docs: add GPU pipeline Phase 2b design — zero-roundtrip CUDA experience collection
Full design for monolithic CUDA kernel that runs the entire DQN experience collection pipeline on GPU: Dueling Q-network inference (online + target), epsilon-greedy action selection, portfolio simulation, barrier tracking, diversity entropy, curiosity inference, and reward combination. 128 parallel episodes × 500 timesteps = 64,000 experiences per kernel launch with zero CPU roundtrips. Key decisions: - Approach A (monolithic kernel) for speed + accuracy - Parallel episode architecture (N threads × L timesteps) - Q-network + curiosity inference in raw CUDA (matrix-vector multiply) - Curiosity training stays on Candle (periodic batch updates) - TD error pre-computed for priority replay - Weight sync after gradient updates (~1.2MB upload) Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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jgrusewski
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docs/plans/2026-02-28-gpu-pipeline-phase2b-design.md
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# GPU Pipeline Phase 2b: Zero-Roundtrip CUDA Experience Collection
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**Date:** 2026-02-28
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**Branch:** `feature/cuda-pipeline-phase2b`
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**Status:** Design approved
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**Depends on:** Phase 2 (complete — GPU batch states + structural portfolio sim)
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## Problem
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Phase 2 achieved GPU batch state construction (eliminating 1.8M Vec allocations/epoch) and structural GPU portfolio simulation. However, the DQN trainer's inner loop still:
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1. Runs portfolio simulation on CPU (duplicating GPU work)
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2. Computes barrier tracking, curiosity, diversity, and reward combination on CPU per bar
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3. Performs 15,600+ kernel launches per epoch with PCIe roundtrips each
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4. Achieves only ~23% GPU utilization on L4
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## Solution
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A single monolithic CUDA kernel that runs the **entire experience collection pipeline** — portfolio simulation, Q-network inference (Dueling architecture), epsilon-greedy action selection, barrier tracking, diversity entropy, curiosity inference, and reward combination — with zero CPU roundtrips during execution.
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128 parallel episodes × 500 timesteps = 64,000 experiences per kernel launch.
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## Architecture
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### Approach: Extended Monolithic Kernel + Parallel Episodes
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**Why monolithic:** The kernel loop is inherently sequential per episode (portfolio state depends on previous bar). Adding more sequential logic (barrier, diversity, Q-network forward) per iteration costs negligible extra ALU ops. One kernel launch eliminates 15,600 launches worth of overhead.
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**Why parallel episodes:** Each episode has independent portfolio state. N=128 threads process 128 episodes simultaneously. This is the standard GPU RL environment pattern (Isaac Gym, EnvPool).
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### Data Flow (One Training Round)
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```
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CPU: Generate 128 random episode_starts
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CPU: Upload episode_starts [128] to GPU (512B)
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GPU: Launch dqn_full_experience_kernel(N=128, L=500) (ONE launch)
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+-- Each of 128 threads independently:
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| for t in 0..500:
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| 1. Read market features[bar, 51]
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| 2. Compute portfolio features [3] from local state
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| 3. Concatenate -> state [54]
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| 4. Dueling Q-network forward (online) -> 45 Q-values
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| 5. Epsilon-greedy action selection (LCG RNG)
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| 6. Portfolio simulation (trade execution)
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| 7. Barrier tracking (profit/stop/time)
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| 8. Diversity entropy (sliding window)
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| 9. Curiosity inference (forward model MSE)
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| 10. Mark-to-market -> next_state
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| 11. Dueling Q-network forward (target) -> target Q-values
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| 12. Compute TD error for priority replay
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| 13. Combine reward (PnL + barrier + risk + diversity + curiosity)
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| 14. Write experience to output buffers
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| 15. Handle episode reset if done
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CPU: Download outputs (~16MB) (ONE transfer, <1ms)
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CPU: Add 64,000 experiences to replay buffer (with TD priorities)
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CPU: Sample batch, Candle gradient update (standard DQN)
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CPU: Every K rounds: Candle curiosity training batch
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CPU: After gradient update: sync weights to GPU (1.2MB, <1ms)
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```
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### Performance Comparison
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| Metric | Current (Phase 2) | Phase 2b |
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|--------|-------------------|----------|
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| Kernel launches/epoch | ~15,600 | ~31 |
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| PCIe roundtrips/epoch | ~31,200 | ~62 |
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| Experiences/launch | 128 | 64,000 |
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| GPU compute utilization | ~23% | ~95%+ |
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| CPU inner loop | Full per-bar processing | Replay buffer + gradient only |
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## Kernel Components
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### 1. Dueling Q-Network (Online + Target)
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Production architecture: Dueling DQN with 54-dim input, 45-action output.
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```
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Shared pathway: [54] -> [256] LeakyReLU(0.01) -> [256] LeakyReLU(0.01)
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Value stream: [256]->[128] LReLU -> [1] Advantage: [256]->[128] LReLU -> [45]
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Q(s,a) = V(s) + [A(s,a) - mean(A)]
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```
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Each thread performs matrix-vector multiplies (not matrix-matrix):
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- Layer 1: 54x256 = 13,824 FMA
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- Layer 2: 256x256 = 65,536 FMA
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- Value/Advantage streams: 2x(256x128 + 128x1/45) = ~66K FMA
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- Total per forward pass: ~145K FMA per thread
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- Two passes (online + target): ~290K FMA per thread per timestep
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With 128 threads x 500 timesteps: ~18.6B FMA per kernel launch. Trivial for L4.
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**Weight layout:** Candle convention `[out_features, in_features]`. Kernel accesses as:
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```c
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output[j] = bias[j];
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for (int i = 0; i < in_dim; i++)
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output[j] += input[i] * weight[j * in_dim + i];
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```
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### 2. Curiosity Forward Model (Inference Only)
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Architecture: 35 -> 64 LeakyReLU -> 32
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Input encoding: 32 state features + 3 action one-hot (Long/Flat/Short mapping from 45 actions).
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```c
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int exposure = action_idx / 9; // 0-4
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int cat = (exposure <= 1) ? 0 : (exposure == 2) ? 1 : 2; // Short/Flat/Long
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```
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Curiosity bonus = clamped MSE between predicted and actual next state embedding.
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**Training:** Runs periodically on Candle (every K rounds). Download batch of transitions, run backward + AdamW step, re-upload weights to GPU.
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### 3. Portfolio Simulation
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Existing logic from Phase 2 kernel: trade execution with two-phase reversals, cash reserves, transaction costs. Per-thread portfolio state [8]: cash, position, entry_price, initial_capital, spread, last_price, reserve_pct, cum_costs.
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### 4. Barrier Tracking
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Per-thread barrier state [5]: entry_price_raw, upper_barrier, lower_barrier, expiry_step, barrier_result.
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On position open: set barriers based on config (profit_target_bps, stop_loss_bps, max_holding_bars).
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Per bar: check price against upper/lower barriers, check step against expiry.
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Output: barrier_label (-1 stop, 0 time/none, +1 profit) for reward scaling.
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### 5. Diversity Entropy
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Per-thread sliding window [100 action indices] with circular buffer.
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Shannon entropy: H = -sum(p_i * log2(p_i)) over 3-action histogram (Long/Flat/Short).
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Penalty: -0.1 if H < 1.0 (low diversity).
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### 6. Reward Combination
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```c
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float reward = raw_pnl_return; // (next_norm - curr_norm) / curr_norm
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reward *= (1.0f + barrier_scale); // +0.5 profit, -0.5 stop, 0.0 time/none
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if (abs_pos_norm > 0.8f)
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reward -= (abs_pos_norm - 0.8f) * 5.0f * 0.1f; // risk penalty
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reward += diversity_bonus; // -0.1 if low entropy
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reward += fminf(curiosity_mse, curiosity_max_reward); // novelty bonus
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```
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### 7. TD Error for Priority Replay
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```c
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float td_target = reward + gamma * max_target_q * (1 - done);
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float td_error = fabsf(td_target - online_q_selected);
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td_error_out[idx] = td_error;
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```
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### 8. Epsilon-Greedy RNG
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LCG per thread, seeded from host:
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```c
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__device__ float gpu_random(unsigned int* state) {
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*state = *state * 1664525u + 1013904223u;
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return (float)(*state & 0x7FFFFF) / (float)0x7FFFFF;
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}
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```
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## Memory Budget
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### GPU Buffers
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| Category | Size |
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|----------|------|
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| Market features [total_bars, 51] | ~400MB (2M bars) |
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| Targets [total_bars, 4] | ~32MB |
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| Online Q-network weights | 592KB |
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| Target Q-network weights | 592KB |
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| Curiosity weights | 17KB |
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| Per-episode state (128 episodes: portfolio + barrier + diversity) | 57KB |
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| Output buffers (128 x 500 x (54+1+1+1+1+1)) | ~16MB |
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| RNG states [128] | 512B |
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| Barrier config [4] | 16B |
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| **Total** | **~450MB** |
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Fits comfortably in L4 24GB or RTX 3050 Ti 4GB (with smaller dataset).
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### Per-Thread Scratch (Registers/Local Memory)
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| Buffer | Per Thread | 128 Threads |
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|--------|-----------|-------------|
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| shared_h1 [256] | 1KB | 128KB |
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| shared_h2 [256] | 1KB | 128KB |
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| value_h/adv_h [128+128] | 1KB | 128KB |
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| q_values [45] | 180B | 22KB |
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| target_q [45] | 180B | 22KB |
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| curiosity_h [64] | 256B | 32KB |
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| state [54] | 216B | 27KB |
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| next_state [54] | 216B | 27KB |
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| **Total** | **~4.1KB** | **~514KB** |
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## Weight Synchronization
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### Online Network Sync
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After each gradient update round:
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1. `vs.get("shared_layers.0.weight")` -> `Tensor` -> `.to_vec1::<f32>()`
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2. Repeat for all 12 weight/bias tensors
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3. `stream.memcpy_htod(&weights, &mut gpu_buffer)`
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4. Total: 592KB upload per sync
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### Target Network Sync
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Every `target_update_freq` gradient steps:
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1. Same extraction process from target VarMap
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2. 592KB upload
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3. Infrequent (typically every 1000-10000 steps)
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### Curiosity Weight Sync
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Every K training rounds:
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1. Download batch of transitions from replay buffer
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2. Run Candle backward + AdamW step on curiosity model
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3. Re-upload 17KB weights to GPU
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4. K chosen to balance curiosity signal freshness vs overhead
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## Training Loop Changes
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### Current (Phase 2)
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```
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for epoch in 0..num_epochs:
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for batch_idx in 0..num_batches: // ~15,600 batches
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for bar in batch: // 128 bars per batch
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CPU: portfolio sim
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CPU: barrier tracking
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CPU: Q-network forward (Candle GPU)
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CPU: curiosity
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CPU: diversity
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CPU: reward combination
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CPU: store experience
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GPU: build_batch_states // Phase 2 optimization
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for update in 0..gradient_steps:
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sample replay buffer
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Candle: forward + backward + step
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```
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### New (Phase 2b)
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```
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for epoch in 0..num_epochs:
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for round in 0..num_rounds: // ~31 rounds
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CPU: generate 128 episode_starts
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GPU: ONE kernel launch -> 64,000 experiences
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CPU: download experiences (~16MB)
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CPU: add to replay buffer (with TD priorities)
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for update in 0..gradient_steps:
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sample replay buffer
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Candle: forward + backward + step
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every target_freq: sync target weights -> GPU
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sync online weights -> GPU
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every K rounds: Candle curiosity training + sync
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```
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## Concerns and Mitigations
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### Training Dynamics (Sequential -> Random Episode Sampling)
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**Risk:** Low. Random episode sampling reduces temporal correlation, which is desirable for DQN (replay buffer exists to break correlations). With ~31 rounds per epoch, data coverage is equivalent.
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### Barrier Timing Precision
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**Risk:** Solvable. GPU uses bar indices (i32) not nanosecond timestamps. `max_holding_bars` replaces `max_holding_period_ns`. No precision loss.
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### Curiosity Signal Freshness
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**Risk:** Moderate. Curiosity model trains periodically (every K rounds) instead of per-bar. More data per training step (batch of 1024 vs single transition) may compensate. Monitor curiosity bonus distribution during training.
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### Done-Episode Idling
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**Risk:** Minor. When a barrier terminates an episode at step 200/500, that thread idles for 300 steps. ~30% waste on average. Acceptable vs complexity of dynamic episode recycling.
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### Epsilon Decay
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**Risk:** None. Epsilon is a scalar parameter passed to kernel. CPU decrements between rounds. All threads use same epsilon per launch.
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## Future Phase 3 Optimization
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If >95% GPU utilization is insufficient:
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- **Dynamic episode recycling:** When a thread's episode ends, start a new episode from a new random bar instead of idling. Eliminates 30% waste.
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- **Multi-stream pipelining:** Launch experience collection on stream A while gradient updates run on stream B. Overlaps compute phases.
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- **Larger N:** Scale from 128 to 512+ parallel episodes if GPU memory allows.
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