📝 docs: add GPU-only sparse policy integration baseline

Document baseline performance before integrating sparse attention
to GPU-only mode:
- GPU-only Full Attention: 4869 tok/s (32K prefill)
- CPU Offload Full Attention: 1500 tok/s (3.2x slower)

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via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

docs/gpu_only_sparse_integration.md

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+# GPU-only Sparse Policy 整合
+
+本文档记录将 sparse attention 策略整合到 GPU-only 模式的过程和性能对比。
+
+## 背景
+
+当前 sparse policy（Quest、XAttention）仅在 CPU offload 路径中实现。目标是将其扩展到 GPU-only 模式，以提升长上下文场景下的性能。
+
+## 基准性能（优化前）
+
+**测试环境**:
+- GPU: NVIDIA A100-SXM4-80GB
+- 模型: Llama-3.1-8B-Instruct
+- 上下文长度: 32K tokens
+- 日期: 2026-01-27
+
+### Prefill Benchmark (32K context)
+
+| 模式 | Throughput | Time | KV Cache 分配 |
+|------|------------|------|---------------|
+| **GPU-only (Full Attention)** | 4869.67 tok/s | 6.73s | 438 blocks (56GB GPU) |
+| CPU Offload (Full Attention) | 1500.29 tok/s | 21.84s | 4 blocks GPU + 32 blocks CPU |
+
+**性能比**: GPU-only 比 CPU Offload 快 **3.2x**
+
+### 配置详情
+
+**GPU-only 模式**:
+```bash
+CUDA_VISIBLE_DEVICES=0 python bench.py \
+    --model ~/models/Llama-3.1-8B-Instruct \
+    --max-len 32768
+```
+
+**CPU Offload 模式**:
+```bash
+CUDA_VISIBLE_DEVICES=0 python bench_offload.py \
+    --model ~/models/Llama-3.1-8B-Instruct \
+    --max-len 32768
+```
+
+### KV Cache 配置
+
+| 参数 | GPU-only | CPU Offload |
+|------|----------|-------------|
+| block_size | 1024 tokens | 1024 tokens |
+| per-token KV | 128 KB | 128 KB |
+| per-block KV | 128 MB | 128 MB |
+| GPU blocks | 438 | 4 |
+| CPU blocks | 0 | 32 |
+| Total memory | 56 GB | 4.6 GB |
+
+## 目标
+
+将以下 sparse policy 整合到 GPU-only 模式：
+
+| Policy | 阶段 | 描述 |
+|--------|------|------|
+| Quest | Decode | Top-K block selection based on query-key scores |
+| XAttention BSA | Prefill | Block sparse attention with cumulative threshold |
+
+## 实现进度
+
+- [ ] 分析现有 sparse policy 代码结构
+- [ ] 设计 GPU-only sparse policy 接口
+- [ ] 实现 GPU-only Quest decode
+- [ ] 实现 GPU-only XAttention prefill
+- [ ] 性能测试和对比
+
+## 优化后性能
+
+*待测试*
+
+| 模式 | Throughput | Speedup vs Full |
+|------|------------|-----------------|
+| GPU-only + Quest (decode) | TBD | TBD |
+| GPU-only + XAttn (prefill) | TBD | TBD |