[None][test] Disable ctx overlap scheduler for deepseek-v4-pro con8 disagg config

[chenfeiz0326]

Description

Sets worker_config.ctx.disable_overlap_scheduler: true for
gb300_deepseek-v4-pro-fp4_8k1k_con8_ctx1_dep4_gen4_tep8_eplb0_mtp3_ccb-NIXL.

#18346 removed the unconditional ctx_config["disable_overlap_scheduler"] = True from
the ctx_only path on the rationale that "the overlap scheduler works fine for a
context-only run". That holds at high concurrency, but not for this config. Measured on
oci-aga GB300, 3 arms x 2 reps, base 21dc97fbc8 (pre-#18457) with #18457 applied as the
third arm:

ctx disable_overlap_scheduler	#18457	iterations	req/s
true (this PR)	–	22	7.82 / 7.75
false (today, pre-#18457)	–	34	6.00 / 5.99
false (today, main)	applied	33	6.20 / 6.25

+25% over current main for the ctx_only case. #18457 is merged and active in the
false arm above (verified: KVCM V2 gate passes, IndexMapper capacity 3 -> 5), so this
is not something the merged fix already covers.

Why the overlap scheduler cannot win here

perf-sanity drives --max-concurrency 8 at request_rate=inf, i.e. a closed loop:
nothing arrives except as an echo of a completion. With per-iteration/per-rank counters
added to py_executor.py (forwarded, router in/assigned/scheduled, plus each rank's
pre-router active-list size and how many of those are GENERATION_TO_COMPLETE), the
steady state is:

• A(N) = F(N) — no real backlog; everything resident is a retiring request
• R(N) = F(N-1) — deferred teardown: the request is still GENERATION_TO_COMPLETE at
  N+1 and only retires there, so its response leaves one iteration late
• A(N) = R(N-2) — permit release, new POST, next fetch

so F(N) = F(N-3): a 3-iteration permit round-trip, against 2 with overlap disabled
(confirmed by E2E latency in iteration units, 2.00 vs 2.78). Sustained cadence is then
fixed by Little's Law on the fetch grid:

in-flight = cadence x period
    8     =  4.00   x   2     overlap off   (measured 4.000)
    8     =  2.67   x   3     overlap on    (measured 2.696 / 2.727)
   12     =  4.00   x   3     what overlap-on would need to match

7456-token prompts against max_num_tokens: 8192 allow 1 ctx request per rank per
iteration, so the cap is 4/iteration at dep4; both overlap-on arms average 2.67 of 4,
i.e. the scheduler is never the binding constraint — concurrency is. Matching
overlap-off at dep4 would need 12 concurrent requests and the config specifies 8. No
scheduler or balancing change closes that; #18457 removes the phantom admission charge
(measured: summed charge 86 -> 3, exactly the overlap-off value) but not the extra
iteration of residency, which is why it buys 1 iteration of 34.

Scope

This config generates four CI tests, and the ctx worker's config is shared by all of
them, so all four baselines shift once after merge:

test	measured here
aggr_upload-ctx_only-...	yes, +25%
disagg_upload-e2e-...	no
disagg_upload-gen_only-...	no
disagg_upload-e2e-time_breakdown-...	no

Only ctx_only was measured. The three disagg modes are expected to be neutral-to-better
(the same deferred-teardown residency applies to their ctx worker) but that is a
prediction, not a measurement — happy to measure them before merge if reviewers prefer.

A per-config yaml value is used rather than restoring the ctx_only override because the
override would re-disable overlap for all 24 ctx_only configs including the
high-concurrency ones, where overlap plus #18457 beats overlap-off by ~2%. Six
gb300_nemotron-ultra-v3-fp4_* configs already set ctx disable_overlap_scheduler: true,
so this needs no new grammar.

Follow-ups, not addressed here

• The threshold is concurrency < 3 x ctx_dp, so five other ctx_only configs are
  predicted affected — the three con1 cases most strongly (a 3/2 period ratio on a
  single-user latency measurement). They are left alone pending measurement.
• The underlying product issue stays open on nvbug 6778910. The lever is residency, not
  accounting: emitting the response before the following _fetch_new_requests would make
  R(N) = F(N) and drop the period to 2. Overlap-on iterations are already cheaper than
  overlap-off (457 vs 502 ms), so fixing residency should land this config ~10% above
  the overlap-off number this PR restores, rather than merely matching it.

Test Coverage

Config-only change; no product code touched. The emitted value was verified through the
ctx_only build path, which copies worker_config.ctx wholesale
(ctx_config = dict(worker_config.get("ctx", {})), test_perf_sanity.py:3511) into
ServerConfig.disable_overlap_scheduler (:1530) and reports it to the perf DB as
b_disable_overlap_scheduler (:1690), so the shift is attributable in the baseline
history:

ctx.disable_overlap_scheduler = True    # this PR
gen.disable_overlap_scheduler = False   # unchanged

The gen: block of this yaml carries its own disable_overlap_scheduler at line 76 and
is deliberately left at false; only the ctx: occurrence at line 106 is changed.

PR Checklist

• Single-line config change, verified to land in the ctx: block only
• Measured, 2 reps per arm, counters bit-identical across reps
• Perf baselines for the four affected tests to be refreshed after merge

🤖 Generated with Claude Code

Dev Engineer Review

The GB300 configuration now sets worker_config.ctx.disable_overlap_scheduler: true. The change affects only the context worker; the generation worker remains unchanged. Reported measurements show approximately 25% higher ctx_only throughput. Three related disaggregated baselines require refresh after merge.

QA Engineer Review

One performance-sanity configuration file changed. No test functions, test IDs, selectors, or waivers changed. The ctx_only case was measured and the emitted configuration value was verified. Three related disaggregated tests require baseline refreshes. Coverage verdict: needs follow-up.

Per-File QA Perspective

• tests/scripts/perf-sanity/disaggregated/gb300_deepseek-v4-pro-fp4_8k1k_con8_ctx1_dep4_gen4_tep8_eplb0_mtp3_ccb-NIXL.yaml: This configuration disables overlap scheduling for the context worker only. QA should verify the emitted value and refresh the related performance baselines after merge.

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Walkthrough

The context worker configuration changes disable_overlap_scheduler from false to true.

Changes

Scheduler Configuration

Layer / File(s)	Summary
Context worker scheduler setting tests/scripts/perf-sanity/disaggregated/gb300_deepseek-v4-pro-fp4_8k1k_con8_ctx1_dep4_gen4_tep8_eplb0_mtp3_ccb-NIXL.yaml	The context worker now sets disable_overlap_scheduler to true.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~2 minutes

Change: Other

Suggested reviewers: bowenfu

Merge Risk: ⚪ Minimal · up to 8b0f7

No actionable current-head risk is established; the remaining baseline refreshes are planned validation follow-up.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the configuration change: disabling the context overlap scheduler for the specified DeepSeek disaggregated test configuration. It uses the required ticket and type format.
Description check	✅ Passed	The description is complete and relevant. It explains the change, performance rationale, measurements, scope, test coverage, unchanged generation settings, and follow-up considerations. It also includ…

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