[https://nvbugs/6647405][fix] Do not sleep out the KV transfer poll interval with no in-flight session

[chuangz0]

Description

Fixes the disagg gen_only perf regression bisected to #17535 (nvbugs 6627789 / 6647405: disagg_upload-gen_only-gb200_deepseek-r1-fp4_8k1k_con1_ctx1_dep4_gen1_tep8_eplb0_mtp3_ccb-NIXL, total token throughput −27.7%).

Root cause

The idle executor loop calls check_context_transfer_status(1) on every iteration where no batch is scheduled (_check_disagg_transfer_progress_when_idle, py_executor.py). In KvCacheTransceiverV2._poll_sessions_for_interval the exit condition — completed + failed >= wait_num — can only ever count in-flight sessions. Once _ever_had_send_session is set and _send_sessions is empty, the target is unsatisfiable and the helper sleeps out the full kv_transfer_sender_future_timeout_ms (default 1000 ms) on every idle iteration.

A newly arrived request then waits for the current sleep to expire before _schedule() can pick it up, so prefill start is delayed by up to one full interval per request. #17535 made this path reachable by (deliberately, for a hang fix) dropping TRTLLM_DISABLE_KV_CACHE_TRANSFER_OVERLAP=1 from the CTX worker, whose early-return guard had hidden the defect. The artifacts quantize to exactly one interval per request handover:

measurement	good (ec3e1a13)	bad (71f025e9)
CTX post-prefill idle iter host_step_time	210 ms	1151 ms
GEN idle wait before next request's KV (num_scheduled_requests = 0 iter)	480 ms	1457 ms
benchmark duration	2.46 s	3.31 s

Beyond the benchmark, any Python-transceiver disagg deployment without the CTX overlap-disable env var pays up to 1000 ms of extra TTFT per request whenever the CTX worker goes idle between requests.

Fix

Clamp wait_num to len(sessions) at the top of _poll_sessions_for_interval and return immediately when nothing is in flight.

• Multi-rank safety: the clamp is purely local — the helper contains no collectives, so ranks with divergent session counts (e.g. DP4 CTX with a single request) return at different times without mismatching the _ctx_consensus / _ctx_consensus_outcome collectives that follow in check_context_transfer_status. This is deliberately not a gate on the live session dict at the top of check_context_transfer_status, which the existing comment there rules out as rank-unsafe (a cancel clears the dict per-rank).
• [None][chore] Scope TRTLLM_DISABLE_KV_CACHE_TRANSFER_OVERLAP to the gen worker in disagg gen_only #17535 semantics preserved: with sessions in flight the poll still waits, so KV blocks keep being released and the gen_only hang fix stands. The CTX flag is not re-added anywhere.
• Parity with the C++ runtime: CacheTransceiver::checkContextTransferStatus iterates mSenderFutures and exits immediately when it is empty; the Python V2 runtime now behaves the same.
• check_gen_transfer_status shares the helper via _poll_gen_sessions_for_poll_interval (kv_transfer_poll_interval_ms, default 5000 ms), so its empty-session case is covered by the same clamp.

Complementary to #18011, which makes the d_mean_gen_worker_per_iter_device_step_time gate honest by excluding idle-successor iterations; this PR removes the underlying scheduling delay that gate accidentally caught.

Test Coverage

Seven new cases appended to the existing tests/unittest/disaggregated/test_transceiver_bounded_polling.py (collected both by the unittest/disaggregated directory entry in l0_cpu.yml and the per-file entry in l0_h100.yml):

• empty session dict with wait_num=1 returns immediately (the regression scenario);
• wait_num above the session count waits only for what can complete;
• an in-flight session is still awaited until completion ([None][chore] Scope TRTLLM_DISABLE_KV_CACHE_TRANSFER_OVERLAP to the gen worker in disagg gen_only #17535 semantics);
• a never-completing session is still released at the deadline;
• a failed session satisfies the exit condition;
• an already-completed session satisfies wait_num=1 despite an in-flight peer;
• completion observed only through the wait_complete(blocking=False) pump (not wall clock) exits the poll.

Full file passes: 50 passed, 1 skipped.

An adversarial multi-agent review (4 lenses: caller behavior changes, multi-rank collective safety, #17535 semantics preservation, test quality; each finding challenged by 2 independent skeptics) raised 10 candidate issues and confirmed none — notably: the PP scheduler retry loop at py_executor.py:2541 is outcome-equivalent (its pre-fix 1 s sleeps could not free KV with no sends in flight, and the C++ runtime it was written against already returns instantly), and the fast idle loop matches existing C++/V1 behavior on main.

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Dev Engineer Review

• _poll_sessions_for_interval clamps wait_num to the number of active sessions.
• The helper returns immediately when no sessions are active.
• In-flight sessions continue polling until completion, failure, or timeout.
• The change is rank-local and preserves existing polling semantics.
• The shared helper also updates generation transfer polling.
• State-cache layer groups now use CacheKind.STATE and carry the request Mamba state slot in KVSlice.block_ids_per_layer_groups.
• State transfer byte accounting includes valid state slots across local layers and pool views.
• No public API, configuration, or test-list files changed.
• The implementation is consistent with the stated performance fix. No correctness or regression issues were identified.

QA Engineer Review

• Added coverage in tests/unittest/disaggregated/test_transceiver_bounded_polling.py.
• Tests cover:
  • Empty sessions.
  • Wait counts larger than the active session count.
  • Completed sessions.
  • Failed sessions.
  • Completion through nonblocking polling.
  • Timeout-bounded polling.
  • An already-completed session satisfying the wait count while another session remains in flight.
• The existing bounded-polling suite reports 50 passing tests and 1 skipped test.
• The tests run through the existing per-file l0_h100 entry.
• No test-list files were modified.
• Verdict: sufficient.

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---

Walkthrough

The transceiver now carries Mamba state slots in state cache groups and counts state bytes across local layers and pool views. Session polling now clamps wait targets to active sessions and returns immediately for zero targets. Tests cover bounded polling behavior.

Changes

Transceiver updates

Layer / File(s)	Summary
Mamba state transfer and byte accounting tensorrt_llm/_torch/disaggregation/transceiver.py	The transceiver resolves Mamba slots for V1 and V2 cache managers, stores slots in state cache groups, and counts state bytes across local layers and pool views.
Bounded session polling and validation tensorrt_llm/_torch/disaggregation/transceiver.py, tests/unittest/disaggregated/test_transceiver_bounded_polling.py	_poll_sessions_for_interval clamps the wait target to active sessions and returns immediately for zero targets. Tests cover completion, failure, deadlines, already-completed sessions, and pump-driven completion.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 90572

The change prevents unnecessary idle polling delays when no KV-transfer session is in flight and adds focused regression coverage; no actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: bo-nv, pcastongay

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 51.85% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 27 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title follows the required NVBugs and fix format and clearly identifies the main change: preventing the KV transfer poll interval from sleeping when no session is in flight.
Description check	✅ Passed	The description is complete and relevant. It explains the regression, root cause, fix, safety considerations, preserved semantics, test coverage, and checklist status.
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.

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[coderabbitai]

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Inline comments:
In `@tests/unittest/disaggregated/test_poll_sessions_interval.py`:
- Line 26: Update the test functions in test_poll_sessions_interval.py to
include the required -> None return annotations, and replace Optional[float]
with float | None. Remove the Optional import if it is no longer used, while
preserving the existing test behavior.

Apply the same fix in
`@tests/unittest/disaggregated/test_poll_sessions_interval.py` around lines 61 -
107: The same annotation cleanup applies to the remaining test functions.

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[chienchunhung]

Thanks for the PR! LGTM.

The local clamp fixes the unsatisfiable empty-session wait while preserving in-flight polling and the outer rank-consensus sequence. The focused regression coverage is sufficient.

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