小结：
1、最为基础的召回链路就是要保证召回层的相关性，但是相关性高的广告并不一定具有很高的商业价值，所以开始尝试将一些商业化业务指标作为召回的依据
百度凤巢新一代广告召回系统——“莫比乌斯” - 知乎 https://zhuanlan.zhihu.com/p/146210155
MOBIUS: Towards the Next Generation of Query-Ad Matching in Baidu's Sponsored Search http://research.baidu.com/Public/uploads/5d12eca098d40.pdf
一、创新点
1、在召回层保证相关性的同时引入了CPM等业务指标作为召回的依据
2、将以往的CTR预估模型融合到召回层中，提出一种全新的多目标商业召回系统架构
二、论文背景
在大部分公司的商业广告系统架构中，都会采用经典的“漏斗”结构，即召回——粗排——精排——重排序等模块，在现有的召回模块中，应该首先保证召回内容的相关性，保证用户的搜索query和候选广告之间的匹配程度，最为基础的召回链路就是要保证召回层的相关性，但是相关性高的广告并不一定具有很高的商业价值，所以开始尝试将一些商业化业务指标作为召回的依据，比如CPM（可以理解为变现能力）。但是如果单纯的依靠CPM来进行召回的话又无法保证召回的准确性，会出现大量的badcase，即“所答非所问”，这种情况对于用户的体验来说是非常不友好的，所以召回层基本都是在保证相关性的大前提下尽量的筛选出一些变现能力较强的候选广告进入排序阶段。
KEYWORDS Sponsored search; query-ad matching; active learning; click-through rate (CTR) prediction; approximate nearest neighbor (ANN) search
Figure 4: The distinct objectives of matching and ranking layer lead to lower CPM which is one of the key business indicators of a sponsored search engine. Therefore, we are engaged in building a high-efficient query-ad matching system (i.e., Mobius) in Baidu sponsored search. Mobius is expected to unify the learning objectives of the query-ad relevance and many other business indicators together, subject to lower response latency, limitation of computation resources and tiny adverse impact on user experience. For now, we have deployed the first version of Mobius (Mobius-V1) which can more accurately predict CTR for billions of user query & ad pairs.