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Gahow Wang 31cf8c9b11 DR-fix A/B: env-gate hash sync drops slope from +81 to -0.7 μs/1k blocks

Adds an env-gated skip for the per-step `set(cache.keys())` walk in
MooncakeConnectorScheduler.build_connector_meta() that was introduced
in our own commit a7df84b (Direct RDMA read). Re-runs the cache_sweep
A/B with three configs: plain (control), mooncake_both (baseline), and
mooncake_both_drfix (VLLM_MOONCAKE_DISABLE_DIRECT_READ_SYNC=1).

Files:
  apply_direct_read_fix.py  one-line env-gate patch (markered revert)
  run_drfix.sh              orchestrator for plain + mooncake_both + drfix
  analyze.py                extended to compare mooncake_both_drfix vs plain
                            and mooncake_both vs mooncake_both_drfix
  REPORT_DRFIX.md           findings
  results/20260526_1543_drfix/ run artifacts

Headline:

  config                | slope (μs/1k blocks) | step_dur p50 @ 16.6k
  ----------------------|----------------------|---------------------
  mooncake_both         | +81.0                | 1 550 μs
  mooncake_both_drfix   | -0.7  (≈ 0)          |    95 μs
  plain (control)       | -1.8  (≈ 0)          |    72 μs

  build_meta p50 @ 16.6k blocks:
    mooncake_both        = 1 459 μs
    mooncake_both_drfix  =     6 μs    (residual loop bookkeeping)

  worker get_finished p50:
    mooncake_both        = 178 μs    (unchanged; this fix doesn't touch it)
    mooncake_both_drfix  = 183 μs

The fix recovers 1 453 μs (99.6 %) of the scheduler-side cost at
|cache|=16.6k blocks. drfix's per-bin step_dur tracks plain within
±50 μs across the full cache range — that's noise-level. The slope
goes from +81 to essentially zero.

Worker-side get_finished (180 μs constant) is unchanged because the
DR-fix touches scheduler.build_connector_meta only. That's the next
target if we want to bring kv_both fully back to plain-level.

Extrapolation to trace-replay (|cache|≈13k, APC≈79%):
  before: build_meta 1 060 μs + get_finished 180 μs = 1.24 ms/step
  after DR-fix: build_meta 6 μs + get_finished 180 μs = ~0.19 ms/step
  → 85% reduction in per-step connector cost
  → TPOT inflation drops from ~+18% to ~+3% on a 7 ms decode step

Confirms: the entire O(|cache|) slope was introduced by our own
direct-RDMA-read implementation (commit a7df84b), not upstream
Mooncake. Production fix: gate the sync on the presence of any
direct_read consumer, or replace per-step diff with an incremental
delta listener fed by block_pool add/remove callbacks.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

2026-05-27 00:03:23 +08:00