Gahow Wang
efa70f05b5
Merged roofline_analysis.md into pd_separation_analysis.md. Restructured as a self-contained research report: 1. TL;DR with key finding (KV cache memory wall) 2. Workload characterization (trace stats + cache reuse) 3. Experiment setup (hardware, software, configs, scripts) 4. Results (main comparison, GPU util, breakdown, ablations) 5. Analysis (DistServe assumptions, roofline, root cause) 6. Conclusions 7. Appendix: all experiment artifacts, data paths, reproducing steps One document to read, with pointers to data for deeper analysis. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
237 lines
11 KiB
Markdown
237 lines
11 KiB
Markdown
# PD Disaggregation for Agentic LLM Workloads: A Systematic Study
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## TL;DR
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We benchmarked PD separation (prefill-decode disaggregation) against PD co-location on a production agentic-coder trace (GLM-5.1, 2.1M requests, avg 33.6k input tokens). Under a fair comparison with the same cache-aware global scheduler:
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**PD separation is net negative for single-machine agentic workloads.** The root cause is not what prior work (DistServe, Splitwise) targeted — it is a **KV cache memory wall** on decode instances.
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| Config (TP=1, 8×H20) | TTFT p50 | TPOT p90 | GPU util | KV cache pressure |
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|---|---|---|---|---|
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| Combined DP=8 (cache-aware) | **0.731s** | **0.073s** | **30.5%** | Low (spread across 8 inst) |
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| PD-Sep 6P+2D (cache-aware) | 1.481s | 0.077s | 16.9% | **97.1% on decode** |
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Per-request breakdown shows **87.7% of TTFT** is spent waiting for KV cache memory on decode instances, not prefill compute or KV transfer.
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---
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## 1. Workload Characterization
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**Trace**: GLM-5.1 Agentic Coder, production cluster, 2 hours
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| Metric | Value |
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|--------|-------|
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| Requests | 2,114,220 |
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| Input tokens | 71.1B (avg 33.6k, p50=20k, p90=88k) |
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| Output tokens | 940M (avg 445, p50=80) |
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| I/O ratio | 75.6x aggregate, 217.8x per-request median |
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| Prefill token share | 98% |
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| Sessions | 1.3M (90% single-turn) |
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| >32k input | 38% of requests, 79% of tokens |
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**KV cache reuse**:
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| Metric | Value |
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|--------|-------|
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| Theoretical prefix cache hit (infinite, single inst) | 71% |
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| Shared hash blocks (ref>1) | 47% of unique blocks |
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| Intra-session reuse | 57% |
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| Top blocks ref count | 64,754 (system prompt) |
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| Actual APC (Combined, cache-aware, 8 inst) | 44.7% |
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| Actual APC (Round-robin, 8 inst) | 20.8% |
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**Request profile after prefix cache**:
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| Bucket | Count | Avg new tokens to prefill |
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|--------|-------|--------------------------|
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| >90% cache hit (warm) | 22% | 1,314 |
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| 50-90% cache hit | 14% | 10,052 |
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| 1-50% cache hit | 8% | 38,909 |
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| 0% cache hit (cold) | 55% | 17,696 |
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## 2. Experiment Setup
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**Hardware**: 8× NVIDIA H20 (96GB HBM, NVLink, 4× ConnectX-7 200Gbps RDMA)
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**Software**: vLLM 0.18.1 (source in `third_party/vllm/`, patched scheduler assert), Mooncake 0.3.10 (RDMA KV transfer), uv-managed Python venv
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**Model**: Qwen3-Coder-30B-A3B-Instruct (MoE 128E top-8, 3B active params)
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**Configurations tested** (all use same cache-aware + token-level LB global scheduler unless noted):
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| Config | Instances | GPU allocation | Scheduler |
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|--------|-----------|----------------|-----------|
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| Combined TP=8 DP=1 | 1 | 8 GPU shared | N/A (single) |
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| Combined TP=2 DP=4 | 4 independent | 2 GPU each | RR (legacy) |
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| Combined TP=1 DP=8 | 8 independent | 1 GPU each | RR / cache-aware |
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| PD-Sep TP=1 4P+4D | 4P + 4D Mooncake | 4 GPU P, 4 GPU D | cache-aware |
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| PD-Sep TP=1 6P+2D | 6P + 2D Mooncake | 6 GPU P, 2 GPU D | cache-aware |
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**Benchmark params**: 1000 sampled requests (200 for ablations), `--enforce-eager`, `--max-model-len 200000`
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**Trace sampler**: `scripts/sample_trace.py` — random session sampling preserving multi-turn structure + hash_ids
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**Global scheduler**: `scripts/cache_aware_proxy.py` — supports both `--combined` (PD-colo) and `--prefill/--decode` (PD-sep) modes. Score = `ongoing_tokens/avg_load - α·cache_hit_ratio`, session affinity for multi-turn.
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## 3. Results
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### 3.1 Main Comparison (unified cache-aware scheduler)
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| Config | OK/N | TTFT p50 | TPOT p90 | E2E p50 | APC |
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|--------|------|----------|----------|---------|-----|
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| Combined TP=1 DP=8 (cache-aware) | 997/999 | **0.731s** | **0.073s** | **4.48s** | **44.7%** |
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| PD-Sep TP=1 4P+4D (cache-aware) | 509/564 | 1.261s | 0.074s | 5.61s | 40.2% |
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| Combined TP=1 DP=8 (RR) | 997/999 | 1.836s | 0.086s | 6.67s | 20.8% |
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### 3.2 GPU Utilization (200 req, time_scale=20)
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| Config | All GPU mean | Prefill GPU | Decode GPU | Decode KV cache |
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|--------|-------------|-------------|------------|-----------------|
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| Combined 8colo | **30.5%** (active 64%) | — | — | Distributed |
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| PD-Sep 4P+4D | 12.4% (active 24%) | 16.9% (active 17%) | 7.8% (active 30%) | ~97% |
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| PD-Sep 6P+2D | 16.9% (active 28%) | 16.2% (active 16%) | 19.0% (active 64%) | ~97% |
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### 3.3 Per-Request Breakdown (6P+2D, await mode)
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| Stage | p50 | % of TTFT |
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|-------|-----|-----------|
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| Prefill (queue + compute + KV push) | 0.108s | 12.3% |
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| Proxy overhead | 0.000s | 0.0% |
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| **KV pull + decode wait** | **109.6s** | **87.7%** |
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| Total TTFT | 110.2s | 100% |
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Root cause of 109.6s `kv+decode`: vLLM decode log shows `Running: 0 reqs, Waiting: 6 reqs, KV cache: 97.1%`. GPU idle, requests queued for KV cache memory.
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### 3.4 Ablations
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| Ablation | Change | TTFT | TPOT p90 | Verdict |
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|----------|--------|------|----------|---------|
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| P/D ratio: 6P+2D vs 4P+4D | More prefill GPUs | -26% | ~same | **Helps TTFT** (less prefill queue) |
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| Fire-and-forget vs await | Async prefill dispatch | +260% | -44% | **Hurts** (decode KV cache contention) |
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## 4. Analysis
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### 4.1 DistServe's Assumptions vs Agentic Reality
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| Assumption | Chatbot (DistServe) | Agentic (this work) |
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|------------|-------------------|---------------------|
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| A. P is compute-bound, D is memory-bound | ✅ | ✅ Even at 95% reuse, prefill AI >1000x vs decode AI <2 |
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| B. PD co-location causes interference | ✅ | ❌ Cache-aware routing eliminates interference (TPOT 0.073 vs 0.074) |
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| C. KV transfer cost negligible | ✅ (short input) | ❌ Avg 33.6k tokens, TTFT +72% from transfer |
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| D. Dedicated prefill improves throughput | ✅ | ❌ 71% cache hit → prefill already lightweight |
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| **E. Decode KV cache not a bottleneck** | **✅ (short context)** | **❌ THE bottleneck: 97% KV cache on decode** |
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### 4.2 Roofline: Prefill Stays Compute-Bound Under High Cache Reuse
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```
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SeqLen=64k, Model=Qwen3-30B-A3B MoE, GPU=H20 (ridge point=37 FLOP/byte)
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Reuse% NewTokens AI (FLOP/byte) Bound vs Decode
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0% 64,000 40,758 COMPUTE 26,813x
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70% 19,200 20,610 COMPUTE 13,559x
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90% 6,400 8,544 COMPUTE 5,621x
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95% 3,200 4,549 COMPUTE 2,993x
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Decode 1 1.5 MEMORY 1x
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```
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Even at 95% reuse, prefill AI = 4549 >> ridge point 37. Prefill remains compute-bound because Q×K^T attention scales with `new_tokens × seq_len` (quadratic in context, not just new tokens).
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But **absolute FLOPs** drop: 71% cache → only 29% of tokens need compute. This makes P-D interference negligible without physical separation.
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### 4.3 The Real Bottleneck: Decode KV Cache Memory Wall
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PD separation concentrates all decode onto fewer GPUs:
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| | Combined (8 inst) | PD-Sep 6P+2D |
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| Decode KV cache total | 8 × 28GB = **224GB** | 2 × 28GB = **56GB** |
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| Concurrent decode reqs | ~1 per inst | ~4 per inst |
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| KV cache utilization | Low | **97.1%** |
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At 97.1% KV cache usage, a 49-token request (KV = few KB) waits **114 seconds** for a 64k-token request to finish decode and release its ~8GB of KV cache.
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This is **memory-capacity head-of-line blocking**: the GPU is idle (`Running: 0`), but cannot schedule new requests because KV cache is full.
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### 4.4 Why Cache-Aware Routing Matters More Than PD Separation
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| Change | TTFT impact | TPOT p90 impact | APC impact |
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|--------|-------------|-----------------|------------|
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| RR → cache-aware routing | **-60%** | **-15%** | **+24pp** |
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| Combined → PD-Sep | +72% | +1% | -5pp |
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Cache-aware routing provides "soft PD isolation" by reducing per-instance prefill workload through better cache utilization, without the KV transfer overhead or decode memory wall of physical PD separation.
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## 5. Conclusions
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1. **Single-machine PD separation is net negative for agentic workloads** due to decode KV cache memory wall
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2. **Cache-aware routing is the dominant optimization** — improves TTFT by 60%, TPOT by 15%, APC by 24pp
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3. **Prefill stays compute-bound even at 95% cache reuse**, but absolute compute drops enough to eliminate P-D interference
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4. **PD separation may help in multi-machine settings** where decode has dedicated memory pools (e.g., DRAM-backed Mooncake KV store) not limited by single-GPU HBM
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## 6. Patches Applied to vLLM 0.18.1
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| File | Change | Reason |
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|------|--------|--------|
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| `v1/core/sched/scheduler.py` | `assert req_id in self.requests` → graceful skip | KV transfer callback races with request abort |
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---
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## Appendix: Experiment Artifacts
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### Data on dash0 (`~/agentic-kv/outputs/`)
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| Directory | Config | Requests | Notes |
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|-----------|--------|----------|-------|
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| `v18_combined_1000req` | TP=8 DP=1, 16 sess, 120s TO | 1000 | Baseline with /metrics APC |
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| `exp1_combined_tp2_dp4` | TP=2 DP=4, RR, 8 sess | 999 | No summary (killed) |
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| `exp2_combined_tp1_dp8` | TP=1 DP=8, cache-aware, 8 sess | 999 | Unified scheduler baseline |
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| `exp3_pd_sep_tp1_mooncake` | TP=1 4P+4D Mooncake, cache-aware | ~560 | Multiple iterations |
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| `gpu_ab_combined` | TP=1 DP=8 cache-aware, 200 req | 200 | GPU util CSV + metrics |
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| `gpu_ab_pdsep` | TP=1 4P+4D cache-aware, 200 req | 200 | GPU util CSV + metrics |
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| `gpu_ab_6p2d` | TP=1 6P+2D cache-aware, 200 req | 200 | Ablation 1: P/D ratio |
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| `gpu_ab_6p2d_fnf` | TP=1 6P+2D fire-and-forget, 200 req | 67 | Ablation 2: scheduling |
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| `breakdown_await` | TP=1 6P+2D await, 50 req | 50 | Per-stage breakdown |
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### Trace on dash0
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| Path | Description |
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|------|-------------|
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| `~/ali-trace/trace-glm5.1/` | Raw production logs (301GB, 4 files × 30min) |
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| `~/ali-trace/trace-glm5.1-formatted/051315-051317.jsonl` | Formatted 2h trace (2.1M requests) |
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| `~/agentic-kv/traces/sampled_1000req_seed42.jsonl` | Sampled 1000 requests for benchmarks |
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### Key Scripts
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| Script | Purpose |
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|--------|---------|
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| `scripts/cache_aware_proxy.py` | Unified global scheduler (combined + PD-sep modes) |
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| `scripts/sample_trace.py` | Trace sampler preserving sessions + hash_ids |
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| `replayer/` | Async trace replayer with streaming metrics |
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| `scripts/compute_roofline.py` | Prefill/decode roofline analysis |
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| `scripts/analyze_cache_hit.py` | Theoretical vs actual KV cache hit ratio |
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| `scripts/analyze_breakdown.py` | Per-request stage breakdown from proxy |
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| `scripts/gpu_monitor.sh` | 5s-interval GPU utilization sampling |
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### Reproducing
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```bash
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# On dash0, activate env
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cd ~/agentic-kv && source .venv/bin/activate
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# Sample trace
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python scripts/sample_trace.py --input ~/ali-trace/trace-glm5.1-formatted/051315-051317.jsonl \
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--output traces/sampled_1000req_seed42.jsonl --target-requests 1000 --seed 42
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# Combined TP=1 DP=8 + cache-aware scheduler
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for i in $(seq 0 7); do
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MASTER_PORT=$((29500+i)) CUDA_VISIBLE_DEVICES=$i vllm serve $MODEL \
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--port $((8000+i)) --tp 1 --enable-prefix-caching --enforce-eager &
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done
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python scripts/cache_aware_proxy.py --combined http://127.0.0.1:800{0..7} --port 9090
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python -m replayer --trace traces/sampled_1000req_seed42.jsonl \
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--endpoint http://localhost:9090 --time-scale 10 --max-inflight-sessions 8
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# Breakdown data
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curl http://localhost:9090/breakdown | python scripts/analyze_breakdown.py /dev/stdin
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```
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