抖音品质建设 - iOS启动优化《实战篇》

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"前言"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"启动是 App 给用户的第一印象，启动越慢，用户流失的概率就越高，良好的启动速度是用户体验不可缺少的一环。启动优化涉及到的知识点非常多，面也很广，一篇文章难以包含全部，所以拆分成两部分：原理和实战，本文是实战篇。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"原理篇："},{"type":"link","attrs":{"href":"http:\/\/mp.weixin.qq.com\/s?__biz=MzI1MzYzMjE0MQ==&mid=2247486932&idx=1&sn=eb4d294e00375d506b93a00b535c6b05&chksm=e9d0c636dea74f20ec800af333d1ee94969b74a92f3f9a5a66a479380d1d9a4dbb8ffd4574ca&scene=21#wechat_redirect","title":null,"type":null},"content":[{"type":"text","marks":[{"type":"underline"}],"text":"抖音品质建设-iOS 启动优化《原理篇》"}],"marks":[{"type":"underline"}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"如何做启动优化？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"文章的正式内容开始之前，大家可以思考下，如果自己去做启动优化的，会如何去开展？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这其实是一个比较大的问题，遇到类似情况，我们都可以去把大问题拆解成几个小的问题："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线上用户究竟启动状况如何？"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如何去找到可以优化的点？"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"做完优化之后，如何保持？"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有没有一些成熟的经验可以借鉴，业界都是怎么做的？"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"对应着本文的三大模块："},{"type":"text","marks":[{"type":"strong"}],"text":"监控"},{"type":"text","text":"，"},{"type":"text","marks":[{"type":"strong"}],"text":"工具"},{"type":"text","text":"和"},{"type":"text","marks":[{"type":"strong"}],"text":"最佳实践。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"监控"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"启动埋点"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"既然要监控，那么就要能够在代码里获取到启动时长。启动的起点大家采用的方案都一样：进程创建的时间。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"启动的终点对应用户感知到的 Launch Image 消失的第一帧，抖音采用的方案如下："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"iOS 12 及以下：root viewController 的 viewDidAppear"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"iOS 13+：applicationDidBecomeActive"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Apple 官方的统计方式是第一个 "},{"type":"codeinline","content":[{"type":"text","text":"CA::Transaction::commit"}]},{"type":"text","text":"，但对应的实现在系统框架内部，抖音的方式已经非常接近这个点了。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"分阶段"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"只有一个启动耗时的埋点在排查线上问题的时候显然是不够的，可以通过"},{"type":"text","marks":[{"type":"strong"}],"text":"分阶段和单点埋点"},{"type":"text","text":"结合，下面是这是目前抖音的监控方案："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/03\/03b9fc13a2d74d0359b7fa8d602aaefc.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"+load、initializer 的调用顺序和链接顺序有关，链接顺序默认按照 CocoaPod 的 Pod 命名升序排列，所以取一个命名为 AAA 开头既可以让某个 +load、initializer 第一个被执行。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"无侵入监控"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"公司的 APM 团队提供了一种无侵入的启动监控方案，方案将启动流程拆分成几个粒度比较粗的与业务无关的阶段：进程创建，最早的 +load，didFinishLuanching 开始和首屏首次绘制完成。"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/11\/1190e2c9f436b237a620440e09d84677.jpeg","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"前三个时间点无侵入获取较为简单"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"进程创建：通过 "},{"type":"codeinline","content":[{"type":"text","text":"sysctl"}]},{"type":"text","text":" 系统调用拿到进程创建的时间戳"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最早的 +load：和上面的分阶段监控一样，通过 AAA 为前缀命名 Pod，让 +load 第一个被执行"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"didFinishLaunching：监控 SDK 初始化一般在启动的很早期，用监控 SDK 的初始化时间作为 didFinishLaunching 的时间"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首屏渲染完成时间我们希望和 "},{"type":"codeinline","content":[{"type":"text","text":"MetricKit"}]},{"type":"text","text":" 对齐，即获取到 "},{"type":"codeinline","content":[{"type":"text","text":"CA::Transaction::commit()"}]},{"type":"text","text":"方法被调用的时间。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通过 Runloop 源码分析和线下调试，我们发现 "},{"type":"codeinline","content":[{"type":"text","text":"CA::Transaction::commit()"}]},{"type":"text","text":"，"},{"type":"codeinline","content":[{"type":"text","text":"CFRunLoopPerformBlock"}]},{"type":"text","text":"，"},{"type":"codeinline","content":[{"type":"text","text":"kCFRunLoopBeforeTimers"}]},{"type":"text","text":" 这三个时机的顺序从早到晚依次是："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/f1\/f19fa58a373a8f6810bd0d8c722f98c4.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"可以通过在 didFinishLaunch 中向 Runloop 注册 block 或者 BeforeTimer 的 Observer 来获取上图中两个时间点的回调，代码如下："}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"\/\/注册block\nCFRunLoopRef mainRunloop = [[NSRunLoop mainRunLoop] getCFRunLoop];\nCFRunLoopPerformBlock(mainRunloop,NSDefaultRunLoopMode,^(){\n    NSTimeInterval stamp = [[NSDate date] timeIntervalSince1970];\n    NSLog(@\"runloop block launch end:%f\",stamp);\n});\n\/\/注册kCFRunLoopBeforeTimers回调\nCFRunLoopRef mainRunloop = [[NSRunLoop mainRunLoop] getCFRunLoop];\nCFRunLoopActivity activities = kCFRunLoopAllActivities;\nCFRunLoopObserverRef observer = CFRunLoopObserverCreateWithHandler(kCFAllocatorDefault, activities, YES, 0, ^(CFRunLoopObserverRef observer, CFRunLoopActivity activity) {\n    if (activity == kCFRunLoopBeforeTimers) {\n        NSTimeInterval stamp = [[NSDate date] timeIntervalSince1970];\n        NSLog(@\"runloop beforetimers launch end:%f\",stamp);\n        CFRunLoopRemoveObserver(mainRunloop, observer, kCFRunLoopCommonModes);\n    }\n});\nCFRunLoopAddObserver(mainRunloop, observer, kCFRunLoopCommonModes);\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"经过实测，我们最后选择的无侵入获取首屏渲染方案是："}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"iOS13（含）以上的系统采用 "},{"type":"codeinline","content":[{"type":"text","text":"runloop"}]},{"type":"text","text":" 中注册一个 "},{"type":"codeinline","content":[{"type":"text","text":"kCFRunLoopBeforeTimers"}]},{"type":"text","text":" 的回调获取到的 App 首屏渲染完成的时机更准确。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"iOS13 以下的系统采用 "},{"type":"codeinline","content":[{"type":"text","text":"CFRunLoopPerformBlock"}]},{"type":"text","text":" 方法注入 block 获取到的 App 首屏渲染完成的时机更准确。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"监控周期"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/6c\/6cd6fc7436cee58e1448389b9fa520cf.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"App 的生命周期可以分为三个阶段：研发，灰度和线上，不同阶段监控的目的和方式都不一样。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"研发阶段"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"研发阶段的监控主要目的是防止劣化，对应着会有线下的自动化监控，通过实际的启动性能测试来尽早地发现和解决问题，抖音的线下自动化监控流程图如下："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/ca\/ca53981858370fa22b076c13233fb331.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"由定时任务触发，先 release 模式下打包，接着跑一次自动化测试，测试完毕后会上报测试结果，方便通过看板来跟踪整体的变化趋势。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果发现有劣化，会先发一条报警信息，接着通过二分查找的方式找到对应的劣化 MR，然后自动跑火焰图和 Instrument 来辅助定位问题。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那么如何保证测试的结果是稳定可靠的呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"答案就是控制变量："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"关闭 iCloud & 不登录 AppleID & 飞行模式"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"风扇降温，且用 MFI 认证数据线"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"重启手机和开始下一次测试之前静置一段时间"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"多次测量取平均值 & 计算方差"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Mock 启动过程中的 AB 变量"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"实践下来，我们发现 iPhone 8 的稳定性最好，其次是 iPhone X，iPhone 6 的稳定性很差。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了自动化测试，在研发流程上还可以加一些准入，来防止启动劣化，这些准入包括"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"新增动态库"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"新增 +load 和静态初始化"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"新增启动任务 Code Review"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"不建议做细粒度的 Code Review，除非对相关业务很了解，否则一般肉眼看不出会不会有劣化。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"线上 & 灰度"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"灰度和线上的策略是相似的，"},{"type":"text","marks":[{"type":"strong"}],"text":"主要看的是大盘数据和配置报警"},{"type":"text","text":"，大盘监控和报警和公司的基建有很大关系，如果没有对应基建 Xcode MetricKit 本身也可以看到启动耗时：打开 Xcode -> Window -> Origanizer -> Launch Time"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"大盘数据本身是统计学的，会有些统计学的规律："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"发版本的前几天启动速度比较慢"},{"type":"text","text":"，这是因为 iOS 13 后更新 App 的第一次启动要创建启动闭包，这个过程是比较慢的"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"新版本发布会导致老版本 pct50 变慢"},{"type":"text","text":"，因为性能差的设备升级速度慢，导致老版本性能差设备比例变高"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"采样率调整会影响 pct50"},{"type":"text","text":"，比如某些地区的 iPhone 6 比例较高，如果这些地区采样率提高会导致大盘性能差的设备比例提高。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"基于这些背景，我们一般会通过控制变量的方式：拆地区，机型，版本，有时候甚至要根据时间看启动耗时的趋势。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"工具"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"完成了监控之后，我们需要找到一些可以优化的点，就需要用到工具。主要包括两大类："},{"type":"text","marks":[{"type":"strong"}],"text":"Instrument 和自研"},{"type":"text","text":"。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"Time Profiler"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Time Profiler 是大家日常性能分析中用的比较多的工具，通常会选择一个时间段，然后聚合分析调用栈的耗时。但"},{"type":"text","marks":[{"type":"strong"}],"text":"Time Profiler 其实只适合粗粒度的分析"},{"type":"text","text":"，为什么这么说呢？我们来看下它的实现原理："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"默认 Time Profiler 会 1ms 采样一次，只采集在运行线程的调用栈，最后以统计学的方式汇总"},{"type":"text","text":"。比如下图中的 5 次采样中，method3 都没有采样到，所以最后聚合到的栈里就看不到 method3。所以 Time Profiler 中的看到的时间，并不是代码实际执行的时间，而是栈在采样统计中出现的时间。"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/73\/739e1bd94c59375e82c4f3314db3dcc7.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Time Profiler 支持一些额外的配置，"},{"type":"text","marks":[{"type":"strong"}],"text":"如果统计出来的时间和实际的时间相差比较多，可以尝试开启"},{"type":"text","text":"："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"High Frequency，降低采样的时间间隔"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Record Kernel Callstacks，记录内核的调用栈"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Record Waiting Thread，记录被 block 的线程"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"System Trace"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"既然 Time Profiler 支持粗粒度的分析，那么有没有什么精细化的分析工具呢？答案就是 System Trace。"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/61\/61e910d07456afc09a5ab4c1d8b54821.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"既然要精细化分析，那么我们就需要标记出一小段时间，可以用 Point of interest 来标记。除此之外，System Trace 分析虚拟内存和线程状态都很管用："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Virtual Memory："},{"type":"text","marks":[{"type":"strong"}],"text":"主要关注 Page In"},{"type":"text","text":"这个事件，因为启动路径上有很多次 Page In，且相对耗时"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Thread State："},{"type":"text","marks":[{"type":"strong"}],"text":"主要关注挂起和抢占两个状态，记住主线程不是一直在运行的"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"System Load 线程有优先级，高优先级的线程不应该超过系统核心数量"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"os_signpost"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"os_signpost 是 iOS 12 推出的用于在 instruments 里标记时间段的 API，性能非常高，可以认为对启动无影响。结合最开始讲的分阶段监控，我们可以在 Instrument 把启动划分成多个阶段，和其他模板一起分析具体问题："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/56\/561f084fb626488fb30e632bf397f5aa.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"结合 swizzle，os_signpost 可以发挥出意想不到的效果，比如 hook 所有的 load 方法，来分析对应耗时，又比如 hook UIImage 对应方法，来统计启动路径上用到的图片加载耗时。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"其他 Instrument 模板"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了这些，还有几个模板是比较常用的："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Static Initializer"},{"type":"text","text":"：分析 C++ 静态初始化"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"App Launch"},{"type":"text","text":"：Xcode 11 后新出的模板，可以认为是 Time Profiler + System Trace"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Custom Instrument"},{"type":"text","text":"：自定义 Instrument，最简单是用 os_signpost 作为模板的数据源，自己做一些简单的定制化展示，具体可参考 WWDC 的相关 Session。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"火焰图"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"火焰图用来分析时间相关的性能瓶颈非常有用"},{"type":"text","text":"，可以直接把业务代码的耗时绘制出来。此外，火焰图可以自动化生成然后 diff，所以可以用于自动化归因。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"火焰图有两种常见实现方式"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"hook objc_msgSend"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"编译期插桩"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本质上都是在方法的开始和末尾打两个点，就知道这个方法的耗时，然后转换成 Chrome 的标准的 json 格式就可以分析了。注意就算用 mmap 来写文件，仍然会有一些误差，所以找到的问题并不一定是问题，需要二次确认。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"最佳实践"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"整体思路"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"优化的整体思路其实就四步："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/93\/93421fba08590c93d8878225011e5c99.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"删"},{"type":"text","text":"掉启动项，最直接"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"如果不能删除，尝试"},{"type":"text","marks":[{"type":"strong"}],"text":"延迟"},{"type":"text","text":"，延迟包括第一次访问以及启动结束后找个合适的时间预热"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"不能延迟的可以尝试"},{"type":"text","marks":[{"type":"strong"}],"text":"并发"},{"type":"text","text":"，利用好多核多线程"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"如果并发也不行，可以尝试让代码执行"},{"type":"text","marks":[{"type":"strong"}],"text":"更快"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"这块会以 Main 函数做分界线，看下 Main 函数前后的优化方案；接着介绍如何优化 Page In；最后讲解一些非常规的优化方案，这些方案对架构的要求比较高"},{"type":"text","text":"。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"Main 之前"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Main 函数之前的启动流程如下："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"加载 dyld"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"创建启动闭包（更新 App\/重启手机需要）"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"加载动态库"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Bind & Rebase & Runtime 初始化"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"+load 和静态初始化"}]}]}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/a2\/a278c98fb2a7e24002687330213a95a5.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"动态库"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"减少动态库数量可以加减少启动闭包创建和加载动态库阶段的耗时，官方建议动态库数量小于 6 个。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"推荐的方式是动态库转静态库"},{"type":"text","text":"，因为还能额外减少包大小。另外一个方式是合并动态库，但实践下来可操作性不大。最后一点要提的是，"},{"type":"text","marks":[{"type":"strong"}],"text":"不要链接那些用不到的库"},{"type":"text","text":"（包括系统），因为会拖慢创建闭包的速度。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"下线代码"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下线代码可以减少 Rebase & Bind & Runtime 初始化的耗时。那么如何找到用不到的代码，然后把这些代码下线呢？"},{"type":"text","marks":[{"type":"strong"}],"text":"可以分为静态扫描和线上统计两种方式"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最简单的静态扫描是基于 AppCode，但是项目大了之后 AppCode 的索引速度非常慢，另外的一种静态扫描是基于 Mach-O 的："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"_objc_selrefs"}]},{"type":"text","text":" 和"},{"type":"codeinline","content":[{"type":"text","text":"_objc_classrefs"}]},{"type":"text","text":" 存储了引用到的 sel 和 class"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"__objc_classlist"}]},{"type":"text","text":" 存储了所有的 sel 和 class"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"二者做个差集就知道那些类\/sel 用不到，但"},{"type":"text","marks":[{"type":"strong"}],"text":"objc 支持运行时调用，删除之前还要在二次确认"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"还有一种统计无用代码的方式是用线上的数据统计，主流的方案有三种："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"ViewConteroller 渗透率，hook 对应的声明周期方法即可统计"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Class 渗透率，遍历运行时的所有类，通过 Objective C Runtime 的标志位判断类是否被访问"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"行级渗透率，需要用编译期插桩，对包大小和执行速度均有损。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"前两种是 ROI 较高的方案，绝大多数时候 Class 级别的渗透率足够了。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"+load 迁移"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"+load 除了方法本身的耗时，还会引起大量 Page In"},{"type":"text","text":"，另外 +load 的存在对 App 稳定性也是冲击，因为 Crash 了捕获不到。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"举个例子，很多 DI 的容器需要把协议绑定到类，所以需要在启动的早期(+load)里注册："}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"+ (void)load\n{\n    [DICenter bindClass:IMPClass toProtocol:@protocol(SomeProcotol)]\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本质上只要知道协议和类的对应关系即可，利用 clang attribute，这个过程可以迁移到编译期："}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"typedef struct{\n    const char * cls;\n    const char * protocol;\n}_di_pair;\n#if DEBUG\n#define DI_SERVICE(PROTOCOL_NAME,CLASS_NAME)\\\n__used static Class _DI_VALID_METHOD(void){\\\n    return [CLASS_NAME class];\\\n}\\\n__attribute((used, section(_DI_SEGMENT \",\" _DI_SECTION ))) static _di_pair _DI_UNIQUE_VAR = \\\n{\\\n_TO_STRING(CLASS_NAME),\\\n_TO_STRING(PROTOCOL_NAME),\\\n};\\\n#else\n__attribute((used, section(_DI_SEGMENT \",\" _DI_SECTION ))) static _di_pair _DI_UNIQUE_VAR = \\\n{\\\n_TO_STRING(CLASS_NAME),\\\n_TO_STRING(PROTOCOL_NAME),\\\n};\\\n#endif\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"原理很简单："},{"type":"text","marks":[{"type":"strong"}],"text":"宏提供接口，编译期把类名和协议名写到二进制的指定段里，运行时把这个关系读出来就知道协议是绑定到哪个类了"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有同学会注意到有个无用的方法"},{"type":"codeinline","content":[{"type":"text","text":"_DI_VALID_METHOD"}]},{"type":"text","text":" ，这个方法只在 debug 模式下存在，为了让编译器保证类型安全。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"静态初始化迁移"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"静态初始化和 +load 方法一样也会引起大量 Page In，一般来自 C++代码，比如网络或者特效的库。另外"},{"type":"text","marks":[{"type":"strong"}],"text":"有些静态初始化是通过头文件引入进来的"},{"type":"text","text":"，可以通过预处理来确认。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"几个典型的迁移思路："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"std:string 转换成 const char *"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"静态变量移动到方法内部，因为方法内部的静态变量会在方法第一次调用的时候初始化"}]}]}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"\/\/Bad\nnamespace {\n    static const std::string bucket[] = {\"apples\", \"pears\", \"meerkats\"};\n}\nconst std::string GetBucketThing(int i) {\n     return bucket[i];\n}\n\/\/Good\nstd::string GetBucketThing(int i) {\n  static const std::string bucket[] = {\"apples\", \"pears\", \"meerkats\"};\n  return bucket[i];\n}\n"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"Main 之后"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"启动器"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"启动是需要一个框架来管控的，抖音采用了轻量级的中心式方案："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有个启动任务的配置仓，里面只包含启动任务的顺序和线程"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"业务仓实现协议 BootTask，表明这是个启动任务"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"启动任务的执行流程如下："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/8b\/8b6cab47601257d5aa1707b88bdc47c7.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"为什么需要启动器呢？"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"全局并发调度"},{"type":"text","text":"：比如 AB 任务并发，C 任务等待 AB 执行完毕，框架调度还能减少线程数量和控制优先级"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"延迟执行"},{"type":"text","text":"：提供一些时机，业务可以做预热性质的初始化"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"精细化监控"},{"type":"text","text":"：所有任务的耗时都能监控到，线下自动化监控也能受益"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"管控"},{"type":"text","text":"：启动任务的顺序调整，新增\/删除都能通过 Code Review 管控"}]}]}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"三方 SDK"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有些三方 SDK 的启动耗时很高，比如 Fabric，抖音下线了 Fabric 后启动速度 pct50 快了 70ms 左右。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了下线，很多 SDK 是可以延迟的，比如分享和登录的 SDK。此外，在接入 SDK 之前可以先评估下对启动性能的影响，如果影响较大是可以反馈给 SDK 的提供方去修改的，尤其是付费的 SDK，他们其实很愿意配合做一些修改。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"高频次方法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"有些方法的单个耗时不高，但是在启动路径上会调用很多次的，这种累计起来的耗时也不低，比如读 Info.plist 里面的配置："}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"+ (NSString *)plistChannel\n{\n    return [[[NSBundle mainBundle] infoDictionary] objectForKey:@\"CHANNEL_NAME\"];\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"修改的方式很简单，加一层内存缓存即可，这种问题在 TimeProfiler 里时间段选长一些往往就能看出来。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"锁"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"锁之所以会影响启动时间，是因为有时候子线程先持有了锁，"},{"type":"text","marks":[{"type":"strong"}],"text":"主线程就需要等待子线程锁释放。还要警惕系统会有很多隐藏的全局锁"},{"type":"text","text":"，比如 dyld 和 Runtime。举个例子："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下图是 "},{"type":"codeinline","content":[{"type":"text","text":"UIImage imageNamed"}]},{"type":"text","text":" 引起的主线程 block:"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/40\/401cf0e99e4bc721071f361924313ec3.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通过右侧的堆栈能看到，imageNamed 触发了 dlopen，dlopen 会等待 dyld 的全局锁。"},{"type":"text","marks":[{"type":"strong"}],"text":"通过 System Trace 的 Thread State Event，可以找到线程被 blocked 的下一个事件"},{"type":"text","text":"，这个事件表明了线程重新可以运行，原因就是其他线程释放了锁："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/a6\/a650762b992f474082270947869751b8.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"接下来通过分析后台线程这个时间在做什么，就知道为什么会持有锁，如何优化了。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"线程数量"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程的数量和优先级都会影响启动时间。可以通过设置 QoS 来配置优先级，两个高优的 QoS 是 User Interactive\/Initiated，启动的时候，"},{"type":"text","marks":[{"type":"strong"}],"text":"需要主线程等待的子线程任务都应该设置成高优的"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"高优的线程数量不应该多于 CPU 核心数量"},{"type":"text","text":"，可以通过 System Trace 的 System Load 来分析这种情况。"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"\/GCD\ndispatch_queue_attr_t attr = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_UTILITY, -1);\ndispatch_queue_t queue = dispatch_queue_create(\"com.custom.utility.queue\", attr);\n\/\/NSOperationQueue\noperationQueue.qualityOfService = NSQualityOfServiceUtility\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程的数量也会影响启动时间，但 iOS 中是不太好全局管控线程的，比如二\/三方库要起后台线程就不太好管控，不过业务上的线程可以通过启动任务管控。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"线程多没关系，只要同时"},{"type":"text","marks":[{"type":"strong"}],"text":"并发执行的不多就好"},{"type":"text","text":"，大家可以利用 System Trace 来看看上下文切换耗时，确认线程数量是否是启动的瓶颈。"}]}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"图片"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"启动难免会用到很多图，有没有办法优化图片加载的耗时呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"用 Asset 管理图片而不是直接放在 bundle 里"},{"type":"text","text":"。Asset 会在编译期做优化，让加载的时候更快，此外在 Asset 中加载图片是要比 Bundle 快的，因为 UIImage imageNamed 要遍历 Bundle 才能找到图。"},{"type":"text","marks":[{"type":"strong"}],"text":"加载 Asset 中图的耗时主要在在第一次张图，因为要建立索引"},{"type":"text","text":"，可以通过把启动的图放到一个小的 Asset 里来减少这部分耗时。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"每次创建 UIImage 都需要 IO，在首帧渲染的时候会解码。所以可以通过提前子线程预加载（创建 UIImage）来优化这部分耗时。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如下图，启动只有到了比较晚的阶段“RootWindow 创建”和“首帧渲染”才会用到图片，"},{"type":"text","marks":[{"type":"strong"}],"text":"所以可以在启动的早期开预加载的子线程启动任务"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/de\/de59949e75227977f9c0bbeaf31e7d6a.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"Fishhook"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"fishhook 是一个用来 hook C 函数的库，但这个库的第一次调用耗时很高，最好"},{"type":"text","marks":[{"type":"strong"}],"text":"不要带到线上"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Fishhook 是按照下图的方式遍历 Mach-O 的多个段来找函数指针和函数符号名的映射关系，带来的"},{"type":"text","marks":[{"type":"strong"}],"text":"副作用就是要大量的 Page In，对于大型 App 来说在 iPhone X 冷启耗时 200ms+。"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/ff\/ff06d8e01166bf7d05d88f183969a535.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果不得不用 fishhook，"},{"type":"text","marks":[{"type":"strong"}],"text":"请在子线程调用，且不要在在"},{"type":"codeinline","content":[{"type":"text","text":"_dyld_register_func_for_add_image"}],"marks":[{"type":"strong"}]},{"type":"text","marks":[{"type":"strong"}],"text":"直接调用 fishhook"},{"type":"text","text":"。因为这个方法会持有 dyld 的一个全局互斥锁，主线程在启动的时候系统库经常会调用 "},{"type":"codeinline","content":[{"type":"text","text":"dlsym"}]},{"type":"text","text":" 和 "},{"type":"codeinline","content":[{"type":"text","text":"dlopen"}]},{"type":"text","text":"，其内部也需要这个锁，造成上文提到的子线程阻塞主线程。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"首帧渲染"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"不同 App 的业务形态不同，首帧渲染优化方式也相差的比较多，几个常见的优化点："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"LottieView：lottie 是 airbnb 用来做 AE 动画的库，但是加载动画的 json 和读图是比较慢的，可以"},{"type":"text","marks":[{"type":"strong"}],"text":"先显示一帧静态图，启动结束后再开始动画，或者子线程预先把图和 json 设置到 lottie cache 里"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Lazy 初始化 View："},{"type":"text","marks":[{"type":"strong"}],"text":"不要先创建设置成 hidden，这是很不好的习惯"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"AutoLayout：AutoLayout 的耗时也是比较高的，但这块往往历史包袱比较重，可以"},{"type":"text","marks":[{"type":"strong"}],"text":"评估 ROI 看看要不要改成 frame"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Loading 动画：App 一般都会有个 loading 动画表示加载中，这个"},{"type":"text","marks":[{"type":"strong"}],"text":"动画最好不要用 gif"},{"type":"text","text":"，线下测量一个 60 帧的 gif 加载耗时接近 70ms"}]}]}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"其他 Tips"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"启动优化里有一些需要注意的 Tips："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"不要删除"},{"type":"codeinline","content":[{"type":"text","text":"tmp\/com.apple.dyld"}],"marks":[{"type":"strong"}]},{"type":"text","marks":[{"type":"strong"}],"text":"目录"},{"type":"text","text":"，因为这个目录下存储着 iOS 13+的启动闭包，如果删除了下次启动会重新创建，创建闭包的过程是很慢的。接下来是 IO 优化，常见的方式是用 "},{"type":"codeinline","content":[{"type":"text","text":"mmap"}]},{"type":"text","text":" 让 IO 更快一些，也可以在启动的早期预加载数据。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"还有一些 iPhone 6 上耗时会明显增加的点："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"WebView User Agent：第一次在启动时获取，之后缓存，每次启动结束后刷新"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"KeyChain：可以延迟获取或者预加载"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"VolumeView：建议直接删掉"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"iPhone 6 是个分水岭，性能会断崖式下跌，可以在 iPhone 6 上下掉部分用户交互来换取核心体验（记得 AB 验证）"},{"type":"text","text":"。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"Page In 耗时"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"启动路径上会触发大量 Page In，有没有办法优化这部分耗时呢？"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"段重命名"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"App Store 会对上传的 App 的 TEXT 段加密，在发生 Page In 的时候会解密，解密的过程是很耗时的。既然会 TEXT 段加密，那么直接的思路就是把 TEXT 段中的内容移动到其它段，ld 也有个参数 "},{"type":"codeinline","content":[{"type":"text","text":"rename_section"}]},{"type":"text","text":" 支持重命名："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/b4\/b4d65ad298147c9b8700ba31b0c4f354.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"抖音重命名方案："}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"\"-Wl,-rename_section,__TEXT,__cstring,__RODATA,__cstring\",\n\"-Wl,-rename_section,__TEXT,__const,__RODATA,__const\",\n\"-Wl,-rename_section,__TEXT,__gcc_except_tab,__RODATA,__gcc_except_tab\",\n\"-Wl,-rename_section,__TEXT,__objc_methname,__RODATA,__objc_methname\",\n\"-Wl,-rename_section,__TEXT,__objc_classname,__RODATA,__objc_classname\",\n\"-Wl,-rename_section,__TEXT,__objc_methtype,__RODATA,__objc_methtype\"\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个优化方式在 iOS 13 下有效，因为 "},{"type":"text","marks":[{"type":"strong"}],"text":"iOS 13 优化了解密流程，Page In 的时候不需要解密了"},{"type":"text","text":"，这是 iOS 13 启动速度变快的原因之一。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"二进制重排"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"既然启动的路径上会触发大量的 Page In，那么有没有什么办法优化呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"启动具有局部性特征，即只有少部分函数在启动的时候用到，这些函数在中的分布是零散的，所以 Page In 读入的数据利用率并不高。如果我们可以把启动用到的函数排列到二进制的连续区间，那么就可以减少 Page In 的次数，从而优化启动时间："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"以下图为例，方法 1 和方法 3 是启动的时候用到的，为了执行对应的代码，就需要两次 Page In。假如我们把方法 1 和 3 排列到一起，那么只需要一次 Page In，从而提升启动速度。"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/6c\/6cbb077d6599ffd6a3f355baa6067c4d.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"链接器 ld 有个参数-order_file 支持按照符号的方式排列二进制。获取启动时候用到的符号主流有两种方式："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"抖音方案：静态扫描获取 +load 和 C++静态初始化，hook objc_msgSend 获取 Objective C 符号。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Facebook 方案：LLVM 函数插桩，灰度统计启动路径符号，用大多数用户的符号生成 order_file。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Facebook 的 LLVM 函数插桩是针对 order_file 定制，并且代码也是他们自己给 LLVM 开发的，目前已经合并到 LLVM 主分支了。"}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/46\/464885b7e4254e7ea0ccafb842f3c4a8.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Facebook 的方案更精细化，生成的 order_file 是最优解，但是工程量很大。抖音的方案"},{"type":"text","marks":[{"type":"strong"}],"text":"不需要源码编译"},{"type":"text","text":"，"},{"type":"text","marks":[{"type":"strong"}],"text":"不需要对现有编译环境和流程改造"},{"type":"text","text":"，侵入性最小，缺点就是只能覆盖 90%左右的符号。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"- 灰度是任何优化都要利用好的一个阶段，因为很多新的优化方案存在不确定性，需要先在灰度上验证。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"非常规方案"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"动态库懒加载"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最开始我们提到可以通过删代码的方式来减少代码量，那么有没有什么不减少代码总量，就可以减少启动时候要加载代码数量的方式呢？"}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"答案就是动态库懒加载。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"什么是懒加载的动态库呢？正常动态库都是会被主二进制直接或者间接链接的，那么这些动态库会在启动的时候加载。"},{"type":"text","marks":[{"type":"strong"}],"text":"如果只打包进 App，不参与链接，那么启动的时候就不会自动加载，在运行时需要用到动态库里面的内容的时候，再手动懒加载"},{"type":"text","text":"。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"懒加载动态库需要在编译期和运行时都进行改造，编译期的架构："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/a5\/a531fe11744a319ba5c1dec1c32c7d4d.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"像 A.framework 等动态库是懒加载的，因为并没有参与主二进制的直接 or 间接链接。动态库之间一定会有一些共同的依赖，把这些依赖打包成 Shared.framework 解决公共依赖的问题。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"运行时通过"},{"type":"codeinline","content":[{"type":"text","text":"-[NSBundle load]"}]},{"type":"text","text":"来加载，本质上调用的是底层的 "},{"type":"codeinline","content":[{"type":"text","text":"dlopen"}]},{"type":"text","text":"。那么什么时候触发动态库手动加载呢？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"动态库可以分成两种：业务和功能。"},{"type":"text","marks":[{"type":"strong"}],"text":"业务就是 UI 的入口，可以把动态库加载的逻辑收敛到路由内部，这样外部其实并不知道动态库是懒加载的，也能更好地容错"},{"type":"text","text":"。功能库（比如上图的 QR.framework）会有些不一样，因为没有 UI 等入口，需要功能库自己维护 Wrapper："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/b1\/b10bc32136ddceb8bf1451fdfa67c7ee.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"App 对 Wrapper 直接依赖，这样外部并不知道这个动态库是懒加载的"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Wrapper 内部封装了动态调用逻辑，动态调用指的是通过 dlsym 等方式调用"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"动态库懒加载除了启动加载的代码减少，还能长期防止业务增加代码引起启动劣化，因为业务的初始化在第一次访问的时候完成的。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个方案还有其他优点，比如动态库化后本地编译时间会大幅度降低，对其他性能指标也有好处，缺点是会牺牲一定程度的包大小，但可以用段压缩等方式优化懒加载的动态库来打平这部分损耗。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"Background Fetch"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Background Fetch 可以隔一段时间把 App 在后台启动，对于时间敏感的 App（比如新闻）可以在后台刷新数据，这样能够提高 Feed 加载的速度，进而提升用户体验。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那么，这种类似“后台保活”的机制，为什么能提高启动速度呢？我们来看一个典型的 case："}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/56\/562300f16c923df6515a3678e4af225c.png","alt":"图片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"系统在后台启动 App"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"时间长因为内存等原因，后台的 App 被 kill 了"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"这时候用户立刻启动 App，那么这次启动就是一次"},{"type":"text","marks":[{"type":"strong"}],"text":"热启动"},{"type":"text","text":"，因为缓存还在"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"又一次系统在后台启动 App"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","text":"这次用户在 App 在后台的时候点了 App，那么这次启动就是一次"},{"type":"text","marks":[{"type":"strong"}],"text":"后台回前台"},{"type":"text","text":"，因为 App 仍然活着"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通过这两个典型的场景，可以看出来为什么 Background Fetch 能提高启动速度了："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"提高热启动在冷启动的占比"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"后台启动回前台被定义为启动，因为用户的角度来说这就是一次启动"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"后台启动有一些要注意的点，"},{"type":"text","marks":[{"type":"strong"}],"text":"比如日活，广告，甚至是 AB 进组逻辑都会受影响"},{"type":"text","text":"，需要做不少适配。往往需要启动器来支撑，因为正常启动在 didFinishLaunch 执行的任务，在后台启动的时候需要延迟到第一次回前台的时候再执行。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"总结"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"最后提炼出几点我们认为在任何优化中都重要的："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"白盒优化"},{"type":"text","text":"，知道为什么慢，优化的是哪部分。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"线上数据都是优化的指南针"},{"type":"text","text":"，也是衡量优化效果的唯一方式，建议开 AB 实验，验证对业务上的影响。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"不要忽略防劣化的建设"},{"type":"text","text":"，尤其是业务迭代迅速的团队，否则很有可能优化的速度赶不上劣化。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"做长期的架构建设"},{"type":"text","text":"，良好的架构会长期为启动这些基础性能保驾护航。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本文转载自：字节跳动技术团队（ID：toutiaotechblog）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"原文链接："},{"type":"link","attrs":{"href":"https:\/\/mp.weixin.qq.com\/s\/ekXfFu4-rmZpHwzFuKiLXw","title":"xxx","type":null},"content":[{"type":"text","text":"抖音品质建设 - iOS启动优化《实战篇》"}]}]}]}