📈 feat: add MemoryObserver for GPU-CPU communication tracking

Implement MemoryObserver to track memory transfers between GPU and CPU: - H2D (Host to Device): CPU → GPU transfers - D2H (Device to Host): GPU → CPU transfers - D2D (Device to Device): GPU buffer copies - Supports prefill/decode phase separation Integration points in offload_engine.py: - load_to_slot_layer: H2D with is_prefill parameter - offload_slot_layer_to_cpu, offload_prefill_buffer_async: D2H - write_to_prefill_buffer, write_to_decode_buffer: D2D - load_block_sample_from_cpu, load_block_full_from_cpu: H2D Add bench_offload.py integration for memory stats printing. Benchmark results (Llama-3.1-8B, 64K context): - Full Policy: Prefill H2D 262.13 GB - XAttention: Prefill H2D 386.62 GB (1.48x) Generated with [Claude Code](https://claude.ai/code) via [Happy](https://happy.engineering) Co-Authored-By: Claude <noreply@anthropic.com> Co-Authored-By: Happy <yesreply@happy.engineering>
♻️ refactor: restructure Observer as base class with InferenceObserver
2026-01-28 04:06:45 +08:00 · 2026-01-28 03:15:33 +08:00
11 changed files with 614 additions and 40 deletions
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -31,6 +31,8 @@ Nano-vLLM is a lightweight vLLM implementation (~1,200 lines) for fast offline L
 | [`docs/cpu_offload_optimization_strategies.md`](docs/cpu_offload_optimization_strategies.md) | 🚀 OPT: CPU offload 优化策略：chunk size、CUDA Graph、前沿研究(InfiniGen/ShadowKV) |
 | [`docs/gpu_only_xattn_guide.md`](docs/gpu_only_xattn_guide.md) | 🚀 GPU-Only XAttention: 内存预分配、性能分析 (32K +15%, 64K +41%)、CUDA Graph 限制 |
 | [`docs/xattn_performance_analysis.md`](docs/xattn_performance_analysis.md) | 📊 XAttention 性能分析: NVTX 标记、block size 影响、estimate vs compute 耗时对比 |
+| [`docs/observer_architecture.md`](docs/observer_architecture.md) | 📊 Observer 架构: InferenceObserver (TTFT/TPOT)、MemoryObserver (H2D/D2H/D2D) 设计 |
+| [`docs/memory_communication_benchmark.md`](docs/memory_communication_benchmark.md) | 📊 通信量测试: Full vs XAttention 通信量对比 (32K/64K)、阶段分离统计 |

 ## Rules Index

--- a/bench.py
+++ b/bench.py
@@ -2,6 +2,7 @@ import os
 import time
 from random import randint, seed
 from nanovllm import LLM, SamplingParams
+from nanovllm.utils.observer import InferenceObserver


 def bench_decode(llm, num_seqs, input_len, output_len):
@@ -14,13 +15,17 @@ def bench_decode(llm, num_seqs, input_len, output_len):
    llm.generate(prompt_token_ids, sampling_params, use_tqdm=False)
    t = time.time() - t

-    # Calculate metrics
-    prefill_tokens = num_seqs * input_len
+    # Get metrics from InferenceObserver
+    ttft_ms = InferenceObserver.ttft / 1e6
+    tpot_ms = InferenceObserver.tpot / 1e6
+
+    # Calculate throughput from observer metrics
    decode_tokens = num_seqs * output_len
-    decode_throughput = decode_tokens / t
+    decode_throughput = 1000.0 / tpot_ms if tpot_ms > 0 else 0  # tokens/s per sequence

    print(f"[Decode] Input: {num_seqs}x{input_len}tok, Output: {decode_tokens}tok, Time: {t:.2f}s")
-    print(f"         Throughput: {decode_throughput:.2f} tok/s (includes prefill overhead)")
+    print(f"         TTFT: {ttft_ms:.2f}ms, TPOT: {tpot_ms:.2f}ms")
+    print(f"         Decode Throughput: {decode_throughput:.2f} tok/s (from observer)")


 def bench_prefill(llm, num_seqs, input_len):
@@ -33,9 +38,19 @@ def bench_prefill(llm, num_seqs, input_len):
    t = time.time()
    llm.generate(prompt_token_ids, sampling_params, use_tqdm=False)
    t = time.time() - t
+
+    # Get TTFT from InferenceObserver
+    ttft_ms = InferenceObserver.ttft / 1e6
+    ttft_s = ttft_ms / 1000.0
+
    total_input_tokens = num_seqs * input_len
-    throughput = total_input_tokens / t
-    print(f"[Prefill] Input: {total_input_tokens}tok ({num_seqs}x{input_len}), Time: {t:.2f}s, Throughput: {throughput:.2f}tok/s")
+    # Use observer TTFT for accurate prefill throughput
+    throughput_observer = total_input_tokens / ttft_s if ttft_s > 0 else 0
+    throughput_external = total_input_tokens / t
+
+    print(f"[Prefill] Input: {total_input_tokens}tok ({num_seqs}x{input_len})")
+    print(f"          External Time: {t:.2f}s, Throughput: {throughput_external:.2f}tok/s")
+    print(f"          Observer TTFT: {ttft_ms:.2f}ms, Throughput: {throughput_observer:.2f}tok/s")


 def main():
--- a/bench_offload.py
+++ b/bench_offload.py
@@ -2,6 +2,15 @@ import os
 import time
 from random import randint, seed
 from nanovllm import LLM, SamplingParams
+from nanovllm.utils.observer import InferenceObserver
+from nanovllm.utils.memory_observer import MemoryObserver
+
+
+def print_memory_stats():
+    """Print MemoryObserver communication statistics"""
+    fmt = MemoryObserver._fmt_bytes
+    print(f"[Memory] Prefill H2D: {fmt(MemoryObserver.prefill_h2d_bytes)}, D2H: {fmt(MemoryObserver.prefill_d2h_bytes)}")
+    print(f"         Decode  H2D: {fmt(MemoryObserver.decode_h2d_bytes)}, D2H: {fmt(MemoryObserver.decode_d2h_bytes)}")


 def bench_decode(llm, num_seqs, input_len, output_len):
@@ -14,16 +23,18 @@ def bench_decode(llm, num_seqs, input_len, output_len):
    llm.generate(prompt_token_ids, sampling_params, use_tqdm=False)
    t = time.time() - t

-    # Calculate metrics
-    prefill_tokens = num_seqs * input_len
-    decode_tokens = num_seqs * output_len
+    # Get metrics from InferenceObserver
+    ttft_ms = InferenceObserver.ttft / 1e6
+    tpot_ms = InferenceObserver.tpot / 1e6

-    # Approximate: assume prefill takes ~input_len/prefill_speed, rest is decode
-    # For more accurate measurement, we'd need internal timing
-    decode_throughput = decode_tokens / t  # This includes prefill time, so it's a lower bound
+    # Calculate throughput from observer metrics
+    decode_tokens = num_seqs * output_len
+    decode_throughput = 1000.0 / tpot_ms if tpot_ms > 0 else 0  # tokens/s per sequence

    print(f"[Decode] Input: {num_seqs}x{input_len}tok, Output: {decode_tokens}tok, Time: {t:.2f}s")
-    print(f"         Throughput: {decode_throughput:.2f} tok/s (includes prefill overhead)")
+    print(f"         TTFT: {ttft_ms:.2f}ms, TPOT: {tpot_ms:.2f}ms")
+    print(f"         Decode Throughput: {decode_throughput:.2f} tok/s (from observer)")
+    print_memory_stats()


 def bench_prefill(llm, num_seqs, input_len):
@@ -36,9 +47,20 @@ def bench_prefill(llm, num_seqs, input_len):
    t = time.time()
    llm.generate(prompt_token_ids, sampling_params, use_tqdm=False)
    t = time.time() - t
+
+    # Get TTFT from InferenceObserver
+    ttft_ms = InferenceObserver.ttft / 1e6
+    ttft_s = ttft_ms / 1000.0
+
    total_input_tokens = num_seqs * input_len
-    throughput = total_input_tokens / t
-    print(f"[Prefill] Input: {total_input_tokens}tok ({num_seqs}x{input_len}), Time: {t:.2f}s, Throughput: {throughput:.2f}tok/s")
+    # Use observer TTFT for accurate prefill throughput
+    throughput_observer = total_input_tokens / ttft_s if ttft_s > 0 else 0
+    throughput_external = total_input_tokens / t
+
+    print(f"[Prefill] Input: {total_input_tokens}tok ({num_seqs}x{input_len})")
+    print(f"          External Time: {t:.2f}s, Throughput: {throughput_external:.2f}tok/s")
+    print(f"          Observer TTFT: {ttft_ms:.2f}ms, Throughput: {throughput_observer:.2f}tok/s")
+    print_memory_stats()


 def main():
@@ -76,6 +98,9 @@ def main():
    path = os.path.expanduser(args.model)
    max_len = args.max_len

+    # Enable MemoryObserver for communication stats
+    MemoryObserver._enabled = True
+
    # Setup policy configuration
    if args.enable_quest:
        sparse_policy = SparsePolicyType.QUEST
--- a/docs/memory_communication_benchmark.md
+++ b/docs/memory_communication_benchmark.md
@@ -0,0 +1,82 @@
+# Memory Communication Benchmark
+
+GPU-CPU 通信量测试结果，对比 Full Policy 和 XAttention BSA Policy。
+
+## 测试环境
+
+- **模型**: Llama-3.1-8B-Instruct
+- **GPU**: RTX 3090 (24GB)
+- **配置**: `num_gpu_blocks=4`, `block_size=1024`, `enable_cpu_offload=True`
+- **XAttention 参数**: `threshold=0.95`, `stride=8`
+
+## 32K 上下文测试结果
+
+| 指标 | Full Policy | XAttention | 比率 |
+|------|-------------|------------|------|
+| **Prefill H2D** | 66.57 GB | 111.12 GB | **1.67x** |
+| Prefill D2H | 4.29 GB | 4.29 GB | 1.00x |
+| TTFT | 8473 ms | 10367 ms | 1.22x |
+
+### XAttention Block Selection (32K)
+
+| 指标 | 数值 |
+|------|------|
+| 可用 blocks | 465 |
+| 选中 blocks | 374 |
+| 选择密度 | 80.4% |
+
+## 64K 上下文测试结果
+
+| 指标 | Full Policy | XAttention | 比率 |
+|------|-------------|------------|------|
+| **Prefill H2D** | 262.13 GB | 386.62 GB | **1.48x** |
+| Prefill D2H | 8.46 GB | 8.46 GB | 1.00x |
+| Decode H2D (32 tokens) | 262.13 GB | 262.13 GB | 1.00x |
+| TTFT | 27081 ms | 33634 ms | 1.24x |
+
+## 通信量比率对比
+
+| 上下文长度 | XAttn/Full Prefill H2D 比率 |
+|------------|----------------------------|
+| 32K | 1.67x |
+| 64K | 1.48x |
+
+### 分析
+
+1. **XAttention 通信量增加原因**：
+   - Estimate 阶段：加载 **100%** 历史 blocks（用于 attention score 估计）
+   - Compute 阶段：加载 **选中的** blocks（约 70-80%）
+   - 理论比率：`1 + selection_density`
+
+2. **64K 比率更低的原因**：
+   - 更长上下文时，attention 分布更稀疏
+   - XAttention 的 block 选择更有效（选中比例更低）
+   - First/last block 强制包含的影响相对减小
+
+3. **Decode 阶段通信量相同**：
+   - XAttention 仅支持 prefill 阶段
+   - Decode 阶段 fallback 到 Full Policy
+
+## 测试命令
+
+```bash
+# 32K Full Policy
+python bench_offload.py --max-len 32768 --input-len 32000
+
+# 32K XAttention
+python bench_offload.py --max-len 32768 --input-len 32000 --enable-xattn
+
+# 64K Full Policy
+python bench_offload.py --max-len 65536 --input-len 64000
+
+# 64K XAttention
+python bench_offload.py --max-len 65536 --input-len 64000 --enable-xattn
+
+# 包含 decode 测试
+python bench_offload.py --max-len 65536 --input-len 64000 --bench-decode --output-len 32
+```
+
+## 相关文档
+
+- [`observer_architecture.md`](observer_architecture.md) - Observer 架构设计
+- [`xattn_bsa_policy_design.md`](xattn_bsa_policy_design.md) - XAttention BSA 算法设计
--- a/docs/observer_architecture.md
+++ b/docs/observer_architecture.md
@@ -0,0 +1,194 @@
+# Observer Architecture
+
+nanovllm 的 Observer 架构用于统计推理过程中的关键指标，采用类变量（class variable）模式实现全局状态管理。
+
+## 架构概览
+
+```
+Observer (基类)
+├── InferenceObserver - 推理时间指标 (TTFT, TPOT)
+└── MemoryObserver - 内存传输统计 (H2D, D2H, D2D)
+```
+
+## 设计原则
+
+### 1. 类变量模式
+
+所有 Observer 使用类变量（而非实例变量）存储状态：
+
+```python
+class Observer:
+    """Observer 基类"""
+    _enabled: bool = True  # 类变量，控制是否启用
+
+class InferenceObserver(Observer):
+    ttft: int = 0          # 类变量，全局共享
+    tpot: int = 0
+    ttft_start: int = 0
+    tpot_start: int = 0
+```
+
+**优点**：
+- 无需实例化，任何地方都可以直接访问
+- 避免跨模块传递 observer 实例
+- 适合全局统计场景
+
+### 2. 启用/禁用控制
+
+每个 Observer 可独立启用/禁用：
+
+```python
+# 启用 MemoryObserver
+MemoryObserver._enabled = True
+
+# 禁用后，record_* 方法不会记录
+MemoryObserver._enabled = False
+```
+
+### 3. 阶段分离
+
+MemoryObserver 支持 prefill/decode 阶段分离统计：
+
+```python
+@classmethod
+def record_h2d(cls, num_bytes: int, is_prefill: bool = True) -> None:
+    if not cls._enabled:
+        return
+    cls.h2d_bytes += num_bytes
+    cls.h2d_count += 1
+    if is_prefill:
+        cls.prefill_h2d_bytes += num_bytes
+    else:
+        cls.decode_h2d_bytes += num_bytes
+```
+
+## Observer 实现
+
+### InferenceObserver
+
+**位置**: `nanovllm/utils/observer.py`
+
+**统计指标**：
+| 指标 | 说明 | 单位 |
+|------|------|------|
+| `ttft` | Time To First Token | 纳秒 |
+| `tpot` | Time Per Output Token | 纳秒 |
+| `ttft_start` | TTFT 计时开始点 | 纳秒 |
+| `tpot_start` | TPOT 计时开始点 | 纳秒 |
+
+**统计位置**：
+| 位置 | 代码 | 说明 |
+|------|------|------|
+| `scheduler.py:add()` | `InferenceObserver.ttft_start = perf_counter_ns()` | 开始计时 |
+| `llm_engine.py:step()` | `InferenceObserver.ttft = ... - ttft_start` | Prefill 完成后计算 TTFT |
+| `llm_engine.py:step()` | `InferenceObserver.tpot = ... - tpot_start` | Decode 时计算 TPOT |
+
+### MemoryObserver
+
+**位置**: `nanovllm/utils/memory_observer.py`
+
+**统计指标**：
+| 指标 | 说明 |
+|------|------|
+| `h2d_bytes` / `h2d_count` | Host to Device 传输量/次数 |
+| `d2h_bytes` / `d2h_count` | Device to Host 传输量/次数 |
+| `d2d_bytes` / `d2d_count` | Device to Device 复制量/次数 |
+| `prefill_h2d_bytes` / `prefill_d2h_bytes` | Prefill 阶段 H2D/D2H |
+| `decode_h2d_bytes` / `decode_d2h_bytes` | Decode 阶段 H2D/D2H |
+
+**统计位置** (均在 `offload_engine.py`)：
+
+| 方法 | 传输类型 | 说明 |
+|------|----------|------|
+| `load_to_slot_layer()` | H2D | 从 CPU 加载 block 到 GPU slot |
+| `load_block_sample_from_cpu()` | H2D | 采样加载（Quest） |
+| `load_block_full_from_cpu()` | H2D | 完整加载 block |
+| `offload_slot_layer_to_cpu()` | D2H | GPU slot 卸载到 CPU |
+| `offload_prefill_buffer_async()` | D2H | Prefill buffer 异步卸载 |
+| `write_to_prefill_buffer()` | D2D | 写入 prefill buffer |
+| `write_to_decode_buffer()` | D2D | 写入 decode buffer |
+
+**重置位置**：
+| 位置 | 代码 |
+|------|------|
+| `llm_engine.py:generate()` | `MemoryObserver.complete_reset()` |
+| `llm_engine.py:generate()` | `InferenceObserver.complete_reset()` |
+
+## 使用示例
+
+### 1. 启用并统计
+
+```python
+from nanovllm.utils.memory_observer import MemoryObserver
+
+# 启用统计
+MemoryObserver._enabled = True
+
+# 运行推理
+outputs = llm.generate(prompts, sampling_params)
+
+# 获取结果
+print(f"Prefill H2D: {MemoryObserver.prefill_h2d_bytes / 1e9:.2f} GB")
+print(f"Decode H2D: {MemoryObserver.decode_h2d_bytes / 1e9:.2f} GB")
+
+# 或使用 print_summary
+MemoryObserver.print_summary()
+```
+
+### 2. 在 bench_offload.py 中
+
+```python
+from nanovllm.utils.memory_observer import MemoryObserver
+
+# 启用
+MemoryObserver._enabled = True
+
+# benchmark 结束后打印
+def print_memory_stats():
+    fmt = MemoryObserver._fmt_bytes
+    print(f"[Memory] Prefill H2D: {fmt(MemoryObserver.prefill_h2d_bytes)}")
+    print(f"         Decode  H2D: {fmt(MemoryObserver.decode_h2d_bytes)}")
+```
+
+### 3. 获取结构化数据
+
+```python
+summary = MemoryObserver.get_summary()
+# {
+#     "total": {"h2d_bytes": ..., "d2h_bytes": ..., "d2d_bytes": ...},
+#     "prefill": {"h2d_bytes": ..., "d2h_bytes": ...},
+#     "decode": {"h2d_bytes": ..., "d2h_bytes": ...}
+# }
+```
+
+## 添加新 Observer
+
+1. 继承 `Observer` 基类
+2. 定义类变量存储统计数据
+3. 实现 `record_*` 方法（需检查 `_enabled`）
+4. 实现 `complete_reset()` 方法
+5. 在相关代码位置添加 `record_*` 调用
+6. 在 `llm_engine.py:generate()` 中添加 reset 调用
+
+```python
+from nanovllm.utils.observer import Observer
+
+class MyObserver(Observer):
+    _enabled: bool = False
+    my_metric: int = 0
+
+    @classmethod
+    def record_event(cls, value: int) -> None:
+        if not cls._enabled:
+            return
+        cls.my_metric += value
+
+    @classmethod
+    def complete_reset(cls) -> None:
+        cls.my_metric = 0
+```
+
+## 相关文档
+
+- [`memory_communication_benchmark.md`](memory_communication_benchmark.md) - 通信量测试结果
+- [`architecture_guide.md`](architecture_guide.md) - 整体架构指南
--- a/nanovllm/engine/llm_engine.py
+++ b/nanovllm/engine/llm_engine.py
@@ -10,7 +10,8 @@ from nanovllm.sampling_params import SamplingParams
 from nanovllm.engine.sequence import Sequence
 from nanovllm.engine.scheduler import Scheduler
 from nanovllm.engine.model_runner import ModelRunner
-from nanovllm.utils.observer import Observer
+from nanovllm.utils.observer import InferenceObserver
+from nanovllm.utils.memory_observer import MemoryObserver


 class LLMEngine:
@@ -58,15 +59,18 @@ class LLMEngine:
            print(f"[DEBUG LLMEngine.step] Mode={mode}, active_sequences={len(seqs)}")

        if not is_prefill:
-            # The end of the prefill mode. Get TTFT.
-            if Observer.ttft_start != 0:
-                Observer.ttft = perf_counter_ns() - Observer.ttft_start
-                Observer.reset_ttft()
-            # The start of the decode mode. Get TPOT.
-            if Observer.tpot_start != 0:
-                Observer.tpot = perf_counter_ns() - Observer.tpot_start
-            Observer.tpot_start = perf_counter_ns()
+            # Decode mode: calculate TPOT from previous decode step
+            if InferenceObserver.tpot_start != 0:
+                InferenceObserver.tpot = perf_counter_ns() - InferenceObserver.tpot_start
+            InferenceObserver.tpot_start = perf_counter_ns()
+
        token_ids = self.model_runner.call("run", seqs, is_prefill)
+
+        if is_prefill:
+            # Calculate TTFT after prefill completes (including chunked prefill)
+            if InferenceObserver.ttft_start != 0:
+                InferenceObserver.ttft = perf_counter_ns() - InferenceObserver.ttft_start
+                InferenceObserver.reset_ttft()
        self.scheduler.postprocess(seqs, token_ids)
        outputs = [(seq.seq_id, seq.completion_token_ids) for seq in seqs if seq.is_finished]

@@ -91,7 +95,8 @@ class LLMEngine:
        log_level = os.environ.get('NANOVLLM_LOG_LEVEL', 'INFO')
        debug_enabled = log_level.upper() == 'DEBUG'

-        Observer.complete_reset()
+        InferenceObserver.complete_reset()
+        MemoryObserver.complete_reset()
        if use_tqdm:
            pbar = tqdm(total=len(prompts), desc="Generating", dynamic_ncols=True)
        if not isinstance(sampling_params, list):
@@ -128,8 +133,8 @@ class LLMEngine:
                pbar.set_postfix({
                    "Prefill": f"{int(prefill_throughput)}tok/s",
                    "Decode": f"{int(decode_throughput)}tok/s",
-                    "ttft": f"{float(Observer.ttft) / 1e6}ms",
-                    "tpot": f"{float(Observer.tpot) / 1e6}ms",
+                    "ttft": f"{float(InferenceObserver.ttft) / 1e6}ms",
+                    "tpot": f"{float(InferenceObserver.tpot) / 1e6}ms",
                })
            for seq_id, token_ids in output:
                outputs[seq_id] = token_ids
--- a/nanovllm/engine/scheduler.py
+++ b/nanovllm/engine/scheduler.py
@@ -4,7 +4,7 @@ from typing import TYPE_CHECKING

 from nanovllm.config import Config
 from nanovllm.engine.sequence import Sequence, SequenceStatus
-from nanovllm.utils.observer import Observer
+from nanovllm.utils.observer import InferenceObserver

 if TYPE_CHECKING:
    from nanovllm.kvcache import KVCacheManager
@@ -32,8 +32,8 @@ class Scheduler:
        num_seqs = 0
        num_batched_tokens = 0
        while self.waiting and num_seqs < self.max_num_seqs:
-            if Observer.ttft_start == 0:
-                Observer.ttft_start = perf_counter_ns()
+            if InferenceObserver.ttft_start == 0:
+                InferenceObserver.ttft_start = perf_counter_ns()
            seq = self.waiting[0]

            # Check if sequence is too large
--- a/nanovllm/kvcache/offload_engine.py
+++ b/nanovllm/kvcache/offload_engine.py
@@ -17,6 +17,7 @@ from dataclasses import dataclass
 from nanovllm.kvcache.kernels import gathered_copy_kv
 from nanovllm.comm import memcpy_2d_async
 from nanovllm.utils.logger import get_logger
+from nanovllm.utils.memory_observer import MemoryObserver

 # Import for type hints only (avoid circular import)
 from typing import TYPE_CHECKING
@@ -376,7 +377,8 @@ class OffloadEngine:
        self.ring_slot_compute_done[slot_idx].record()

    def load_to_slot_layer(
-        self, slot_idx: int, layer_id: int, cpu_block_id: int, chunk_idx: int = -1
+        self, slot_idx: int, layer_id: int, cpu_block_id: int, chunk_idx: int = -1,
+        is_prefill: bool = True,
    ) -> None:
        """
        Async load a single CPU block to a ring buffer slot for one layer.
@@ -393,6 +395,7 @@ class OffloadEngine:
            layer_id: Layer index to load (for CPU cache indexing)
            cpu_block_id: Source CPU block ID
            chunk_idx: Optional chunk index for NVTX labeling (-1 means not specified)
+            is_prefill: True if in prefill phase, False if in decode phase (for MemoryObserver)
        """
        logger.debug(f"Ring load: layer={layer_id}, CPU[{cpu_block_id}] -> GPU slot[{slot_idx}]")

@@ -425,6 +428,9 @@ class OffloadEngine:
            self.ring_slot_ready[slot_idx].record(stream)
        nvtx.pop_range()

+        # Record H2D transfer: K + V = 2 * block_bytes
+        MemoryObserver.record_h2d(2 * self.gpu_block_bytes, is_prefill=is_prefill)
+
    def wait_slot_layer(self, slot_idx: int) -> None:
        """
        Wait for a slot's loading to complete.
@@ -499,6 +505,9 @@ class OffloadEngine:
            self.ring_slot_offload_done[slot_idx].record(self.transfer_stream_main)
        nvtx.pop_range()

+        # Record D2H transfer: K + V = 2 * block_bytes
+        MemoryObserver.record_d2h(2 * self.gpu_block_bytes, is_prefill=is_prefill)
+
    # ----- KV access methods for ring buffer -----

    def get_kv_for_slot(self, slot_idx: int) -> Tuple[Tensor, Tensor]:
@@ -745,6 +754,10 @@ class OffloadEngine:
        self.prefill_v_buffer[layer_id, :num_tokens].copy_(v)
        torch.cuda.nvtx.range_pop()

+        # Record D2D transfer: K + V
+        transfer_bytes = 2 * k.numel() * k.element_size()
+        MemoryObserver.record_d2d(transfer_bytes)
+
    def write_to_decode_buffer(
        self,
        layer_id: int,
@@ -768,6 +781,10 @@ class OffloadEngine:
        self.decode_v_buffer[layer_id, pos_in_block].copy_(v)
        torch.cuda.nvtx.range_pop()

+        # Record D2D transfer: K + V (single token)
+        transfer_bytes = 2 * k.numel() * k.element_size()
+        MemoryObserver.record_d2d(transfer_bytes)
+
    def offload_prefill_buffer_async(
        self,
        layer_id: int,
@@ -813,6 +830,9 @@ class OffloadEngine:
            self.prefill_offload_events[layer_id].record(stream)
        nvtx.pop_range()

+        # Record D2H transfer: K + V = 2 * block_bytes
+        MemoryObserver.record_d2h(2 * self.gpu_block_bytes, is_prefill=True)
+
    def wait_all_prefill_offloads(self) -> None:
        """Wait for all prefill buffer offloads to complete."""
        for stream in self.prefill_offload_streams:
@@ -851,6 +871,11 @@ class OffloadEngine:
        v_sample = self.v_cache_cpu[
            layer_id, cpu_block_id, :num_samples
        ].clone().cuda()
+
+        # Record H2D transfer: K + V samples
+        transfer_bytes = 2 * k_sample.numel() * k_sample.element_size()
+        MemoryObserver.record_h2d(transfer_bytes, is_prefill=True)
+
        return k_sample, v_sample

    def load_block_full_from_cpu(
@@ -877,4 +902,8 @@ class OffloadEngine:
        v_full = self.v_cache_cpu[
            layer_id, cpu_block_id
        ].clone().cuda()
+
+        # Record H2D transfer: K + V full block
+        MemoryObserver.record_h2d(2 * self.gpu_block_bytes, is_prefill=True)
+
        return k_full, v_full
--- a/nanovllm/kvcache/sparse/full_policy.py
+++ b/nanovllm/kvcache/sparse/full_policy.py
@@ -422,7 +422,7 @@ class FullAttentionPolicy(SparsePolicy):
        num_preload = min(num_slots, num_blocks)
        for i in range(num_preload):
            cpu_block_id = cpu_block_table[i]
-            offload_engine.load_to_slot_layer(load_slots[i], layer_id, cpu_block_id, chunk_idx=cpu_block_id)
+            offload_engine.load_to_slot_layer(load_slots[i], layer_id, cpu_block_id, chunk_idx=cpu_block_id, is_prefill=False)

        # Phase 2: Process blocks with pipeline
        for block_idx in range(num_blocks):
@@ -456,7 +456,7 @@ class FullAttentionPolicy(SparsePolicy):
            next_block_idx = block_idx + num_slots
            if next_block_idx < num_blocks:
                next_cpu_block_id = cpu_block_table[next_block_idx]
-                offload_engine.load_to_slot_layer(current_slot, layer_id, next_cpu_block_id, chunk_idx=next_cpu_block_id)
+                offload_engine.load_to_slot_layer(current_slot, layer_id, next_cpu_block_id, chunk_idx=next_cpu_block_id, is_prefill=False)

            # Merge with accumulated
            with torch.cuda.stream(compute_stream):
--- a/nanovllm/utils/memory_observer.py
+++ b/nanovllm/utils/memory_observer.py
@@ -0,0 +1,133 @@
+"""
+MemoryObserver - 内存传输统计 Observer。
+
+统计 GPU-CPU 间的数据传输量：
+- H2D (Host to Device): CPU → GPU
+- D2H (Device to Host): GPU → CPU
+- D2D (Device to Device): GPU → GPU (buffer copy)
+"""
+
+from nanovllm.utils.observer import Observer
+
+
+class MemoryObserver(Observer):
+    """
+    内存传输 Observer，统计 GPU-CPU 间的数据传输量。
+
+    统计类型：
+    - H2D (Host to Device): CPU → GPU
+    - D2H (Device to Host): GPU → CPU
+    - D2D (Device to Device): GPU → GPU (buffer copy)
+
+    统计位置（均在 offload_engine.py）：
+    - H2D: load_to_slot_layer(), load_block_sample_from_cpu(), load_block_full_from_cpu()
+    - D2H: offload_slot_layer_to_cpu(), offload_prefill_buffer_async()
+    - D2D: write_to_prefill_buffer(), write_to_decode_buffer()
+    - 重置: llm_engine.py:generate() - 与 InferenceObserver 一起重置
+    """
+
+    _enabled: bool = False  # 默认禁用，需要显式启用
+
+    # H2D 统计
+    h2d_bytes: int = 0
+    h2d_count: int = 0
+
+    # D2H 统计
+    d2h_bytes: int = 0
+    d2h_count: int = 0
+
+    # D2D 统计
+    d2d_bytes: int = 0
+    d2d_count: int = 0
+
+    # 按阶段统计
+    prefill_h2d_bytes: int = 0
+    prefill_d2h_bytes: int = 0
+    decode_h2d_bytes: int = 0
+    decode_d2h_bytes: int = 0
+
+    @classmethod
+    def record_h2d(cls, num_bytes: int, is_prefill: bool = True) -> None:
+        """记录 H2D 传输"""
+        if not cls._enabled:
+            return
+        cls.h2d_bytes += num_bytes
+        cls.h2d_count += 1
+        if is_prefill:
+            cls.prefill_h2d_bytes += num_bytes
+        else:
+            cls.decode_h2d_bytes += num_bytes
+
+    @classmethod
+    def record_d2h(cls, num_bytes: int, is_prefill: bool = True) -> None:
+        """记录 D2H 传输"""
+        if not cls._enabled:
+            return
+        cls.d2h_bytes += num_bytes
+        cls.d2h_count += 1
+        if is_prefill:
+            cls.prefill_d2h_bytes += num_bytes
+        else:
+            cls.decode_d2h_bytes += num_bytes
+
+    @classmethod
+    def record_d2d(cls, num_bytes: int) -> None:
+        """记录 D2D 传输"""
+        if not cls._enabled:
+            return
+        cls.d2d_bytes += num_bytes
+        cls.d2d_count += 1
+
+    @classmethod
+    def complete_reset(cls) -> None:
+        """重置所有统计"""
+        cls.h2d_bytes = cls.h2d_count = 0
+        cls.d2h_bytes = cls.d2h_count = 0
+        cls.d2d_bytes = cls.d2d_count = 0
+        cls.prefill_h2d_bytes = cls.prefill_d2h_bytes = 0
+        cls.decode_h2d_bytes = cls.decode_d2h_bytes = 0
+
+    @classmethod
+    def get_summary(cls) -> dict:
+        """返回统计摘要"""
+        return {
+            "total": {
+                "h2d_bytes": cls.h2d_bytes,
+                "h2d_count": cls.h2d_count,
+                "d2h_bytes": cls.d2h_bytes,
+                "d2h_count": cls.d2h_count,
+                "d2d_bytes": cls.d2d_bytes,
+                "d2d_count": cls.d2d_count,
+            },
+            "prefill": {
+                "h2d_bytes": cls.prefill_h2d_bytes,
+                "d2h_bytes": cls.prefill_d2h_bytes,
+            },
+            "decode": {
+                "h2d_bytes": cls.decode_h2d_bytes,
+                "d2h_bytes": cls.decode_d2h_bytes,
+            },
+        }
+
+    @classmethod
+    def _fmt_bytes(cls, b: int) -> str:
+        """格式化字节数"""
+        if b >= 1e9:
+            return f"{b/1e9:.2f} GB"
+        if b >= 1e6:
+            return f"{b/1e6:.2f} MB"
+        if b >= 1e3:
+            return f"{b/1e3:.2f} KB"
+        return f"{b} B"
+
+    @classmethod
+    def print_summary(cls) -> None:
+        """打印人类可读的摘要"""
+        fmt = cls._fmt_bytes
+        total = cls.h2d_bytes + cls.d2h_bytes + cls.d2d_bytes
+        print(f"[MemoryObserver] Total: {fmt(total)}")
+        print(f"  H2D: {fmt(cls.h2d_bytes)} ({cls.h2d_count} ops)")
+        print(f"  D2H: {fmt(cls.d2h_bytes)} ({cls.d2h_count} ops)")
+        print(f"  D2D: {fmt(cls.d2d_bytes)} ({cls.d2d_count} ops)")
+        print(f"  Prefill - H2D: {fmt(cls.prefill_h2d_bytes)}, D2H: {fmt(cls.prefill_d2h_bytes)}")
+        print(f"  Decode  - H2D: {fmt(cls.decode_h2d_bytes)}, D2H: {fmt(cls.decode_d2h_bytes)}")
--- a/nanovllm/utils/observer.py
+++ b/nanovllm/utils/observer.py
@@ -1,17 +1,106 @@
-class Observer():
-    ttft_start = 0
-    tpot_start = 0
+"""
+Observer 基类和 InferenceObserver 实现。

-    ttft = 0
-    tpot = 0
+Observer 架构：
+- Observer: 基类，定义通用接口
+- InferenceObserver: 推理性能观测（TTFT/TPOT）
+- MemoryObserver: 内存传输观测（在 memory_observer.py 中定义）
+"""
+
+
+class Observer:
+    """
+    Observer 基类，提供通用的启用/禁用、重置、输出接口。
+
+    所有 Observer 子类应继承此类并实现：
+    - complete_reset(): 重置所有统计数据
+    - get_summary(): 返回统计摘要 dict
+    - print_summary(): 打印人类可读的摘要
+    """
+
+    _enabled: bool = True  # 默认启用

    @classmethod
-    def reset_ttft(cls):
+    def enable(cls) -> None:
+        """启用 observer"""
+        cls._enabled = True
+
+    @classmethod
+    def disable(cls) -> None:
+        """禁用 observer"""
+        cls._enabled = False
+
+    @classmethod
+    def is_enabled(cls) -> bool:
+        """检查是否启用"""
+        return cls._enabled
+
+    @classmethod
+    def complete_reset(cls) -> None:
+        """重置所有统计数据（子类实现）"""
+        raise NotImplementedError
+
+    @classmethod
+    def get_summary(cls) -> dict:
+        """返回统计摘要（子类实现）"""
+        raise NotImplementedError
+
+    @classmethod
+    def print_summary(cls) -> None:
+        """打印人类可读的摘要（子类可选覆盖）"""
+        import json
+        print(json.dumps(cls.get_summary(), indent=2))
+
+
+class InferenceObserver(Observer):
+    """
+    推理性能 Observer，统计 TTFT 和 TPOT。
+
+    - TTFT (Time To First Token): 首个 token 生成延迟
+    - TPOT (Time Per Output Token): 每个输出 token 的平均延迟
+
+    统计位置：
+    - TTFT 开始: scheduler.py:35-36 - 第一个 sequence 从 waiting 队列取出时
+    - TTFT 结束: llm_engine.py:69-72 - prefill 完成后（包括 chunked prefill 所有 chunks）
+    - TPOT 开始: llm_engine.py:65 - 每次 decode step 结束时
+    - TPOT 结束: llm_engine.py:62-63 - 下一次 decode step 开始时计算（测量上一次 decode 时间）
+    - 重置: llm_engine.py:97 - generate() 开始时
+
+    注意：TPOT 需要至少 2 个输出 token 才能计算（测量 decode step 间隔）。
+    """
+
+    # 时间戳 (nanoseconds)
+    ttft_start: int = 0
+    tpot_start: int = 0
+
+    # 统计结果 (nanoseconds)
+    ttft: int = 0
+    tpot: int = 0
+
+    @classmethod
+    def reset_ttft(cls) -> None:
+        """重置 TTFT 计时器"""
        cls.ttft_start = 0

    @classmethod
-    def complete_reset(cls):
+    def complete_reset(cls) -> None:
+        """重置所有统计数据"""
        cls.ttft_start = 0
        cls.tpot_start = 0
        cls.ttft = 0
        cls.tpot = 0
+
+    @classmethod
+    def get_summary(cls) -> dict:
+        """返回统计摘要"""
+        return {
+            "ttft_ns": cls.ttft,
+            "ttft_ms": cls.ttft / 1e6,
+            "tpot_ns": cls.tpot,
+            "tpot_ms": cls.tpot / 1e6,
+        }
+
+    @classmethod
+    def print_summary(cls) -> None:
+        """打印摘要"""
+        print(f"[InferenceObserver] TTFT: {cls.ttft / 1e6:.2f}ms, TPOT: {cls.tpot / 1e6:.2f}ms")