Jetsam模型
Jetsam可以简单的抽象为：前台应用程序，在触发某个或多个条件时，触发系统事件，被系统kill掉。而OOM也就是因为触发了内存相关的系统事件，被系统kill掉了。
Memory Footprint
device: (crash amount/total amount/percentage of total)
iPad1: 127MB/256MB/49%
iPad2: 275MB/512MB/53%
iPad3: 645MB/1024MB/62%
iPad4: 585MB/1024MB/57% (iOS 8.1)
iPad Mini 1st Generation: 297MB/512MB/58%
iPad Mini retina: 696MB/1024MB/68% (iOS 7.1)
iPad Air: 697MB/1024MB/68%
iPad Air 2: 1383MB/2048MB/68% (iOS 10.2.1)
iPad Pro 9.7": 1395MB/1971MB/71% (iOS 10.0.2 (14A456))
iPad Pro 10.5”: 3057/4000/76% (iOS 11 beta4)
iPad Pro 12.9” (2015): 3058/3999/76% (iOS 11.2.1)
iPad Pro 12.9” (2017): 3057/3974/77% (iOS 11 beta4)
iPod touch 4th gen: 130MB/256MB/51% (iOS 6.1.1)
iPod touch 5th gen: 286MB/512MB/56% (iOS 7.0)
iPhone4: 325MB/512MB/63%
iPhone4s: 286MB/512MB/56%
iPhone5: 645MB/1024MB/62%
iPhone5s: 646MB/1024MB/63%
iPhone6: 645MB/1024MB/62% (iOS 8.x)
iPhone6+: 645MB/1024MB/62% (iOS 8.x)
iPhone6s: 1396MB/2048MB/68% (iOS 9.2)
iPhone6s+: 1392MB/2048MB/68% (iOS 10.2.1)
iPhoneSE: 1395MB/2048MB/69% (iOS 9.3)
iPhone7: 1395/2048MB/68% (iOS 10.2)
iPhone7+: 2040MB/3072MB/66% (iOS 10.2.1)
iPhone X: 1392/2785/50% (iOS 11.2.1)
OOM触发的其他事件
这个是本文想要讨论的重点。我们已知，Compressed Image是Dirty Memory，也就是在Memory Footprint中的。在最新的Xcode 9.3升级后，发现某些特殊的图片，在加载后并不会引起Memory Footprint的升高，且不会受到Memory Footprint Limit Line的限制。
比如，以iPhone 6+为例，通常Memory Footprint值到达645MB时，就会crash掉，同时Xcode的console中有memory issue的提示。然而，当我们加载特殊的图片时，根据图片加载内存计算，当其加载到1100MB左右时，才会crash，并且同样在console中有memory issuer的提示。简要的说：
通过Xcode9.3打包出的程序，加载特殊图片可以使用更多内存。比如原来加载15张图片就会内存警告，OOM。现在则可以加载20张图片。
上述现象的条件是使用Xcode9.3及以后打包，对iOS9~12有效，怀疑是LLVM的优化。我看了WWDC2018的内存和LLVM的session未发现有效的信息。
PLATFORM AND VERSION
iOS
IDE: Xcode 9.3 and above.
iOS:iOS 9 and above.
Device:iPhone 6p, iPhone 5s, iPhone x, iPhone 6sp, etc.
This appeared after I update Xcode 9.3, and
performed normal when I downgrade to Xcode 9.2.
DESCRIPTION OF PROBLEM
Memory Tool:IDE footprint memory, instruments-Allocations, source code;
Usually, when I load a 6000(px)*4000(px) image, it will cost 96MB memory. But I fount that, when I load some kind of picture, the footprint memory did not change obviously. And I can't find any memory increase in instruments-Allocations.
In addition, I find OOM is closely related to footprint memory level. But when I use this kind of picture, it can breakthroughs footprint memory limit.In other words, I can load more picture in my App when I use this kind of picture.
So, my question is, why this kind of picture has this feature, and what kind of picture will have this feature.
Usual
Unusual
Demo
STEPS TO REPRODUCE
1.Load some kind of picture.
2.Check the footprint memory use Xcode memory tool or source code.
NAME AND APPLE ID OF APP