177 lines
5.3 KiB
Markdown
177 lines
5.3 KiB
Markdown
# Bad-start stop counterexample - 2026-06-26
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本文记录一次有意构造的 adversarial bad-start 测试。它的目的不是证明 harness 已经
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robust，而是攻击当前实现，确认它是否会从明显不合理的初始配置中恢复。
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结论：
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```text
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当前 production/prototype harness 还不能支持 bad-start robustness claim。
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它会在高 GPU、高 TP 的坏起点上被 search_high_saturated_by_incumbent 提前 stop，
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没有测试 topology/resource-efficiency contrast。
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```
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这不是一个需要补 `TP=8 -> TP=4` 特例规则的问题。它暴露的是更基础的 stop authority
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问题：measurement saturation 不能绕过 coverage-relative candidate set。
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## 实验设置
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机器：`dash1`，8x H20。
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目标：从一个故意不合理的初始配置开始：
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```text
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tensor-parallel-size = 8
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data-parallel-size = 1
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gpu-memory-utilization = 0.5
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max-num-seqs = 8
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LLM endpoint disabled
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```
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期望行为：
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- harness 不应只因为 baseline feasible 就停止；
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- 它至少应生成 topology/resource-efficiency contrast candidate；
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- 对 `req/s/GPU` 目标，8 GPU incumbent 需要被低 GPU 或邻域 topology probe 验证。
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## Run A: 低 search.high
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第一轮保留原始 `search.high=0.125`。
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结果：
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```text
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trial-0001 completed
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harness-stop-0002
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tuning_stop_reason = harness_stop
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validator reason = search_high_saturated_by_incumbent
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best request_rate = 1.0333 total
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best request_rate_per_gpu = 0.1292
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pass_rate = 1.0
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```
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解释：这个 run 的 offered-load ceiling 太低，baseline 很容易 saturate `search.high`。
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因此它不能区分“配置真的足够好”和“测量上限太低”。
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## Run B: corrected high search ceiling
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第二轮把 `search.high` 提到 `1.0`，保留同一个 bad-start 配置，`max_trials=3`。
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远端产物：
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```text
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session = adv_badcase_corr_casea_20260626T095356Z
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store = /home/admin/cpfs/wjh/aituner/aituner/.aituner/adversarial-badcase-corrected-casea-20260626T095356Z
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spec = /home/admin/cpfs/wjh/aituner/aituner/.aituner-run-configs/adversarial-badcase-corrected-casea-20260626T095356Z/casea-combined-bad-highsearch.json
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log = /home/admin/cpfs/wjh/aituner/aituner/.aituner/adversarial-badcase-corrected-casea-20260626T095356Z.log
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```
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结果仍然是在 baseline 后 stop：
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```text
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trial-0001 completed
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harness-stop-0002
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no harness-proposal-0002.json
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tuning_stop_reason = harness_stop
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validator reason = search_high_saturated_by_incumbent
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best sampling_u = 0.9375
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best request_rate = 8.033333333333333
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best request_rate_per_gpu = 1.0041666666666667
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pass_rate = 1.0
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```
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Probe trace：
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| sampling_u | request_rate | feasible |
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| --- | ---: | --- |
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| 0.5 | 4.6000 | true |
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| 0.75 | 6.5167 | true |
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| 0.875 | 7.5000 | true |
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| 0.9375 | 8.0333 | true |
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它触发 stop 的原因是当前 guard 计算：
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```text
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binary_probe_resolution = max(tolerance, (high - low) / 2**max_probes)
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= 0.0625
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threshold_gap_to_high = 1.0 - 0.9375
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= 0.0625
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```
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因此当前实现认为 incumbent 已经 saturate `search.high`。
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## 为什么这是反例
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当前 objective 是 SLO-constrained `req/s/GPU`，不是固定 8 GPU 的 total throughput。
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一个 8-GPU incumbent saturate offered-load ceiling，并不能证明：
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- 低 TP / 低 GPU 配置没有更高 `req/s/GPU`；
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- 当前 topology 是资源效率最优；
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- runtime knobs 已经进入合适 trust region；
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- no-LLM harness 能从 bad start 中恢复。
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所以这个 stop 是 unsound 的，至少相对于 bad-start robustness claim 是 unsound。
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更形式化地说：
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```text
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search_high_saturated_by_incumbent
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does not imply
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incumbent_validated(topology/resource-efficiency)
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```
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当目标包含 resource efficiency，并且 parallel-size/topology 仍然 tunable 时，
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`search_high_saturated_by_incumbent` 只能作为 measurement evidence，不能单独作为 stop
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authority。
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## 对新 harness 设计的约束
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这个反例直接约束 declarative harness：
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1. Stop 前必须生成并持久化完整 `CandidateSet`。
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2. Stop proof 必须引用 `candidate_set_hash`。
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3. 如果存在未覆盖的 high-priority topology/resource-efficiency candidate，validator
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必须返回 `eligible_candidates_remain`，即使 incumbent saturate `search.high`。
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4. `search.high` saturation 只能更新 measurement coverage，不能替代
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`incumbent_validated`。
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5. 对 `req/s/GPU` objective，required coverage 必须包含至少一个 topology 或
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resource-efficiency contrast，除非 StudySpec 明确固定 GPU budget 和 topology。
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这也说明当前 repair 方向不能是：
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```text
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if tp == 8 and gmu == 0.5: try tp = 4
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```
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正确方向应该是：
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```text
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ordered topology lattice + resource-efficiency objective
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-> candidate set includes lower/redistributed topology contrast
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-> stop is blocked until that coverage unit is measured or invalidated
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```
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## 当前 verdict
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当前 production harness：
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```text
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prototype, not yet fundamental
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```
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新的 declarative prototype：
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```text
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promising substrate, but not production-proven
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```
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它已经把 `CandidateSet`、`CoverageUnit`、failure region 和 coverage-relative stop 的最小
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接口跑通，但还没接入真实 tuning loop，也还没证明 bad-start distribution 的收敛。
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因此接下来的 P0 gate 是：
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```text
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先实现 coverage-relative stop authority，再重跑 bad-start distribution。
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```
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