Follow-up to the LMetric sweep: rerun with --policy linear (cache-aware
load + sticky session affinity, the cache_aware_proxy default) and cap
each PD-disagg arm at 2x the colo bench wall (SIGTERM bench.sh once cap
is exceeded; the cleanup trap clears vLLM and proxy; capped runs lack
metrics.summary.json so the analysis script computes from raw
metrics.jsonl).
Headline: the success-rate ceiling is policy-invariant.
arm linear (capped at 2x) lmetric (uncapped)
colo 807/807 = 100%, 964s 807/807 = 100%, 1021s
pd6 (6:2) 472/807 = 58%, 2280s ⊗ 474/807 = 59%, 3325s
pd4 (4:4) 349/807 = 43%, 2281s ⊗ 348/807 = 43%, 6850s
pd2 (2:6) 176/807 = 22%, 2280s ⊗ 180/807 = 22%, 19275s
Routing affects only how much wall is wasted timing out unreachable
requests at 600s each. Linear hits the same ceiling in 2280s as
LMetric does in 3300-19000s. This *strengthens* the §5 D-pool
capacity-ceiling thesis -- the cap is structural, not a routing
artifact.
Artifacts:
analysis/v2/fig4r_linear.json -- 4-arm linear summary
analysis/v2/PD_DISAGG_LMETRIC.md -- extended with wall-cap section
figs/v2/fig4_linear_vs_lmetric.png -- 3-panel side-by-side comparison
microbench/fresh_setup/plot_fig4_linear_vs_lmetric.py
Anchor experiment for the clean-stack PD comparison using the canonical
cache-aware proxy with --policy lmetric (scripts/bench.sh harness). Two
traces x four arms = eight runs on dash1.
Headline: with the right routing baseline (LMetric), PD-colo holds 100%
completion on both traces while every static PD-disagg ratio fails
(14-65% completion), and the failure mode rotates with the split --
no static partition has a working operating point on this workload.
LMetric improves colo dramatically (TTFT p50 1.0s vs original §3 RR
7.0s; 7x) but does NOT rescue PD-disagg, confirming the bottleneck is
structural (D-pool admission + multi-turn KV accumulation), not routing.
Completion matrix:
first600s full
colo 100% 100%
pd6 (6:2) 58.7% 65.3% (decode-bound)
pd4 (4:4) 43.1% 43.9% (both bottlenecks)
pd2 (2:6) 22.3% 13.9% (prefill-bound)
The original §3 RR "100% PD completion" appears to be a measurement
artifact of e13391e: producer-KV eviction acted as a relief valve,
letting more requests squeeze under the 600s timeout at the (uncosted)
price of cross-turn re-prefill. With the eviction off, PD-disagg is
worse than §3 advertised, not better.
Artifacts:
analysis/v2/fig4l_lmetric.json -- 8-arm summary data
analysis/v2/PD_DISAGG_LMETRIC.md -- writeup + reproduce recipe
figs/v2/fig4_lmetric_pd_vs_colo.png -- 4-panel comparison figure
microbench/fresh_setup/plot_fig4l_lmetric.py -- plot script
User flagged unified_v2 as a still-buggy build. Regenerate the four
per-policy figures with only the four stable policies:
lmetric, load_only, sticky, unified
Story is now directly comparable to v1: unified still dominates p90
TTFT (8.8s) and E2E p90 (20.0s) over the other three on the fresh run.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
User-provided fresh run with five policies (lmetric, load_only, sticky,
unified, plus a new unified_v2 variant). Reproduces the v1 set under
figs/v2/ so we can A/B the same panels:
f4a_apc_loss.png — APC bars per policy
f4c_per_worker_ttft.png — per-worker TTFT p90 panel per policy
f6_e2e_latency_bars.png — TTFT/TPOT/E2E p90 bars per policy
f6_e2e_latency_full_grid — mean/p50/p90/p99 × TTFT/TPOT/E2E grid
scripts/render_b3_figures_v2.py is a standalone driver that reads each
policy's metrics.summary.json and breakdown.json directly from the run
directory — the breakdown.json `routed_to` field is required to recover
per-worker assignment because the new setup routes every request
through a proxy (127.0.0.1:9300), so metrics.jsonl's endpoint_url no
longer identifies the backend.
Headline numbers, new vs v1:
APC v2: lmetric 57.2% / load_only 53.9% / sticky 77.7%
unified 78.7% / unified_v2 78.4%
v1: lmetric 56.9% / load_only 54.1% / sticky 77.2% / unified 79.4%
TTFT p90 (s) v2: lmetric 14.8 / load_only 20.1 / sticky 14.8 /
unified 8.8 / unified_v2 10.1
v1: lmetric 15.7 / load_only 20.2 / sticky 18.0 / unified 7.3
E2E p90 (s) v2: lmetric 25.4 / load_only 33.9 / sticky 30.3 /
unified 20.0 / unified_v2 24.1
v1: lmetric 24.8 / load_only 33.5 / sticky 34.6 / unified 18.0
Worker p90 (s, median / max)
v2: lmetric 13.3/30.4 · load_only 21.3/29.2 · sticky 13.5/33.0
unified 10.0/35.1 · unified_v2 8.6/34.2
v1: lmetric 13.9/31.3 · load_only 19.4/25.1 · sticky 20.3/55.4
unified 10.3/37.7
Story is unchanged: unified dominates at p90 across TTFT/E2E and on
median-worker latency; unified_v2 is competitive at p50 but slightly
worse than unified at p90.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
