Swept session-affinity P routing (MB5_P_ROUTING=session) across all
four ratios on the metrics-fixed stack. Findings:
- Strictly worse than round-robin at every ratio. 4P+4D: round-robin
100% vs session-affinity 36% completion.
- Success DECREASES monotonically as decode capacity grows
(6P+2D 59% -> 4P+4D 36% -> 3P+5D 24% -> 2P+6D 19%) — refutes the
"session prefill is faster so it needs more D" hypothesis.
- GPUs sit at ~0% utilization (2P+6D entirely idle) — the cluster
stalls on KV-transfer/admission coordination, not compute. This is
the deepest anti-PD argument: paid-for hardware does nothing while
requests pile up; colocation keeps every GPU busy.
- Mechanism: session-affinity pins heavy multi-turn sessions onto
single producers (producer hot-pinning, same pathology as sticky
routing in the colocated §3.3 study); fewer producers -> worse
concentration -> the monotonic decline. Failed transfers also pin
producer KV (kv_load_failure_policy=fail), compounding to deadlock.
Verdict: neither ratio tuning nor routing policy rescues static
PD-disagg for this agentic workload — the failure is structural.
mb5_launch.sh: add 5P+3D / 3P+5D ratios for the sweep.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Gahow Wang
6243b78bba