Evidence-backed analysis with per-request matched comparison:
1. KV CACHE MEMORY WALL (Evidence 3)
Combined: 12% KV cache per instance (comfortable)
PD-Sep 6P+2D: 48-97% on decode instances (saturation -> 100s waits)
2. KV TRANSFER OVERHEAD (Evidence 4, matched requests)
Mean 1.79s extra TTFT per request, 3.3x slower overall
Small requests (<5k) hit 8.0x ratio (transfer dominates prefill)
Large requests (>50k) hit 1.3x ratio (prefill dominates)
3. SESSION AFFINITY BROKEN (Evidence 5)
Combined: turn N+1 hits same GPU -> 80% multi-turn APC
PD-Sep: turn N+1 prefill on P has NO prior KV (sent to D) -> 0% APC on P
Must re-prefill + re-transfer on every turn
4. GPU UNDERUTILIZATION (Evidence 2)
PD-Sep: 12-17% GPU util (decode is memory-bound, wastes GPU compute)
Combined: 28-54% GPU util (flexible P+D on same GPU)
Root cause: agentic workloads break PD-Sep's assumptions (short input,
no prefix sharing, compute-heavy prefill) with long context, 91%
intra-session KV reuse, and lightweight MoE compute.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Gahow Wang
7f93d36970