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>
The upstream mooncake_connector_proxy round-robins both P and D
selection. For agentic multi-turn sessions this destroys prefix-cache
reuse on the producer side — every turn of a session lands on a
different P, so the prefix-cache hit ratio collapses to 0 (observed in
the 6P+2D round-robin baseline) and every turn re-prefills from
scratch, piling extra load on the P pool.
Add an env-gated routing mode so the same proxy serves both arms of a
clean A/B:
MB5_P_ROUTING=rr round-robin (default, = upstream behavior)
MB5_P_ROUTING=session consistent md5 hash on X-Session-Id -> same
producer for all turns of a session
Decode side stays round-robin (load balance) in both modes — decode
KV is freshly transferred per turn, so D gains nothing from affinity
but everything from even load spreading.
mb5_launch.sh threads MB5_P_ROUTING through to the proxy and logs the
active mode. Default path is byte-for-byte the old behavior, so an
in-flight round-robin sweep is unaffected if this is redeployed.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Three independent bugs were blocking PD-disagg smoke; each fix is
isolated so the next PD experiment doesn't re-hit them.
1. mb5_launch.sh
- stop_all() also kills mb5_pd_proxy.py (our vendored copy),
not just the upstream filename, and asserts ports 8000-8007 +
PROXY_PORT are free before launching — stale proxies were
silently passing the readiness check.
- Proxy readiness uses a generic "any HTTP response" probe;
mooncake_connector_proxy only exposes /v1/completions so
/v1/models 404 is expected.
2. mb5_pd_proxy.py (vendored from third_party so deploy.sh ships it)
- Force min_tokens=1 on the prefill leg. Clients that set
min_tokens == max_tokens (our replayer does) collide with
vLLM's min_tokens<=max_tokens check after the proxy caps
max_tokens=1.
3. instrument_kv_snapshot.py
- Adds a second patch target: initialize
MooncakeConnectorWorker.bootstrap_server = None in __init__.
vLLM 0.18.1 only sets it under the is_kv_producer branch, so
kv_consumer hits AttributeError as soon as the first remote
prefill request lands.
- apply/revert refactored to iterate over (path, patches) pairs.
plot_kv_pool_timeline.py also handles snapshot files that never
captured a running request (would otherwise IndexError on an empty
stackplot input).
Smoke: 4P+4D × 20 reqs → 20/20 success, mean 3.9s, p99 17s, 8 PIDs
all writing snapshots (601 total), well above the 8C baseline.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Two bugs caught by 8C smoke:
mb5_launch.sh
${env_bp_arg} expanded as a literal command line prefix doesn't work
when env_bp_arg is itself a variable — bash only treats VAR=val as
an env assignment if it sees the literal in the parsed command, not
after expansion. Fix: always export VLLM_MOONCAKE_BOOTSTRAP_PORT as
a literal, defaulting to 9999 when caller passed no port (consumer
mode ignores the var so the placeholder is harmless).
mb5_run.sh
replayer's actual CLI flags are --trace / --output / --endpoint /
--model, not the --*-path / --*-name variants I had. Plus dash1
has no `bc`; compute wall_clock_s via python instead.
Both fixed; 8C smoke (CONFIG=8C REPS=1 REQUEST_LIMIT=20) now runs
end-to-end in ~30 s:
- 8 vLLM kv_both instances on GPU 0-7 come up
- replayer round-robins 20 reqs across them
- MB5 instrumentation captures 8 snapshot files (one per EngineCore
PID), ranging 7-139 snapshots each = ~10 Hz throttle works
- plot_kv_pool_timeline.py renders the stacked-area + queue-depth
chart cleanly (figs/mb5_smoke/*.png)
Pipeline validated. Ready for the real PD-ratio sweep.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Three new files to drive the PD ratio sweep + per-request KV occupancy
capture, plus a deploy.sh update so the patched replayer rides along
to the fresh-venv host.
mb5_launch.sh
One script handles all four configs we plan to sweep:
CONFIG=8C / 6P+2D / 4P+4D / 2P+6D
- For 8C: 8 vLLM instances with kv_role=kv_both on GPU 0-7. Replayer
talks to them via the existing comma-separated round-robin in
replayer/replay.py — no proxy.
- For PD configs: kv_role=kv_producer for the P pool (with
VLLM_MOONCAKE_BOOTSTRAP_PORT) + kv_role=kv_consumer for the D pool,
routed by the official vLLM example
third_party/vllm/examples/online_serving/disaggregated_serving/
mooncake_connector/mooncake_connector_proxy.py — no policy choice
made by us, per user instruction to use the standard recipe.
- Applies instrument_kv_snapshot.py before launching so every
EngineCore writes its per-step KV snapshot to
$RUN_ROOT/kv_snapshots/mb5_kv_snapshot_pid<pid>.jsonl
- Reverts the patch on stop.
- Emits ENDPOINTS= line on stdout for the orchestrator to read.
mb5_run.sh
For each CONFIG × rep: launch, replay w600 trace via the existing
replayer, capture wall-clock, tear down, cool down 10 s. Defaults:
CONFIGS="8C 6P+2D 4P+4D 2P+6D"
REPS=3
TRACE=traces/w600_r0.0015_st30.jsonl
All artefacts go under $FRESH_ROOT/mb5_runs/$RUN_TAG_${config}_rep${rep}/
(vllm_logs/, kv_snapshots/, replay_metrics.jsonl, wall_clock_s.txt).
plot_kv_pool_timeline.py
Reads one or more mb5_kv_snapshot_pid*.jsonl files and renders a
stacked-area chart per file:
x = wall-clock since first snapshot
y = KV block count, stacked by per-request contribution
overlay: pool-total ceiling, 90% line, waiting-queue depth subplot
Bands are colored by a deterministic hash of request_id so individual
requests are visually tractable across the run.
This is the figure the user asked for — turns headline "PD-disagg is
10× worse" into a system-level picture of *where* the KV pool is
blocked, when, and by which requests.
deploy.sh
Also tar-syncs the local replayer/ dir to
/home/admin/cpfs/wjh/agentic-kv-fresh/replayer/ so mb5_run.sh can
`python -m replayer` against the patched (trace_span_s/amplification)
version, not the older copy under /home/admin/cpfs/wjh/agentic-kv/.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
