c9d350b372
Document the iterative debugging from v1 (broken KVC) through v4
(routing fixed + session cap raised), with code-level analysis of
the two main bugs encountered:
1. v2 root cause (mis-diagnosed previously as `allow_local_prefill`):
`--policy default` for KVC mechanism caused replay's round-robin
policy and the PD router's round-robin to diverge, sending requests
with `session_params` to a D worker that did not have the session
open. Resulted in 56-61% truncation with finish_reason
"session id X does not exist".
Fix: use `--policy kv-aware` (sweep_tp1_v3_kvaware.sh) so replay
emits `x-smg-target-worker` and PD router uses consistent_hashing.
2. v3 new bottleneck: `pd-router-fallback-large-append-session-cap`
dominated 52-65% of requests. Root cause was hardcoded
`min(4, ...)` in `_decode_session_soft_cap`. With 7 D workers x 4
sessions = 28 slots for 52 trace sessions, ~24 sessions starved
permanently (bimodal direct-to-D rate of 0% or 99%).
Fix: raise the cap to 16 (replay.py).
Also includes the v3 finding that direct-to-d-session path P50=0.495s
and TTFT P50=0.043s already beats the 8-way DP baseline (0.65s/0.093s)
- the KVC core mechanism works when fallback paths are avoided.
Files:
- docs/KVC_DEBUG_JOURNEY_V1_TO_V4.md: full journey + code location index
- docs/SWEBENCH_EXPERIMENT_{PROGRESS,RESULTS}.md: prior session notes
- scripts/sweep_tp1_v{2,3,4}*.sh: experiment driver scripts
- src/agentic_pd_hybrid/replay.py: cap 4 -> 16, audit fields
- src/agentic_pd_hybrid/pd_router.py: strip session_params from prefill
- src/agentic_pd_hybrid/metrics.py: truncated_request_count
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
122 lines
4.4 KiB
Markdown
122 lines
4.4 KiB
Markdown
# SWE-Bench PD Hybrid Experiment Results
|
||
|
||
## 实验配置
|
||
|
||
- **模型**: Qwen3.5-35B-A3B (MoE, 35B total / 3B active), TP4
|
||
- **硬件**: 8x H100 80GB, NVLink, 单节点
|
||
- **Transfer backend**: mooncake TCP (loopback)
|
||
- **Trace**: 52 sessions, 4,449 requests (10% sample of SWE-Bench 500 instances)
|
||
- **时间压缩**: time-scale=10, concurrency-limit=32
|
||
|
||
## 结果汇总
|
||
|
||
### Experiment A: pd-disaggregation (baseline)
|
||
|
||
| Metric | Value |
|
||
|--------|-------|
|
||
| Run dir | `pd-disaggregation-default-20260426T202540Z` |
|
||
| Requests | 4,449 / 4,449 (100%) |
|
||
| Errors | 0 |
|
||
| **Mean Latency** | **1.662s** |
|
||
| P50 Latency | 0.973s |
|
||
| P90 Latency | 3.644s |
|
||
| P99 Latency | 7.676s |
|
||
| Mean TTFT | 0.445s |
|
||
| P50 TTFT | 0.340s |
|
||
| P90 TTFT | 0.880s |
|
||
| Mean TPOT | 5.20ms |
|
||
| Cache Hit Rate | 94.4% (4199/4449) |
|
||
| Mean Cached Tokens | 27,794 |
|
||
| KV Transfer Blocks | 105,235 |
|
||
|
||
### Experiment B: pd-colo (colocation) — FAILED
|
||
|
||
| Metric | Value |
|
||
|--------|-------|
|
||
| Run dir | `pd-colo-default-20260426T210129Z` |
|
||
| Status | **CRASHED** |
|
||
| Error | `token_to_kv_pool_allocator memory leak detected!` |
|
||
| Root Cause | SGLang v0.5.10 `--disaggregation-mode null` 与 Qwen3.5-35B-A3B (Mamba/GDN hybrid) 不兼容 |
|
||
| Requests | ~10 / 4,449 (0.2%) |
|
||
|
||
**结论**: 当前 vendored SGLang 不支持此模型的 colocation 模式。需要修复 token_to_kv_pool_allocator 中 Mamba 模型的内存管理。
|
||
|
||
### Experiment C: kvcache-centric (session-aware PD)
|
||
|
||
| Metric | Value |
|
||
|--------|-------|
|
||
| Run dir | `kvcache-centric-default-worker-admission-20260426T210800Z` |
|
||
| Requests | 4,449 total |
|
||
| **Errors** | **4,390 (98.7%)** |
|
||
| Successful | 59 (1.3%) |
|
||
| Mean Latency (success) | 1.238s |
|
||
| P50 Latency (success) | 0.484s |
|
||
| P90 Latency (success) | 2.550s |
|
||
| Mean TTFT (success) | 0.179s |
|
||
| P50 TTFT (success) | 0.081s |
|
||
| Mean TPOT (success) | 4.70ms |
|
||
| Direct-to-D Sessions | 56 |
|
||
| KV Transfer (actual) | 196 blocks (vs 105,235 planned) |
|
||
|
||
**Execution Mode 分布**:
|
||
- `kvcache-centric` (failed): 4,390
|
||
- `kvcache-direct-to-d-session` (success): 56
|
||
- `pd-router-*` variants: 3
|
||
|
||
## 关键分析
|
||
|
||
### 1. pd-disaggregation (A) — 稳定可靠
|
||
|
||
- 100% 成功率，0 错误
|
||
- Mean latency 1.66s 合理 (包含 P→D KV transfer 开销)
|
||
- 94.4% cache hit 说明 prefix cache 在 P 侧工作良好
|
||
- KV transfer 105K blocks = 主要开销来源
|
||
- **适合生产使用**
|
||
|
||
### 2. pd-colo (B) — 不可用
|
||
|
||
- Qwen3.5-35B-A3B 的 Mamba/GDN hybrid 架构在 `disaggregation-mode null` 下触发内存泄漏
|
||
- 这是 SGLang 的 bug，不是 agentic-pd-hybrid 的问题
|
||
- **需要 SGLang 修复后重新测试**
|
||
|
||
### 3. kvcache-centric (C) — Admission 过于保守
|
||
|
||
- 98.7% 错误率说明 admission control 拒绝了几乎所有请求
|
||
- `kvcache-seed-min-turn-id=2` 过滤了 turn 1 的 seed（正确行为）
|
||
- 但绝大多数 turn 2+ 请求也走 `kvcache-centric` 模式后失败
|
||
- 可能原因:
|
||
- Worker admission 查询发现 D 侧没有对应 session 的 KV cache（因为 turn 1 没有 seed）
|
||
- D 侧 transfer queue 积压导致 admission 拒绝
|
||
- 成功的 56 个 `direct-to-d-session` 请求表现优异: TTFT 0.08s (P50), 比 pd-disagg 的 0.34s 快 4x
|
||
- **需要调优 admission 参数，或使用 `kvcache-seed-min-turn-id=1` 允许 turn 1 seed**
|
||
|
||
### 4. kvcache-centric 成功请求 vs pd-disaggregation 对比
|
||
|
||
| Metric | pd-disagg (A) | kvcache-centric (C, success only) | Delta |
|
||
|--------|:---:|:---:|:---:|
|
||
| Mean Latency | 1.662s | 1.238s | **-25.5%** |
|
||
| P50 Latency | 0.973s | 0.484s | **-50.3%** |
|
||
| Mean TTFT | 0.445s | 0.179s | **-59.8%** |
|
||
| P50 TTFT | 0.340s | 0.081s | **-76.2%** |
|
||
| Mean TPOT | 5.20ms | 4.70ms | -9.6% |
|
||
| Actual KV Transfer | 105,235 blk | 196 blk | **-99.8%** |
|
||
|
||
**当 kvcache-centric 成功时，性能提升显著：**
|
||
- TTFT 降低 60-76% (D 侧直接 append，无需 P→D transfer)
|
||
- 端到端 latency 降低 25-50%
|
||
- KV transfer 减少 99.8%
|
||
|
||
## 后续建议
|
||
|
||
1. **修复 pd-colo**: 提交 SGLang issue 关于 Mamba/GDN 模型在 disaggregation-mode null 下的内存泄漏
|
||
2. **调优 kvcache-centric admission**:
|
||
- 尝试 `--kvcache-seed-min-turn-id 1` 允许 turn 1 seed
|
||
- 放宽 `--kvcache-seed-max-decode-transfer-queue-reqs` 阈值
|
||
- 使用 `--kvcache-admission-mode router` (shadow state, 不在 critical path)
|
||
3. **增加 D 侧内存**: 调整 `--mem-fraction-static` 给 KV cache 更多空间
|
||
4. **多 P/D 配置**: 测试 2P2D (TP2) 配置以增加并行度
|
||
|
||
## 实验日期
|
||
|
||
2026-04-27
|