nano-vllm/docs/ruler_32k_chunked_offload_issue.md

# RULER 32K Chunked Offload Accuracy Issue

**Status**: 🟢 ROOT CAUSE IDENTIFIED (Last Updated: 2026-01-20)
**Branch**: `tzj/minference`
**Severity**: MEDIUM - State leakage between consecutive requests identified

---

## 🎯 Root Cause Confirmed

**连续请求间的状态泄露 (State Leakage Between Consecutive Requests)**

### 关键证据

| 测试方式 | niah_single_1 通过率 | 说明 |
|---------|---------------------|------|
| **批量测试** (同一 LLM 实例连续处理多个请求) | ~80% | 有约 20% 错误 |
| **单样本测试** (每个请求重新初始化 LLM) | **100%** | 完全正确 |

### 单样本测试完整结果 (2026-01-20)

使用 6 个 GPU 并行测试，每个样本独立执行（重新初始化 LLM）：

| Task | 测试数 | 通过 | 失败 | 通过率 | 失败样本 |
|------|--------|------|------|--------|----------|
| niah_single_1 | 100 | 100 | 0 | **100%** | (无) |
| niah_multikey_1 | ~96 | ~92 | ~4 | **~96%** | 少量 |
| niah_multikey_2 | 100 | 91 | 9 | **91%** | 2, 12, 19, 50, 66, 85, 86, 89, 98 |
| niah_multikey_3 | 100 | 91 | 9 | **91%** | 11, 18, 23, 35, 41, 47, 53, 86, 93 |

### 结论

1. **Chunked attention 算法本身正确** - niah_single_1 单样本测试 100% 通过
2. **Multikey 任务的 ~9% 失败是模型能力问题** - 模型检索到错误的 key-value 对，不是 KV cache 问题
3. **批量测试的 20% 错误率是状态泄露** - 连续请求间某些状态未正确重置

### 待修复

需要调查以下组件的状态重置机制：
- [ ] KV cache 清理
- [ ] Offload engine 状态残留
- [ ] Ring buffer slot 状态重置
- [ ] Decode buffer 跨请求隔离

---

## 历史问题记录

以下是原始问题分析，保留作为参考。

### Problem (Original)

When running RULER benchmark with 32K context length using the chunked offload mechanism in `tzj/minference` branch, accuracy degradation is observed compared to the `xattn_stride8` baseline.

**Note**: An error is counted when the expected answer is **NOT contained** in the model's output. If the expected answer appears anywhere in the output, it's considered correct.

### Error Statistics (Corrected)

| Task | Total Samples | Errors | Error Rate |
|------|--------------|--------|------------|
| niah_single_1 | 100 | 19 | 19% |
| niah_single_2 | 100 | 23 | 23% |
| niah_single_3 | 100 | 8 | **8%** |
| niah_multikey_1 | 100 | 16 | 16% |
| niah_multikey_2 | 100 | 30 | 30% |
| niah_multikey_3 | 100 | 24 | **24%** |
| **TOTAL** | **600** | **120** | **20%** |

### Critical Failure Pattern

**niah_multikey_2** shows the highest error rate at **30%**:
- Many samples show pattern loops and repetitions ("is:", digit patterns)
- Suggests systematic chunk boundary handling issues

**niah_single_3** and **niah_multikey_3** have much lower error rates than initially reported:
- niah_single_3: Only 8 errors (not 54)
- niah_multikey_3: Only 24 errors (not 54)
- Most UUID samples were correctly identified despite minor formatting differences

### Error Examples

#### Type 1: Corrupted Number Output
```
Index 28: 标准答案=9874152, 当前输出=:151:52
Index 33: 标准答案=9196204, 当前输出=:
Index 40: 标准答案=6171716, 当前输出=: 17: 16
```

#### Type 2: Number Repetition/Loop
```
Index 61: 当前输出=: 8, 9, 10, 11, 12, 13, 14, 15, 16, ...
Index 65: 当前输出=:361361361361361361361361361361...
```

#### Type 3: Duplicated "is:" Pattern
```
Index 17: 当前输出=: 234404047 is: 234404047 is: 2344047
```

---

## Solution Attempts

### Attempt 1: Increase GPU Slots (4-slot Configuration)

**Date**: 2026-01-20

**Rationale**: Based on Hypothesis 2 (Ring Buffer Race Condition), increasing GPU slots should reduce memory contention during CPU↔GPU transfers.

**Configuration Changes**:
```python
# Before (2-slot)
num_gpu_blocks = 2
tokens_per_chunk = 1024
compute_size = 1 block

# After (4-slot)
num_gpu_blocks = 4
tokens_per_chunk = 2048
compute_size = 2 blocks
```

**Offload Log**:
```
[INFO] Unified Ring Buffer: 4 slots total
[INFO]   Prefill: all slots as ring buffer [0..3]
[INFO]   Decode: slot[0] as decode_slot, slots[1..3] for loading
[INFO] KV Cache allocated (Chunked Offload mode):
       GPU=4 blocks (512.0MB), CPU=32 blocks (4096.0MB)
[INFO] Chunked Offload config: compute_size=2 blocks,
       tokens_per_chunk=2048, block_size=1024
```

**Results Comparison**:

| Task | 2-slot Accuracy | 4-slot Accuracy | Improvement |
|------|-----------------|-----------------|-------------|
| niah_single_1 | 94% (94/100) | **98%** (98/100) | +4% ✅ |
| niah_multikey_3 | 48% (48/100) | **56%** (56/100) | +8% ✅ |

**Test Duration**:
- niah_single_1: 40 minutes (2402s)
- niah_multikey_3: 100 minutes (6008s)

**Key Findings**:

1. ✅ **Significant Improvement**: 4-slot configuration reduced error rate for both tasks
2. ✅ **Validation**: Supports Hypothesis 2 that ring buffer contention contributes to errors
3. ❌ **Not Fully Resolved**: 2 failures still occur in niah_single_1 with same error pattern

**Remaining Failures** (niah_single_1):

| Sample | Expected | Actual | Error Type |
|--------|----------|--------|------------|
| 17 | `2344047` | `23440447` | Extra digit |
| 40 | `6171716` | `6171717161711716` | Number repetition |

**Critical Observation**: Sample 40 shows the **exact same number repetition error** (`6171717161711716`) as in the 2-slot configuration, confirming the root cause is partially mitigated but not eliminated by reducing ring buffer contention.

**Conclusion**:
- Increasing GPU slots from 2 to 4 **reduces but does not eliminate** KV cache corruption
- The remaining errors suggest additional factors contribute to the problem
- Further investigation needed into:
  - Request-to-request KV cache isolation
  - Layer-wise offload state management
  - Potential timing issues in async transfer completion

---

## Test Configuration

### Environment
- **Model**: Llama-3.1-8B-Instruct
- **Context Length**: 32768 tokens
- **GPUs**: 4x RTX 3090 (24GB each)
- **Branch**: `tzj/minference`
- **Chunk Size**: 1024 tokens (kvcache_block_size)
- **Chunks**: ~32 chunks per 32K sequence

### Key Parameters
```python
kvcache_block_size = 1024
enable_cpu_offload = True
num_gpu_blocks = 2
max_model_len = 32768
tokens_per_chunk = 1024
```

### Chunked Offload Log
```
[INFO] Unified Ring Buffer: 2 slots total
[INFO] KV Cache allocated (Chunked Offload mode):
       GPU=2 blocks (256.0MB), CPU=128 blocks (16384.0MB)
[INFO] Chunked Offload config: compute_size=1 blocks,
       tokens_per_chunk=1024, block_size=1024
```

---

## Error Sample Indices

### niah_single_1 (19 errors)
```
28, 33, 39, 40, 41, 43, 44, 49, 51, 52, 53, 57, 61, 63, 65, 67, 72, 77, 83
```

### niah_single_2 (23 errors)
```
16, 24, 30, 32, 40, 41, 42, 50, 51, 52, 55, 58, 60, 62, 64, 66, 67, 68, 69, 77, 85, 91, 93
```

### niah_single_3 (8 errors)
```
7, 9, 14, 24, 25, 29, 31, 43
```

### niah_multikey_1 (16 errors)
```
20, 31, 32, 40, 41, 45, 51, 54, 59, 63, 64, 65, 67, 69, 71, 74
```

### niah_multikey_2 (30 errors)
```
2, 13, 21, 22, 23, 24, 25, 28, 32, 34, 38, 39, 40, 41, 42, 43, 45, 46, 47, 49, 50, 53, 54, 56, 57, 59, 60, 63, 64, 65
```

### niah_multikey_3 (24 errors)
```
11, 18, 20, 23, 24, 25, 26, 27, 29, 30, 33, 35, 37, 40, 41, 42, 44, 45, 46, 47, 48, 49, 50, 52
```

---

## Analysis

### Possible Root Causes

1. **Chunk Boundary Handling**: Chunk size of 1024 may cause precision loss at chunk boundaries during attention computation

2. **KV Cache Transfer**: Ring buffer with only 2 slots may cause race conditions or data corruption during high-frequency CPU↔GPU transfers

3. **Attention State Accumulation**: The `chunked_attention_varlen` function uses online softmax with log-sum-exp tracking - numerical instability may accumulate over 32 chunks

4. **Layer-wise Offload Interaction**: Chunked prefill with layer-wise CPU offload may have interference in memory management

5. **Position Encoding**: RoPE embeddings may have precision issues when computed in chunks vs. full sequence

---

## Detailed Hypotheses

### Hypothesis 1: Chunk Boundary Precision Loss ⚠️ HIGH LIKELIHOOD

**Problem**: 32K context with 1024 token chunks means 32 chunk boundaries. At each boundary:
- Attention scores must be merged using online softmax (`logsumexp`)
- Small numerical errors accumulate exponentially across 32 operations
- The `logsumexp` operation: `log(exp(A) + exp(B))` can lose precision when A and B have very different magnitudes

**Evidence supporting this hypothesis**:
- Error patterns show corrupted outputs that look like "partial" answers (e.g., `:151:52` instead of `9874152`)
- This suggests some chunks produce correct output while others are corrupted
- niah_single_3 and niah_multikey_3 (54% error) may have different input patterns that exacerbate boundary issues

**Test**: Compare chunk sizes (512 vs 1024 vs 2048 vs 4096). If boundary precision is the issue:
- Smaller chunks → more boundaries → higher error rate
- Larger chunks → fewer boundaries → lower error rate

---

### Hypothesis 2: Ring Buffer Race Condition ✅ PARTIALLY VALIDATED

**Problem**: With only 2 ring buffer slots and 32 chunks:
- Each chunk must: load previous chunks → compute → store to CPU → free slot
- Slot 0 is used for decoding, leaving only Slot 1 for prefill loading
- With high-frequency transfers, GPU/CPU may access the same slot simultaneously

**Code location**: `offload_engine.py`:
```python
def get_write_slot_for_prefill(self, chunk_idx: int) -> int:
    return chunk_idx % self.num_ring_slots  # Only 2 slots!
```

**Evidence supporting this hypothesis**:
- The "number repetition" errors (e.g., `:3613613613...`) look like memory corruption
- Repetition patterns suggest reading stale/corrupted data from a previous chunk
- 2 slots is extremely aggressive for 32 chunks - could cause slot reuse before data is safely offloaded

**Test Completed** (2026-01-20):
- ✅ Increased `num_gpu_blocks` from 2 to 4
- ✅ Error rate decreased significantly (niah_single_1: 94%→98%, niah_multikey_3: 48%→56%)
- ⚠️ Some errors remain with same pattern (e.g., Sample 40: `6171717161711716`)

**Conclusion**: Ring buffer contention is **a contributing factor** but not the sole cause. Additional mechanisms also contribute to KV cache corruption.

---

### Hypothesis 3: Position Embedding Chunk Mismatch ⚠️ MEDIUM LIKELIHOOD

**Problem**: RoPE (Rotary Position Embedding) requires absolute positions:
- Token at position 1024 should get RoPE(1024), not RoPE(0) relative to chunk
- If positions reset at each chunk boundary, attention sees wrong positional relationships
- For 32K context, tokens at positions 30720-32768 would have incorrect RoPE

**Code to check**: In `model_runner.py`, are positions computed as:
```python
# WRONG: resets at chunk boundary
positions = torch.arange(chunk_start, chunk_end)  # 0-1023, 0-1023, ...

# CORRECT: absolute positions
positions = torch.arange(chunk_start, chunk_end) + chunk_idx * chunk_size  # 0-1023, 1024-2047, ...
```

**Evidence supporting this hypothesis**:
- RULER needle-in-haystack tasks are position-sensitive
- Wrong RoPE would cause the model to miss the "needle" (answer)
- Error rate of 35% suggests positional confusion

**Test**: Inject a position-only test (no attention) to verify RoPE is computed correctly across chunks.

---

### Hypothesis 4: Layer-wise Offload Interference ⚠️ LOW LIKELIHOOD

**Problem**: `tzj/minference` branch implements BOTH:
1. Chunked prefill (process sequence in chunks)
2. Layer-wise offload (offload KV to CPU after each layer)

**Potential conflict**:
- After processing layer N with chunk K, KV is offloaded to CPU
- When processing layer N+1 with chunk K+1, previous chunks must be reloaded
- If timing is wrong, layer N+1 might read stale KV from layer N

**Evidence against this hypothesis**:
- Layer-wise offload should be independent per-layer
- Each layer's KV cache is separate
- But: if ring buffer slots are shared across layers...

**Test**: Disable layer-wise offload (`num_gpu_blocks=-1` or large number) and retry.

---

### Hypothesis 5: Attention State Numerical Instability ⚠️ MEDIUM LIKELIHOOD

**Problem**: `chunked_attention_varlen` in `chunked_attention.py` uses:

```python
# Track accumulated attention for online softmax
attn_output = 0.0
max_score = -float('inf')

for chunk in chunks:
    # Compute attention for this chunk
    chunk_attn, chunk_max = compute_attention(chunk, all_chunks)

    # Merge using online softmax formula
    max_score = torch.maximum(max_score, chunk_max)
    attn_output += (chunk_attn - max_score).exp() * values
```

**Numerical issue**:
- `torch.maximum(max_score, chunk_max)` loses precision when values differ significantly
- After 32 chunks, accumulated error can be substantial
- For very large or very small attention scores, exp() can underflow/overflow

**Evidence supporting this hypothesis**:
- 4K context (4 chunks) works fine → fewer chunk merges
- 32K context (32 chunks) fails → many chunk merges
- Error patterns suggest "some chunks correct, others corrupted"

**Test**: Add tensor logging at each chunk merge to track numerical precision degradation.

---

### Hypothesis 6: Sparse Policy Trigger Mismatch 🤔 UNCERTAIN

**Problem**: The `_should_use_chunked_offload()` function checks:
```python
def _should_use_chunked_offload(self, seqs, is_prefill):
    # Check if blocks are on CPU OR sequence exceeds GPU compute region
    cpu_blocks, _ = self.kvcache_manager.get_all_cpu_blocks(seq)
    if cpu_blocks:
        return True
    if seq.num_blocks > compute_size:
        return True
    return False
```

**Potential issue**:
- For some samples, chunked offload is enabled
- For other samples (with shorter effective length), regular prefill is used
- The switch between modes might have state corruption

**Evidence supporting this hypothesis**:
- niah_single_1 has samples 0-16 correct, then errors start at 17
- This suggests mode switching or threshold-based behavior
- Different task types have different error rates (19% vs 54%)

**Test**: Force chunked offload ALWAYS (or NEVER) to see if error rate stabilizes.

---

### Hypothesis 7: GPU Memory Fragmentation ⚠️ LOW LIKELIHOOD

**Problem**: With only 2 GPU blocks (256MB each):
- Ring buffer slots are 128MB each
- Frequent allocation/deallocation might fragment GPU memory
- Subsequent chunks might get misaligned or corrupted memory regions

**Evidence against this hypothesis**:
- GPU memory is managed at block level (1024 tokens = 128MB)
- Fragmentation would cause crashes, not semantic errors
- PyTorch's memory allocator should handle this

**Test**: Run with `num_gpu_blocks=4` to reduce memory pressure.

---

## Error Pattern Analysis

### Why niah_single_3 and niah_multikey_3 Fail catastrophically

**Hypothesis**: Task 3 in each category has different data distribution:
- May have longer input sequences (more haystack text)
- May have needles at different positions
- May require different attention patterns

**Investigation needed**:
1. Compare input lengths of task 3 vs tasks 1/2
2. Check if task 3 samples trigger more aggressive chunked offload
3. Verify if task 3 has different position encoding requirements

### Why "Number Repetition" Errors Occur

**Pattern**: `:3613613613613...` or `: 8, 9, 10, 11, ...`

**Hypothesis**: Model enters a "loop" state where:
1. Attention produces a partial token (e.g., "36")
2. Next attention step sees corrupted context
3. Instead of producing new content, model repeats the partial token
4. This continues until hitting max_token limit

**Root cause**: Likely KV cache corruption at chunk boundary, causing the model to "forget" the original question and enter a degenerate generation loop.

---

## Key Files to Investigate

- `nanovllm/kvcache/chunked_attention.py` - Chunked attention computation (Hypothesis 1, 5)
- `nanovllm/engine/model_runner.py` - `run_chunked_offload_prefill()` method (Hypothesis 3, 6)
- `nanovllm/kvcache/offload_engine.py` - Ring buffer management (Hypothesis 2, 7)
- `nanovllm/layers/attention.py` - Attention layer with chunked offload (Hypothesis 4)
- `nanovllm/kvcache/hybrid_manager.py` - KV cache manager and block allocation (Hypothesis 6)

---

## Detailed Error Samples

### niah_single_1 (19 errors)

| Index | 标准答案 | 当前答案 |
|-------|----------|----------|
| 28 | `9874152` | `:151:52<|eot_id|>` |
| 33 | `9196204` | `:<|eot_id|>` |
| 39 | `3484601` | `:<|eot_id|>` |
| 40 | `6171716` | `: 17: 16<|eot_id|>` |
| 41 | `4524499` | `:<|eot_id|>` |
| 43 | `3726327` | `: 16: 7<|eot_id|>` |
| 44 | `4009172` | `: 2<|eot_id|>` |
| 49 | `4240180` | `:354:180<|eot_id|>` |
| 51 | `9546409` | `:<|eot_id|>` |
| 52 | `2935113` | `: 29351113.<|eot_id|>` |
| 53 | `5453786` | `:354:678:90<|eot_id|>` |
| 57 | `8315831` | `: 5831<|eot_id|>` |
| 61 | `5960271` | `: 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,...<|eot_id|>` |
| 63 | `6049101` | `: 5 0 4 9 1 0 1<|eot_id|>` |
| 65 | `6406444` | `:361361361361361361361361361361361361361361361361361361361361361361361361361361...<|eot_id|>` |
| 67 | `2422633` | `:31<|eot_id|>` |
| 72 | `7442089` | ` 7953166<|eot_id|>` |
| 77 | `8795419` | `:<|eot_id|>` |
| 83 | `6363836` | `:  2<|eot_id|>` |

### niah_single_2 (23 errors)

| Index | 标准答案 | 当前答案 |
|-------|----------|----------|
| 16 | `2344047` | `: 23440447.<|eot_id|>` |
| 24 | `5449324` | `:<|eot_id|>` |
| 30 | `5727085` | `:<|eot_id|>` |
| 32 | `9196204` | `:<|eot_id|>` |
| 40 | `4524499` | `:460<|eot_id|>` |
| 41 | `7817881` | `:171.<|eot_id|>` |
| 42 | `3726327` | `:<|eot_id|>` |
| 50 | `9546409` | `:<|eot_id|>` |
| 51 | `2935113` | `: 3: 5113<|eot_id|>` |
| 52 | `5453786` | `:354<|eot_id|>` |
| 55 | `4188992` | `: 418899189418899, but it is not explicitly stated in the provided ...` |
| 58 | `6266630` | `:5963<|eot_id|>` |
| 60 | `5960271` | ` 0271<|eot_id|>` |
| 62 | `6049101` | `:<|eot_id|>` |
| 64 | `6406444` | `:<|eot_id|>` |
| 66 | `2422633` | `:5313<|eot_id|>` |
| 67 | `4940441` | `:5311<|eot_id|>` |
| 68 | `3472189` | `:361.<|eot_id|>` |
| 69 | `8971465` | `:361.<|eot_id|>` |
| 77 | `8963715` | `: 0 8 9 7 1 5<|eot_id|>` |
| 85 | `2044645` | `: 20446445.<|eot_id|>` |
| 91 | `7783308` | `:<|eot_id|>` |
| 93 | `1454696` | `:<|eot_id|>` |

### niah_single_3 (8 errors)

| Index | 标准答案 | 当前答案 |
|-------|----------|----------|
| 7 | `ee87905e-4ca4-45ea-8dfa-6a56d12dbc9a` | `: 2010-07-01T00:00:00Z<|eot_id|>` |
| 9 | `b7b56ea7-35eb-432d-9ad6-20ab48212ddb` | `:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0:0<|eot_id|>` |
| 14 | `e767dcea-b0e6-4969-a213-42b0f1eedba3` | `:0e6-4969-a213-42b0f1eedba3<|eot_id|>` |
| 24 | `59e4b671-4774-4c58-85f8-bc16f7860b50` | `:4774:4c58:85f8:bc16f7860b50<|eot_id|>` |
| 25 | `54c63cd8-8945-4f27-97fa-2d8dfb2ca025` | `: 54c63c63cd8-8945-4f27-97fa-2d8dfb2ca025.<|eot_id|>` |
| 29 | `006ed6e3-6fa1-4735-b572-f3d00b5cea6a` | `:6e3-6fa1-4735-b572-f3d00b5cea6a<|eot_id|>` |
| 31 | `e6697833-b841-40a0-9fe7-71d6d9178793` | `: e6697837837833-b841-40a0-9fe7-71d6d9178793.<|eot_id|>` |
| 43 | `d92c9227-eadf-4085-bfcb-75468eb22579` | `: d92c922c9227-eadf-4085-bfcb-75468eb22579.<|eot_id|>` |

### niah_multikey_1 (16 errors)

| Index | 标准答案 | 当前答案 |
|-------|----------|----------|
| 20 | `2171218` | `: 2171212181212181212181218<|eot_id|>` |
| 31 | `9333700` | `:<|eot_id|>` |
| 32 | `7121355` | `:9651<|eot_id|>` |
| 40 | `3112652` | `:285<|eot_id|>` |
| 41 | `3427461` | `:<|eot_id|>` |
| 45 | `8217547` | `:<|eot_id|>` |
| 51 | `1514340` | `: 1514343403361.<|eot_id|>` |
| 54 | `8212753` | `:<|eot_id|>` |
| 59 | `6587964` | `:<|eot_id|>` |
| 63 | `1688246` | `:<|eot_id|>` |
| 64 | `8344365` | `: 834436, but it is not explicitly mentioned.<|eot_id|>` |
| 65 | `6614484` | `: 4367.<|eot_id|>` |
| 67 | `6510922` | `:7780<|eot_id|>` |
| 69 | `6649968` | `: 43610.<|eot_id|>` |
| 71 | `9437374` | `:<|eot_id|>` |
| 74 | `6625238` | `:1472908<|eot_id|>` |

### niah_multikey_2 (30 errors)

| Index | 标准答案 | 当前答案 |
|-------|----------|----------|
| 2 | `1535573` | `: 8651665.<|eot_id|>` |
| 13 | `2794159` | `: 5261593<|eot_id|>` |
| 21 | `8970232` | `:168<|eot_id|>` |
| 22 | `9134051` | `: 381:055: 381:055: 381:055: 381:055: 381:055: 381:055: 381:055: 38...` |
| 23 | `9696620` | `: 969662620969662, which is: 969662920, 96966220 is not actually me...` |
| 24 | `7071187` | ` 055055055.<|eot_id|>` |
| 25 | `5572782` | `: 5342494<|eot_id|>` |
| 28 | `4953027` | `:1687719<|eot_id|>` |
| 32 | `4259234` | `: 425923521250, but not found is: 425923751572250, however is: 4259...` |
| 34 | `3643022` | `: 3957500<|eot_id|>` |
| 38 | `2031469` | `: the text.<|eot_id|>` |
| 39 | `8740362` | `: 8740364 8740364 8740364 8740364 is:  is:  is:  is: 874036...` |
| 40 | `7041770` | `:1682<|eot_id|>` |
| 41 | `1986258` | `:086.<|eot_id|>` |
| 42 | `5668574` | `:055.<|eot_id|>` |
| 43 | `8560471` | `:067<|eot_id|>` |
| 45 | `9973767` | `: 8420273<|eot_id|>` |
| 46 | `3960211` | `:0<|eot_id|>` |
| 47 | `8003271` | `: 60870870870870870870870870870870870870870870870870870870870870870...` |
| 49 | `8632309` | ` 303640 is640 640 640 640 640 640 640 640 640 640 640 640 640 640 640 640 640 640 640 640 640 640 640 6...` |
| 50 | `2318630` | `: 7780552.<|eot_id|>` |
| 53 | `3405052` | `:<|eot_id|>` |
| 54 | `5364945` | `: 536494, which is: 536494, which is: 536494494494494494494494494494494494494494...` |
| 56 | `7319214` | `:7607607607607607607607607607607607607607607607607607607607607607607607607607607...` |
| 57 | `9206104` | `:7607607607607607607607607607607607607607607607607607607607607607607607607607607607607607607...` |
| 59 | `9555385` | `:7095<|eot_id|>` |
| 60 | `5727554` | `: 572755755755755755755755755755755755755755755755755755755755 is: 572...` |
| 63 | `1090767` | `:7607607607607607607607607607607607607607607607607607607607607607607607607607607607607607...` |
| 64 | `6791240` | `:<|eot_id|>` |
| 65 | `7275999` | `:7607607607607607607607607607607607607607607607607607607607607607607607607607607607607...` |

### niah_multikey_3 (24 errors)

| Index | 标准答案 | 当前答案 |
|-------|----------|----------|
| 11 | `c73ed342-6523-4d4b-aa33-beb1c9007315` | `: 1d28b88b-b6a8-46ba-8e8f-56cbafbfd897.<|eot_id|>` |
| 18 | `87b8a762-1d1f-4e85-a5d1-caf284c95aa6` | `: 429a6676-5295-4ea2-a694-6aa949f48e31.<|eot_id|>` |
| 20 | `cce29702-134a-460c-979b-6f7ee7895280` | `:<|eot_id|>` |
| 23 | `ed344bfe-983f-4a21-af44-722e2517244c` | `: aec431e7d880a8dce2c023de24 is: aec43163-061a-4afe-b80a-f5bfb5e3c9...` |
| 24 | `4712ef99-a8d1-4388-8ca7-b08dd3505d77` | `:<|eot_id|>` |
| 25 | `46969ce7-0da0-49f8-87b2-845e7b8ef100` | `:<|eot_id|>` |
| 26 | `7cff3c66-6860-49e6-8ba5-002162c250c0` | `:4c7e-946b-30812edf965e<|eot_id|>` |
| 27 | `b63b4988-40bc-44b2-bf1c-ca95adbca4e9` | `:<|eot_id|>` |
| 29 | `6d94011c-f28a-4b0b-a2e2-fe34bb8b19a1` | `: 6d6d6d6d4b0e-52ce-44d9-a0f6-1ae405825615<|eot_id|>` |
| 30 | `7c33bb00-4ab4-4e4f-a78e-39f8f06d63eb` | `  d7a2-4b23-a2c0-8c859cb1fa96<|eot_id|>` |
| 33 | `b7c6b586-713a-4907-ad24-5c4f25aeb769` | `:1-4d2c-b42b-933ded2633d6<|eot_id|>` |
| 35 | `ac8a317b-a6bb-4327-90db-2a01622cb723` | `: d2f2f2f2f2f2f2f2d2d2f2d2d2d3d2f6b3d2f- is: d2dab is:  is:  is:  i...` |
| 37 | `b187b337-3132-4376-a500-9340102092ae` | `:<|eot_id|>` |
| 40 | `2559fa56-dd0a-48d4-ba82-3ae2bf0a4b33` | `:358fe0e3-724e-4cfc-9ae0-d0873162626b.<|eot_id|>` |
| 41 | `7842feb5-e758-44cd-b73b-8ae08aa33142` | `: 6c6adf83-36a9-4e41-9cbe-60a8c9ffba92.<|eot_id|>` |
| 42 | `a1196139-f6fa-4c18-b3da-b7bd50362ac7` | `: a1196131396131196131399a1196139a1196139a1196139a1196139f6a1196139...` |
| 44 | `7d3d40b2-4594-4573-b267-4c6270dd4425` | `:  613a9e-4e7d-8c9f-740a630e3c53<|eot_id|>` |
| 45 | `500b8a75-8f05-43f5-b9ad-46d47d4e33fc` | `: 500b8a5e0e0e0a500b is: 500b is: 500b-4 is:  is:  is:  is:  is:  i...` |
| 46 | `86a867a7-6a98-4a02-b065-70a33bafafde` | `:6139a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a9a...` |
| 47 | `7c0f7fd2-237e-4c0f-b3f5-f43623551169` | ` 5fb71d2f0f0b4f0 is: 5fb71 is: 5fb71f-4f-4f-4f-4f-4f-4d7 is:  is:  ...` |
| 48 | `b0e1f3f5-6570-437e-b8a1-f1b3f654e257` | `: 500b 500b 500b 500b 500b 500b 500b 500b 500b 500b 500b 500b 500b ...` |
| 49 | `0153722a-70a8-4ec0-9f03-2b0930937e60` | `: 500b 500b 500b 500b 500b 500b 500b 500b 500b 500b 500b ...` |
| 50 | `0a1ead51-0c39-4eeb-ac87-d146acdb1d4a` | `: 500b 500b 500b 500b 500b 500b 500b 500b 500b 500b 500b 500b ...` |
| 52 | `ff686e85-3a9f-4635-95dd-f19e8ca68eb1` | ` ff686e686e686e686e686e686f686e6f686e6fb686f686f686f686f686f- is: f...` |

---

## Multikey 任务失败分析 (单样本测试)

### 失败样本特征

单样本测试中 multikey 任务的失败**不是**状态泄露，而是**模型检索能力问题**。

#### 错误类型

| 类型 | 示例 | 说明 |
|------|------|------|
| **检索错误 key** | Expected `5833597`, Got `8617381` | 返回了上下文中另一个 key 的 value |
| **UUID 检索错误** | Expected `c73ed342-...`, Got `1d28b88b-...` | 返回了错误 key 对应的 UUID |

#### multikey_2 失败样本详情 (单样本测试)

| Sample | Expected | Got | 分析 |
|--------|----------|-----|------|
| 2 | `1535573` | `8651665` | 错误 key |
| 12 | `4641400` | `9390530` | 错误 key |
| 19 | `8591874` | `3853628` | 错误 key |
| 50 | `2318630` | `7780552` | 错误 key |
| 66 | `1926587` | `9249734` | 错误 key |
| 85 | `1253265` | `3263480` | 错误 key |
| 86 | `7772887` | `3762547` | 错误 key |
| 89 | `2266721` | `5873220` | 错误 key |
| 98 | (未记录) | (未记录) | - |

#### multikey_3 失败样本详情 (单样本测试)

| Sample | Expected | Got | 分析 |
|--------|----------|-----|------|
| 11 | `c73ed342-6523-...` | `1d28b88b-b6a8-...` | 错误 key 的 UUID |
| 18 | `87b8a762-1d1f-...` | `429a6676-5295-...` | 错误 key 的 UUID |
| 23 | `ed344bfe-983f-...` | `aec43163-061a-...` | 错误 key 的 UUID |
| 35 | `ac8a317b-a6bb-...` | `d2f22889-5b72-...` | 错误 key 的 UUID |
| 41 | `7842feb5-e758-...` | `fc8e724e-418d-...` | 错误 key 的 UUID |
| 47 | `7c0f7fd2-237e-...` | `5fb71d15-4675-...` | 错误 key 的 UUID |
| 53 | `bccd56fa-8fba-...` | `373cc0cc-6ab7-...` | 错误 key 的 UUID |
| 86 | `68c49603-1d17-...` | `aef58e2e-9e99-...` | 错误 key 的 UUID |
| 93 | `74651292-5664-...` | `4546dd56-fe88-...` | 错误 key 的 UUID |

### 关键发现

1. **格式正确**: 失败样本的输出格式完全正确（7位数字或UUID）
2. **合法 value**: 输出的是上下文中存在的另一个 key-value 对的 value
3. **确定性失败**: 同一样本多次测试返回相同的错误值
4. **模型能力边界**: 这是多 key 检索任务的模型能力上限，~91% 准确率符合预期

---

## Comparison with Working Baseline

### xattn_stride8 (Working)
- **Branch**: `tzj/vs_offload` or earlier
- **Method**: XAttention sparse pattern with stride 8
- **Error Rate**: ~8% (expected RULER baseline)
- **Samples**: 100 samples per task

### Chunked Offload - 批量测试 (Broken)
- **Branch**: `tzj/minference`
- **Method**: Full attention with chunked CPU offload
- **Error Rate**: 20% (120/600) - **状态泄露导致**
- **Samples**: 100 samples per task

### Chunked Offload - 单样本测试 (Working)
- **Branch**: `tzj/minference`
- **Method**: Full attention with chunked CPU offload, 每个请求重新初始化 LLM
- **Error Rate**: 0% (niah_single_1), ~9% (multikey tasks)
- **Samples**: 100 samples per task
- **结论**: 算法正确，multikey 失败是模型能力问题

---

## Next Steps (Updated)

### 已完成 ✅

1. ~~**Reproduce with 4K context**~~ - 不再需要，算法已验证正确
2. ~~**Vary chunk size**~~ - 不再需要，问题不在 chunk 大小
3. ~~**4-slot 配置测试**~~ - 已完成，有改善但不是根本原因

### 待完成 🔧

1. **定位状态泄露组件**: 调查连续请求间哪些状态未正确重置
   - KV cache manager 的 `reset()` 或 `clear()` 方法
   - Offload engine 的 ring buffer slot 状态
   - Decode buffer 的跨请求隔离
   - Sparse policy 的内部状态

2. **实现状态重置修复**: 在每个请求完成后正确清理所有状态

3. **验证修复**: 使用批量测试验证修复后准确率恢复到 ~95%+

4. **Add tensor checkpoints**: Log intermediate attention outputs at chunk boundaries

5. **Compare with non-offload**: Test 32K with GPU-only mode (if memory permits)

6. **Numerical stability**: Add clipping/normalization to online softmax accumulation

---

## Related Documents

- [`architecture_guide.md`](architecture_guide.md) - Chunked attention design
- [`known_issues.md`](known_issues.md) - Previously fixed bugs
- [`ruler_benchmark_results_32k.md`](ruler_benchmark_results_32k.md) - Previous working results

---

**Author**: Zijie Tian
**Reported**: 2026-01-18
**Last Updated**: 2026-01-20 (4-slot test results added)