers
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negative → black checkers
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0 → empty
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```
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Fields 1–6 = player 1's home quarter ("petit jan"), 7–12 = big jan, 13–18 = opponent
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big jan, 19–24 = opponent home quarter.
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Field 12 = White rest corner, field 13 = Black rest corner.
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Mirror (for Black's perspective): negate every element and reverse the array.
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### 2.2 CheckerMove
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```
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CheckerMove { from: Field, to: Field }
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from ∈ [1,24], to ∈ [0,24] (to==0 means bear off)
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(0,0) = EMPTY_MOVE (no-op, used when a player cannot move one die)
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```
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### 2.3 Action Space (training_common.rs)
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514 discrete actions:
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| Index | Meaning |
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|---|---|
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| 0 | `Roll` — trigger a dice roll |
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| 1 | `Go` — take the hole and reset (instead of holding) |
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| 2–257 | `Move` with `dice_order=true` (die1 first): `2 + checker1*16 + checker2` |
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| 258–513 | `Move` with `dice_order=false` (die2 first): `258 + checker1*16 + checker2` |
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`checker1` and `checker2` are 1-indexed ordinal positions of checkers from the
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starting field (not the field index). This avoids a per-position bijection and keeps
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the space fixed-size regardless of where checkers are.
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### 2.4 GameState
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Key fields that affect performance:
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```rust
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pub struct GameState {
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pub board: Board, // 24 bytes
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pub active_player_id: u64, // 1 or 2
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pub players: HashMap<u64, Player>, // 2 entries
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pub history: Vec<GameEvent>, // grows unboundedly
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pub dice: Dice, // 2 bytes
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pub dice_points: (u8, u8),
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pub dice_moves: (CheckerMove, CheckerMove),
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pub dice_jans: PossibleJans, // HashMap<Jan, Vec<(CM,CM)>>
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pub turn_stage: TurnStage,
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pub stage: Stage,
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...
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}
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```
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`history` is the largest field and grows ~3–4 entries per turn.
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A 200-turn game holds ~600 `GameEvent` values.
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---
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## 3. Call Chain: get_legal_actions (C++ → Rust)
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```
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C++ LegalActions()
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└─ engine_->get_legal_actions(player_idx) [cxxengine.rs]
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└─ get_valid_action_indices(&game_state or &mirrored)
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└─ get_valid_actions(state) [training_common.rs]
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└─ MoveRules::get_possible_moves_sequences(true, [])
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├─ get_possible_moves_sequences_by_dices(dice_max, dice_min, ...)
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│ ├─ board.get_possible_moves(dice1) [loop over fields]
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│ └─ for each first_move:
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│ ├─ board.clone() [24-byte copy]
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│ ├─ board2.get_possible_moves(dice2)
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│ └─ for each second_move:
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│ ├─ check_corner_rules()
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│ ├─ check_opponent_can_fill_quarter_rule()
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│ ├─ check_exit_rules() [may recurse!]
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│ └─ check_must_fill_quarter_rule() [recurses!]
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└─ get_possible_moves_sequences_by_dices(dice_min, dice_max, ...)
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[same structure as above]
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```
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Then for each valid `(CheckerMove, CheckerMove)` pair, `checker_moves_to_trictrac_action()`
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maps it back to a `TrictracAction`:
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```
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checker_moves_to_trictrac_action(move1, move2, color, state)
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└─ white_checker_moves_to_trictrac_action(...)
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├─ board.get_field_checker(White, from1) [O(24) scan]
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├─ board.clone()
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├─ board.move_checker(White, move1) [board mutation]
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└─ board.get_field_checker(White, from2) [O(24) scan]
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```
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For **player 2**, an extra `GameState::mirror()` is called before all of this,
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cloning the full state including history.
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---
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## 4. Move Generation Deep Dive
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### 4.1 get_possible_moves_sequences
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```rust
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pub fn get_possible_moves_sequences(
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&self,
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with_excedents: bool,
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ignored_rules: Vec<TricTracRule>,
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) -> Vec<(CheckerMove, CheckerMove)> {
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// called TWICE, once per dice order (max-first, then min-first)
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let mut seqs = self.get_possible_moves_sequences_by_dices(dice_max, dice_min, ...);
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let mut seqs2 = self.get_possible_moves_sequences_by_dices(dice_min, dice_max, ...);
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seqs.append(&mut seqs2);
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// deduplication via HashSet
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}
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```
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The function is **correct but called recursively** through rule validation:
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- `check_must_fill_quarter_rule()` calls `get_quarter_filling_moves_sequences()`
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- `get_quarter_filling_moves_sequences()` calls `get_possible_moves_sequences(true, [Exit, MustFillQuarter])`
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- This inner call's `check_must_fill_quarter_rule()` does **not** recurse further (because `MustFillQuarter` is in ignored_rules)
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So there are at most **2 levels of recursion**, but the second level is invoked once per
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candidate move pair at the outer level. If there are N first-moves × M second-moves,
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`get_quarter_filling_moves_sequences()` is called N×M times and each call triggers a
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full second-level move generation pass.
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### 4.2 get_possible_moves_sequences_by_dices
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```rust
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for first_move in board.get_possible_moves(dice1, ...) {
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let mut board2 = self.board.clone(); // ← clone per first move
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board2.move_checker(color, first_move);
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for second_move in board2.get_possible_moves(dice2, ...) {
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// 4 rule checks, each potentially expensive
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if all_pass {
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moves_seqs.push((first_move, second_move));
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}
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}
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if !has_second {
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// also push (first_move, EMPTY_MOVE) after same 4 checks
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}
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}
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```
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**Board clones**: one per first-move candidate. With 15 checkers on 24 fields, a
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typical position has 5–15 valid first moves → 5–15 board clones per call.
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### 4.3 check_must_fill_quarter_rule
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```rust
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fn check_must_fill_quarter_rule(&self, moves) -> Result<(), MoveError> {
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let filling_moves_sequences = self.get_quarter_filling_moves_sequences();
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if !filling_moves_sequences.contains(moves) && !filling_moves_sequences.is_empty() {
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return Err(MoveError::MustFillQuarter);
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}
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Ok(())
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}
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```
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`get_quarter_filling_moves_sequences()` runs a full pass of move generation and
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applies both checker moves to a board clone for each candidate:
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```rust
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pub fn get_quarter_filling_moves_sequences(&self) -> Vec<(CheckerMove, CheckerMove)> {
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for moves in self.get_possible_moves_sequences(true, [Exit, MustFillQuarter]) {
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let mut board = self.board.clone(); // ← clone per candidate
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board.move_checker(color, moves.0).unwrap();
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board.move_checker(color, moves.1).unwrap();
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if board.any_quarter_filled(Color::White) {
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moves_seqs.push(moves);
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}
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}
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moves_seqs
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}
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```
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If quarter-filling is not relevant to the current position (the common case early in
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the game), this entire function still runs a full move generation pass before returning
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an empty vec.
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### 4.4 check_exit_rules
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```rust
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fn check_exit_rules(&self, moves) -> Result<(), MoveError> {
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if !moves.0.is_exit() && !moves.1.is_exit() { return Ok(()); }
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if self.has_checkers_outside_last_quarter() { return Err(...); }
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let non_excedent_seqs = self.get_possible_moves_sequences(false, [Exit]); // ← full pass
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if non_excedent_seqs.contains(moves) { return Ok(()); }
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if !non_excedent_seqs.is_empty() { return Err(ExitByEffectPossible); }
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// check farthest checker rule ...
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Ok(())
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}
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```
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Called per candidate move pair during the inner loop. Triggers another full
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`get_possible_moves_sequences(false, [Exit])` pass whenever a move involves bearing off.
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---
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## 5. Jan (Points) Computation
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### 5.1 get_jans (game_rules_points.rs)
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Called once per dice roll via `game.rs: consume(RollResult)`. Not on the MCTS hot path
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(MCTS does not need to compute points, only moves). However it is called during
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`get_possible_moves_sequences()` indirectly via `get_scoring_quarter_filling_moves_sequences()`.
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`PossibleJans = HashMap<Jan, Vec<(CheckerMove, CheckerMove)>>` — with only 13 possible
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enum keys this is overkill. A fixed-size array would be faster.
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`PossibleJansMethods::push()` uses `ways.contains(&cmoves)` — O(n) linear search on
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the existing moves list to avoid duplicates. For small lists this is fine, but it can
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be called dozens of times per position.
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`PossibleJansMethods::merge()` for `TrueHitBigJan`/`TrueHitSmallJan` does two O(n)
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scans per entry (one `contains`, one `retain`).
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---
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## 6. State Encoding
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### 6.1 to_vec() — neural-network input (36 i8 values)
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```
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[0..23] board positions (i8, negative = black)
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[24] active player color (0=white, 1=black)
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[25] turn stage (u8 cast to i8)
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[26] dice.values.0
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[27] dice.values.1
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[28..31] white player: points, holes, can_bredouille, can_big_bredouille
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[32..35] black player: same
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```
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Simple, allocation-heavy (returns `Vec<i8>`). For the MCTS hot path, returning a
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`[i8; 36]` stack array would avoid the heap allocation entirely.
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### 6.2 GameState::mirror()
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```rust
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pub fn mirror(&self) -> GameState {
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// Mirrors board (O(24))
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// Swaps and mirrors two player entries in a new HashMap
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// Clones and mirrors the entire history Vec (O(history.len())) ← expensive
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// Mirrors dice_moves (O(1))
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// Mirrors dice_jans (clone + HashMap iteration)
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...
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}
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```
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`mirror()` is called on every `get_legal_actions()` invocation for player 2 (Black).
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Cloning the history is **O(history.len())** and history grows through the entire game.
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---
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## 7. Action Encoding/Decoding
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### 7.1 TrictracAction::to_event() — decode action index → GameEvent
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For `Move` actions, `to_event()`:
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1. Reads `checker1` / `checker2` ordinal positions
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2. Calls `board.get_checker_field(color, checker1)` — O(24) scan
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3. Clones the board
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4. Applies the first move on the clone
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5. Calls `board.get_checker_field(color, checker2)` — O(24) scan on the clone
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6. Adjusts for "prise par puissance"
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7. Constructs the `GameEvent::Move`
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This is called once per `apply_action()` call from C++, so it is not as hot as legal
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action generation.
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### 7.2 white_checker_moves_to_trictrac_action() — encode (CheckerMove, CheckerMove) → TrictracAction
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Called for **every valid move** during `get_valid_actions()`:
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1. Computes `diff_move1` to identify which die was used first
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2. Calls `board.get_field_checker(White, from1)` — O(24) scan
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3. Clones the board
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4. Calls `board.move_checker(White, move1)` — mutation on clone
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5. Calls `board.get_field_checker(White, from2)` — O(24) scan on clone
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With 20 valid moves, this is 20 board clones + 40 O(24) scans per
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`get_valid_actions()` call.
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---
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## 8. Identified Performance Bottlenecks
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Ordered by estimated impact on MCTS training throughput:
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### 8.1 [HIGH] Recursive move generation in check_must_fill_quarter_rule
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**Problem**: `get_possible_moves_sequences()` is called O(F₁ × F₂) times where F₁ and F₂ are
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the number of first- and second-move candidates. Each inner call runs a complete move
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generation pass.
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**When it triggers**: Only when at least one valid move would fill/preserve a quarter
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of the board. This is a common situation especially in mid/late game.
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**Fix direction**: Precompute `get_quarter_filling_moves_sequences()` once per
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`get_possible_moves_sequences()` call and pass it in, instead of recomputing it for
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each candidate pair.
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### 8.2 [HIGH] GameState::mirror() copies history
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**Problem**: `mirror()` clones + iterates `self.history` on every `get_legal_actions()`
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call for Black. The history grows proportionally to game length and serves no purpose
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for action generation.
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**Fix direction**: Either skip history in `mirror()` (pass an empty `Vec` for that
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field) or refactor `cxxengine.rs` to mirror only the board and thread color perspective
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through the functions that need it.
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### 8.3 [MEDIUM] Board clones in get_possible_moves_sequences_by_dices
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**Problem**: One `board.clone()` per first-move candidate (5–15 per call). Each clone
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is 24 bytes, but the allocator round-trip costs more than the copy.
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**Fix direction**: Apply and undo moves on a single mutable board (move + undo pattern)
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rather than cloning. Board mutation is O(1) and undoing is symmetric.
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### 8.4 [MEDIUM] Board clones in get_quarter_filling_moves_sequences
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**Problem**: One `board.clone()` per candidate move sequence, plus two `move_checker()`
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calls (which can return Err — currently `.unwrap()`ed). These clones are nested inside
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the already-expensive recursive path described in 8.1.
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**Fix direction**: Same move + undo pattern as 8.3.
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### 8.5 [MEDIUM] Board clones and O(24) scans in checker_moves_to_trictrac_action
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**Problem**: Called for every valid move in `get_valid_actions()`. With 20 valid moves,
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this is 20 clones + 40 O(24) scans.
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**Fix direction**: Pass the checker ordinal index directly from `get_board_exit_farthest`
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or store it alongside the CheckerMove when generating moves. This avoids re-scanning
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to convert back.
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### 8.6 [MEDIUM] check_exit_rules triggers a full move generation pass
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**Problem**: Called for every candidate move pair that involves bearing off. Runs
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`get_possible_moves_sequences(false, [Exit])` → another full move generation.
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**Fix direction**: Precompute non-excedant moves once if any move in the candidate set
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involves bearing off, and pass the result in.
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### 8.7 [LOW] PossibleJans backed by HashMap with linear-search deduplication
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**Problem**: Only 13 `Jan` variants exist. A `HashMap` adds hashing overhead and
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pointer indirection. `Vec::contains()` for deduplication is O(n).
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**Fix direction**: Use `[Option<SmallVec<...>>; 13]` with `Jan as usize` index. For
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deduplication use a `BTreeSet` or just sort + dedup (the lists are short).
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### 8.8 [LOW] to_vec() returns a heap-allocated Vec
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**Problem**: Called from C++ via `get_tensor()` on every MCTS rollout observation.
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**Fix direction**: Return `[i8; 36]` or write directly into a pre-allocated C++-owned
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buffer (possible with cxx.rs via `Pin<&mut CxxVector<i8>>`).
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### 8.9 [LOW] get_color_fields() allocates a Vec on every call
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**Problem**: Called repeatedly in move generation (once per first-move enumeration,
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once in is_quarter_fillable per field, etc.).
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**Fix direction**: Return a fixed-size `ArrayVec<(Field, i8), 24>` (using the
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`arrayvec` crate) or add a small-vec optimisation.
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### 8.10 [LOW] unwrap() calls in hot paths (correctness concern)
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Currently `.unwrap()` in:
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- `get_quarter_filling_moves_sequences()` line 529-530: `board.move_checker(...).unwrap()`
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- Multiple places in `game_rules_points.rs`
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With `catch_unwind` wrapping at the FFI boundary these panics are now caught, but
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they still abort the move and propagate an error to C++ rather than producing a clean
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`SpielFatalError` message. These should be `?` with proper `Result` propagation or
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`.expect()` with descriptive messages.
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---
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## 9. Key Correctness Observations
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### 9.1 Game ends during chance action (fixed in alpha_zero.cc)
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`game.rs::consume(RollResult)` sets `stage = Stage::Ended` (line 795) when a player's
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jan score grants their 13th hole on the dice roll itself. This makes a **chance action**
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terminal. The OpenSpiel `PlayGame` loop in `alpha_zero.cc` did not check `IsTerminal()`
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after chance actions — this was the root cause of the SIGSEGV crash. Fixed by adding:
|
||||
```cpp
|
||||
state->ApplyAction(action); // chance action
|
||||
if (state->IsTerminal()) { trajectory.returns = state->Returns(); break; }
|
||||
```
|
||||
|
||||
### 9.2 current_player_idx() panics on underflow
|
||||
|
||||
```rust
|
||||
fn current_player_idx(&self) -> u64 {
|
||||
self.game_state.active_player_id - 1 // u64 underflow if id == 0
|
||||
}
|
||||
```
|
||||
|
||||
`active_player_id` is always 1 or 2 during normal play (set by `BeginGame`), so this
|
||||
is safe in practice. However it should be wrapped in `catch_unwind` or guarded by an
|
||||
explicit check for robustness:
|
||||
```rust
|
||||
self.game_state.active_player_id.saturating_sub(1)
|
||||
```
|
||||
|
||||
### 9.3 Mirror is always from White's perspective
|
||||
|
||||
`MoveRules` and `PointsRules` always reason from White's point of view. For Black, the
|
||||
caller must mirror the board beforehand. The comment at the top of `game_rules_moves.rs`
|
||||
documents this. `cxxengine.rs` correctly mirrors the `GameState` for player 2 before
|
||||
calling `get_valid_action_indices()`.
|
||||
|
||||
---
|
||||
|
||||
## 10. Summary Table
|
||||
|
||||
| Bottleneck | Location | Estimated Severity | Fix Complexity |
|
||||
|---|---|---|---|
|
||||
| Recursive `get_possible_moves_sequences` in `check_must_fill_quarter_rule` | `game_rules_moves.rs:272` | High | Medium |
|
||||
| `GameState::mirror()` clones history | `game.rs:159` | High | Low |
|
||||
| Board clones per first-move in `get_possible_moves_sequences_by_dices` | `game_rules_moves.rs:555` | Medium | Medium |
|
||||
| Board clones in `get_quarter_filling_moves_sequences` | `game_rules_moves.rs:528` | Medium | Medium |
|
||||
| `check_exit_rules` triggers full move generation | `game_rules_moves.rs:335` | Medium | Medium |
|
||||
| `checker_moves_to_trictrac_action` clones per valid move | `training_common.rs:326` | Medium | Low–Medium |
|
||||
| `PossibleJans` HashMap + linear dedup | `game_rules_points.rs:67` | Low | Low |
|
||||
| `to_vec()` heap allocation | `game.rs:213` | Low | Low |
|
||||
| `get_color_fields()` Vec allocation | `board.rs:405` | Low | Medium |
|
||||
| `unwrap()` in hot paths | multiple | Correctness | Low |
|
||||
Loading…
Add table
Add a link
Reference in a new issue