如何让 Xcode 在读写上提速100倍？

雨点打透心脏的1/2处 2022-06-06 09:49 174阅读 0赞

本文作者：   [伯乐在线][Link 1]  -   [故胤道长][Link 2]  。未经作者许可，禁止转载！  
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![111721232-e5dcce65c3e2c402][]

上个月参加了一场西雅图当地的线下 iOS 开发者聚会。Jeff Szuhay 作为一个有20+年开发经验的资深程序员，跟我讲了一套提高 iOS 开发效率的方法。相比于其他程序员在 App 启动时间、架构优化方面的经验，老爷子 Jeff 的优化基于硬件层面，匠心独运，极客风十足。以下是他的经验分享和我个人的实测。

# 问题来源 #

我们都知道 Xcode 在运行或编译时，会有大量的读写操作。例如从硬盘中调用图片，我们会这么操作：

这时候 Xcode 就会去电脑的硬盘中去找到图片，完成读写操作。类似的操作还有存取文件等等。如果这类读取数量比较少，那么无伤大雅，但是一旦多起来，尤其是大项目在后期产生了大量的 DerivedData 存在硬盘上，Xcode 在编译时就会花大量时间去硬盘（Disk）上完成读写这些数据的操作。更不幸的是有时候还会遇到硬盘故障等问题。

# 解决思路 #

正所谓“哪里需要优化，哪里就需要程序员”，Jeff 在这个时候作为一名白衣骑士登场了。多年的计算机研究让他对整个计算机架构非常熟悉。下图是他展示的计算机结构简图。

![121721232-adf5f8a841eb5116][]

计算机结构简图

此图简洁明了得说明了计算机的基本架构。左上角是计算机的大脑，CPU，负责核心计算和处理工作；右上角是内存（RAM），用来运行程序并与 CPU 进行数据交流；中间的线是总线，负责各个模块之间传递信息和信号；图下侧是基本的 System IO。

再回来看我们的问题：Xcode 现在是在 RAM 中运行，然后到 Storage 中读写数据，数据接着再传回 RAM。这种方式有两个瓶颈：

*  Storage 速度很慢。即使是最先进的 SSD，其速度也比 RAM 慢了400倍。也就是无论你怎么在软件层优化，其速度也无法突破 SSD 的瓶颈；
 *  数据要不停的在各个模块之间传递。传递过程中亦有延时和无谓的时间消耗。

针对以上两个瓶颈，Jeff 认为，如果我们可以让所有的读写操作都在内存（RAM）中完成，那么必然能大幅提高 Xcode 的工作效率。问题是，怎么实现？

# 实现方法 #

方法的思路很简单，大概可以分两步：

1.  配置 RAM。在内存中专门开出一块让 Xcode 使用。
2.  连接 Xcode。让 Xcode 连接到我们开辟出来的专属内存空间。

下面就是见证奇迹的时刻。

第一步， 创建 .sh 文件。代码如下。

<table style="margin-left:0px; font-size:13px; border:none!important; margin-top:0px!important; margin-right:0px!important; margin-bottom:0px!important; padding:0px!important; border-collapse:collapse!important; border-spacing:0px!important; width:auto!important; table-layout:auto!important"> 
 <tbody style="border:0px; margin:0px; padding:0px"> 
  <tr style="border:none!important; margin:0px!important; padding:0px!important; vertical-align:top!important"> 
   <td style="font-size:13px; text-align:center; border:none!important; margin:0px!important; padding:0px!important; vertical-align:top!important; font-family:Monaco,MonacoRegular,&quot;Courier New&quot;,monospace!important; background-color:rgb(238,238,238)!important"> 
    <div style="border:none; margin:0px; padding-right:0px; padding-left:0px; text-align:left; white-space:nowrap; padding-top:5px!important; padding-bottom:3px!important; line-height:15px!important"> 
     <div style="border-right-width:1px!important; border-top-style:none; border-right-style:solid!important; border-bottom-style:none; border-left-style:none; border-right-color:rgb(222,222,222)!important; margin:0px; padding:0px 5px; font-size:inherit!important; font-family:inherit; text-align:right!important; height:inherit; line-height:inherit!important; font-weight:inherit!important; min-width:1.2em!important; color:rgb(170,170,170)!important">
       1 
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       2 
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       3 
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       4 
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       5 
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    <div style="margin:0px; padding-right:0px; padding-left:0px; text-align:left; white-space:pre; overflow:visible; border:none!important; padding-top:5px!important; padding-bottom:3px!important; line-height:15px!important"> 
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     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

第二步， 运行 .sh 文件。在命令行中敲下。

![131721232-bb3796ea2f216f41][]

之后你会发现你会多出一个叫 ramdisk 的内存空间，有大概 1 GB 大小。

![141721232-14e9522d7e98fd63][]

第三步，连接 Xcode。Xcode -> Preferences -> Locations -> Locations Tab，配置 DerivedData。

![151721232-ba3d35d2401dfc28][]

Advanced… 也要配置成下图所示

![161721232-14fc390ed45351fb][]

以上就是全部步骤。这时候你就可以享受飞一般的开发了。现在 Project 中所有文件都在内存中，相比于 SSD，理论上是要快上一个数量级。

# 注意事项 #

*  合理分配内存空间。我这里分配了 1GB 的内存当硬盘使，是因为我电脑本身有 16GB 内存空间。假如你电脑内存只有 4GB，我不建议你使用这个方法，或者建议只分配 256M 空间给 Xcode。总之，注意内存不足或溢出的情况。
 *  只把 DerivedData 放在 Ram Disk 中。为了极限速度，你当然可以把 App 相关所有的文件都放在内存空间中。但是要知道，我们创造的 Ram Disk 本质是内存，当关机或重启的时候，在 Ram Disk 中的数据是会丢失的。而 DerivedData 是可以重新生成的，所以放在 Ram Disk 中可以最大限度的提高 Xcode 开发中的读取速度，且十分安全。

## 参考 ##

*  [Performance Considerations for macOS/iOS Development in the “New Frontier”][Performance Considerations for macOS_iOS Development in the _New Frontier]
 *  [Cache In Your Pocket: Use a RAM disk for Xcode][Cache In Your Pocket_ Use a RAM disk for Xcode]

[Link 1]: http://ios.jobbole.com/
[Link 2]: http://www.jobbole.com/members/soapyigu
[Link 3]: http://blog.jobbole.com/99322
[111721232-e5dcce65c3e2c402]: /images/20220606/4580b9a725824bf1b86f895487f7b9f9.png
[121721232-adf5f8a841eb5116]: /images/20220606/b3eeee0e02fc41e29babaf9694a8676a.png
[131721232-bb3796ea2f216f41]: /images/20220606/843cbe31d4c946df9635d7ea0de7036a.png
[141721232-14e9522d7e98fd63]: /images/20220606/867d764aa90c45b382dd3677c8955b7e.png
[151721232-ba3d35d2401dfc28]: /images/20220606/a27d0972ab2942bfafcdd96603a4d11e.png
[161721232-14fc390ed45351fb]: /images/20220606/20752b2207e44f0b9d3b303e319e942a.png
[Performance Considerations for macOS_iOS Development in the _New Frontier]: http://www.szuhay.org/jeff/Xcoders/PerformanceInNewFrontier_Presentation.pdf
[Cache In Your Pocket_ Use a RAM disk for Xcode]: http://www.blinddogsoftware.com/goodies/#CacheInYourPocket