2nd/10_Wiki/Topics/Other/Inference-Coupled Persistence.md

---
id: wiki-2026-0508-inference-coupled-persistence
title: Inference-Coupled Persistence
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [ICP, Inference-Time Memory, Coupled Persistence]
duplicate_of: none
source_trust_level: A
confidence_score: 0.85
verification_status: applied
tags: [llm, memory, inference, kv-cache, persistence]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: python
  framework: vllm
---

# Inference-Coupled Persistence

## 매 한 줄
> **"매 KV-cache 의 disk 의 spill 매 conversation 의 resume"**. Inference-Coupled Persistence (ICP) 매 LLM serving system 의 inference state (KV-cache, attention states) 의 durable storage 의 couple 매 pattern. 2026 vLLM 0.7+ / SGLang 매 native support — long conversations cost-effective.

## 매 핵심

### 매 Why ICP
- 매 1M token context 의 KV-cache 매 ~100GB GPU memory 의 consume.
- 매 conversation idle 매 hours / days — GPU memory 매 hold cost-prohibitive.
- ICP: idle 시 disk 의 evict, resume 시 reload — 매 5-50x cost reduction.

### 매 Storage tiers
- **L0 (HBM)**: active inference, < 1ms access.
- **L1 (CPU RAM)**: 매 minutes idle, ~10ms reload.
- **L2 (NVMe)**: 매 hours idle, ~100ms reload.
- **L3 (Object store / S3)**: 매 days idle, ~1-5s reload.

### 매 Coupling guarantees
- **Bit-exact resume**: 매 KV-cache 매 quantization-aware serialization.
- **Causal consistency**: 매 token N 의 KV 매 strictly token <N 의 reflect.
- **Atomic checkpoint**: partial-write 의 detect 의 crash recovery.

### 매 응용
1. Long-running coding agent (multi-day session).
2. Customer support bot (hours-long conversation history).
3. Research assistant (multi-week project context).
4. Multi-tenant LLM serving (100K concurrent idle sessions).

## 💻 패턴

### Pattern 1: vLLM KV-cache offload (2026)
```python
from vllm import LLM, SamplingParams
from vllm.engine.arg_utils import EngineArgs

engine_args = EngineArgs(
    model="meta-llama/Llama-3.3-70B-Instruct",
    enable_prefix_caching=True,
    kv_cache_dtype="fp8",
    cpu_offload_gb=200,         # CPU RAM tier
    swap_space=400,              # NVMe tier (GB)
    block_size=32,
)
llm = LLM.from_engine_args(engine_args)
```

### Pattern 2: Conversation checkpoint
```python
import torch
from pathlib import Path

class ConversationCheckpoint:
    def __init__(self, store_dir: Path):
        self.store = store_dir
        self.store.mkdir(exist_ok=True, parents=True)

    def save(self, conv_id: str, kv_blocks: list[torch.Tensor], tokens: list[int]):
        path = self.store / f"{conv_id}.pt"
        tmp = path.with_suffix(".tmp")
        torch.save({
            "kv": [b.cpu() for b in kv_blocks],
            "tokens": tokens,
            "version": 2,
        }, tmp)
        tmp.rename(path)  # atomic

    def load(self, conv_id: str) -> dict | None:
        path = self.store / f"{conv_id}.pt"
        if not path.exists():
            return None
        return torch.load(path, map_location="cuda")
```

### Pattern 3: Tiered eviction policy
```python
from dataclasses import dataclass
from time import time

@dataclass
class Session:
    id: str
    last_access: float
    size_gb: float

def evict_tier(sessions: list[Session], capacity_gb: float) -> list[Session]:
    """매 LRU 의 evict — return list of (session, target_tier)."""
    sessions.sort(key=lambda s: s.last_access)
    used = sum(s.size_gb for s in sessions)
    evicted = []
    now = time()
    for s in sessions:
        if used <= capacity_gb:
            break
        idle_min = (now - s.last_access) / 60
        if idle_min < 5:
            target = "HBM"
        elif idle_min < 60:
            target = "CPU"
        elif idle_min < 1440:
            target = "NVMe"
        else:
            target = "S3"
        evicted.append((s, target))
        used -= s.size_gb
    return evicted
```

### Pattern 4: Resume with prefix matching
```python
def resume_with_prefix(checkpoint: dict, new_prompt: str, tokenizer) -> tuple[list, list]:
    """매 checkpoint 의 prefix 의 reuse — 매 prefix mismatch 의 from-scratch."""
    saved_tokens = checkpoint["tokens"]
    new_tokens = tokenizer.encode(new_prompt)
    common = 0
    for i in range(min(len(saved_tokens), len(new_tokens))):
        if saved_tokens[i] != new_tokens[i]:
            break
        common = i + 1
    if common == 0:
        return [], new_tokens
    kept_kv = [k[:, :common] for k in checkpoint["kv"]]
    return kept_kv, new_tokens[common:]
```

### Pattern 5: Quantized serialization
```python
def serialize_kv_int8(kv: torch.Tensor) -> tuple[bytes, dict]:
    """매 fp16 KV 의 int8 의 quantize — 매 50% storage save."""
    scale = kv.abs().amax() / 127
    q = (kv / scale).round().clamp(-128, 127).to(torch.int8)
    return q.numpy().tobytes(), {"scale": scale.item(), "shape": list(q.shape)}

def deserialize_kv_int8(data: bytes, meta: dict) -> torch.Tensor:
    import numpy as np
    arr = np.frombuffer(data, dtype=np.int8).reshape(meta["shape"])
    return torch.from_numpy(arr).to(torch.float16) * meta["scale"]
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Conversation < 5min idle | HBM 만. |
| Long conversation, hours idle | NVMe tier. |
| Multi-day project context | S3 + prefix cache. |
| Cost-sensitive multi-tenant | Aggressive 4-tier ICP. |
| Latency-sensitive (< 10ms) | HBM only — ICP 의 X. |

**기본값**: 4-tier (HBM → CPU → NVMe → S3) 매 LRU eviction, fp8 KV-cache, prefix caching enabled.

## 🔗 Graph
- 부모: [[KV-Cache]]
- 변형: [[Prefix-Caching]] · [[Paged-Attention]]
- 응용: [[LLM_Optimization_and_Deployment_Strategies|vLLM]]
- Adjacent: [[Continuous-Batching]] · [[FlashAttention]]

## 🤖 LLM 활용
**언제**: 매 production LLM serving 매 multi-hour conversations, 매 cost optimization, 매 multi-tenant 100K+ sessions.
**언제 X**: Single-shot inference (no persistence needed), strict-latency RT systems (< 10ms first-token).

## ❌ 안티패턴
- **Naive pickle of KV**: 매 quantization-unaware — 5-10x bigger than needed.
- **No atomic write**: crash 의 corrupted checkpoint 의 unrecoverable.
- **Per-token checkpoint**: 매 IOPS storm — batch 의 N tokens.
- **Resume without prefix check**: silent correctness bug.

## 🧪 검증 / 중복
- Verified: vLLM 0.7 docs (2025), SGLang RadixAttention paper (2024), Mooncake architecture (2024).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — full content with vLLM 2026 patterns, tiered eviction, quantized serialization |