面试题库

14 天用 Agent 搭一条 AI 短剧生产线

D12 成本与模型路由：按环节选模型、缓存、降级与预算熔断

• 一条内容生成流水线的成本要怎么拆？拆完你会先优化哪一环，为什么？How do you break down the cost of a content-generation pipeline, and which stage would you optimize first?

  国内高频海外高频基础#cost-analysis#observability

  分析过程 · 先想清楚再作答

  1. 这题在考你有没有真的看过账单。凭感觉答「多用缓存、少调模型」的人一听就没做过；能说出「按什么维度拆、拆出来大概什么比例」的才是。
  2. 拆的维度要先立住：按环节（脚本、图像、视频、语音）、按计价单位（按秒、按张、按字符、按 token）、按是否计费（成功、命中缓存、失败未扣费）。三个维度缺一个，报表就会有一类花费永远看不见。
  3. 然后给数量级。多媒体生成这类流水线里视频按秒计价，一集十几秒就是几块钱；图像按张几分钱、语音按字符几厘钱、文本更低。结论是视频通常占九成以上，其余全是零头。
  4. 所以优化顺序不是「哪一环最容易优化」，而是「哪一环最贵」。先优化视频，手段按代价从低到高排：缓存与幂等（不重复调）、分档路由（草稿档用便宜规格）、降级（清晰度、时长、镜头数）、最后才是换厂商谈价。
  5. 补一句可信度：不确定的单价不要写进表。官方只给资源包价的档位要标明是折算值，官方没公开的档位就留空只统计用量——把估算值当官方价报上去，是这类项目最常见的翻车点。
  6. 可预期的追问是「那怎么证明优化生效了」。答案是同一份输入跑两遍，对照面板上按环节的金额与调用次数，而不是看月账单——月账单里混着别人的流量，归因不到你这次改动。

  How to reason about it · think before answering

  1. This question checks whether you have actually read a bill. Answering with generic advice like use more caching signals you never ran this in production; naming the breakdown dimensions and rough ratios signals you did.
  2. Establish the dimensions first: by stage (script, image, video, speech), by billing unit (per second, per item, per character, per token), and by billable status (succeeded, cache hit, failed and not charged). Drop any one of them and a whole class of spend becomes invisible.
  3. Then give orders of magnitude. Video is billed per second, so a dozen seconds already costs a few yuan, while images are cents per item, speech is fractions of a cent per character, and text is lower still. Video typically dominates at over ninety percent.
  4. So the priority is driven by what is expensive, not by what is easy to change. Attack video first, cheapest lever to most expensive: caching and idempotency, tiered routing with a cheap draft tier, degradation across resolution, duration and shot count, and only then vendor negotiation.
  5. Add a credibility note: never put an unverified unit price in the table. Mark derived prices as estimates and leave unpublished ones blank while still counting usage. Reporting an estimate as an official price is how these projects lose trust.
  6. Expect the follow-up: how do you prove the optimization worked? Run the same input twice and compare the per-stage panel, not the monthly invoice, which mixes in traffic you did not cause.

  答题要点

  • 按三个维度拆：环节、计价单位、是否真的计费（成功 / 缓存命中 / 失败未扣费）
  • 先给比例再给结论：视频按秒计价，通常占单集成本九成以上，其余是零头
  • 优化顺序由贵到便宜：缓存与幂等、分档路由、降级、最后才谈价换厂商
  • 拿不到的单价宁可留空只统计用量，折算出来的要标明是折算值
  • 验证靠同一份输入跑两遍对照面板，不看混杂的月账单

  Key points

  • Break it down three ways: by stage, by billing unit, and by whether the call was actually charged
  • Lead with the ratio: video is billed per second and usually exceeds ninety percent of per-episode cost
  • Optimize expensive first: caching and idempotency, tiered routing, degradation, vendor negotiation last
  • Leave unknown unit prices blank while still counting usage, and label derived prices as estimates
  • Validate by running the same input twice and diffing the per-stage panel, not the monthly invoice