LLM(Low Level Memory Tracker) 是从 4.18 开始引入的新的内存统计工具，比memreport 统计数据更加详细精确，但又不会像 MallocProfiler 那样有很大的本身开销
通过LLM，可以知道游戏消耗的内存分别来自引擎的哪些模块，让开发者可以对内存占用高的模块进行有针对性优化。
Debug、Development包才会启用ENABLE_LOW_LEVEL_MEM_TRACKER宏，统计逻辑才会编译进可执行二进制中，才能使用LLM相关的功能
Shipping包是没有LLM功能，对于Test包可以#define ALLOW_LOW_LEVEL_MEM_TRACKER_IN_TEST 1来将LLM代码编译进包
相关逻辑在UnrealEngineEngineSourceRuntimeCorePublicHALLowLevelMemTracker.h、UnrealEngineEngineSourceRuntimeCorePrivateHALLowLevelMemTracker.cpp文件中
LLM目前有两个Tracker（跟踪器）：Default Tracker（默认跟踪器）和Platform Tracker（平台跟踪器）。它们都有自己的独立分配映射和标记堆栈。
Default Tracker（ELLMTracker::Default）的级别较高，记录引擎中FMemory的Malloc函数进行的分配。为 stat LLM和 stat LLMFULL控制台命令提供统计信息。
Platform Tracker（ELLMTracker::Platform）的级别较低，记录从OS进行的所有分配。例如，它会跟踪FMemory的Malloc函数进行的内部分配。因此，Default Tracker是Platform Tracker统计信息的子集。
运行开关
在编译时打开 LLM 后，要想在运行时使用 LLM 的功能，还要加上启动命令行 -LLM -LLMCSV
-LLM：运行时打开LLM统计 或者通过定义#define LLM_AUTO_ENABLE 1默认打开
-LLMCSV：将内存统计信息输出到CSV 文件中，CSV 文件保存在 SavedProfilingLLM 目录下 注：设置控制台变量LLM.LLMWriteInterval xx，修改写csv文件的频率为xx秒，默认5秒写一行
会得到两个 csv 文件，一个是 LLM_ 前缀，另一个是 LLMPlatform_前缀，这两个文件分别由两个 Tracker 生成，这两个文件的关注点不同
① LLM 主要关注FMalloc内存的消费者，主要就是引擎内各个部分的内存开销
② LLMPlatform 只关注系统内存的消费者，主要有几大类: ProgramSize, FMalloc, LLMOverhead
-LLMTAGSETS=Assets：显示每个资源分配的统计，不只是有大类别的数据。 注：需打开LLM_ALLOW_ASSETS_TAGS 宏
-LLMTAGSETS=AssetClasses：按UObject类别进行分类统计
注：写CSV时，如果开启了-LLMTAGSETS 命令行，CSV 文件可能会错乱。
因为写 CSV 文件时预留了第一行标题栏内容的缓冲区，当每个资源或资源类很多，会导致标题栏内容溢出，覆盖了后面的数据区域，所以这里需要自己按需求修改一下
具体的控制台命令为：LLM.LLMHeaderMaxSize 500000
运行时开关 LLM 的处理见 FLowLevelMemTracker::ProcessCommandLine 函数
在UE4CommandLine.txt文件中添加命令行参数：
../../../MyGame/MyGame.uproject -LLM -LLMCSV
Android需要将该文件push到手机的UE4Game/MyGame/UE4CommandLine.txt
iOS需要将ue4commandline.txt（注：需要全小写）放在如下目录中：
从csv生成html图表
PerfreportTool.exe -csv F:UAGameSavedProfilingLLMLLM_Pid23989_2021.03.30-14.14.55.csv -o c:LLMOutput -reporttype LLM -topng. -graphxml LLMReportGraphs.xml -reportxml LLMReportTypes.xml -nostripevents
PerfreportTool.exe -csv F:UAGameSavedProfilingLLMLLMPlatform_Pid23989_2021.03.30-14.14.55.csv -o c:LLMOutput -reporttype LLM -topng. -graphxml LLMReportGraphs.xml -reportxml LLMReportTypes.xml -nostripevents
生成的html文件如下：
统计项说明
LLM_Pid23989_2021.03.30-14.14.55内容如下：
Untagged,RenderingThread,Audio/AudioMisc,EngineInit,Stats,LoadMapMisc,Audio/AudioMixer,GC,Audio,AsyncLoading,UObject,FileSystem,Materials,Shaders,TextureMetaData,InitUObject,UI,EnginePreInit,Textures,Meshes,SceneRender,RenderTargets,RHIMisc,Meshes/StaticMesh,FName,MaterialInstance,PSO,Particles,Localization,ConfigSystem,TaskGraphMiscTasks,Physics/PhysXAllocator,Physics,AssetRegistry,CsvProfiler,Audio/AudioMixerPlugins,Physics/PhysX,NavigationRecast,EngineMisc,ProgramSize,GenericPlatformMallocCrash,FMallocUnused,TrackedTotal,Total,Untracked,StreamingManager,Lua,Meshes/Landscape,Animation,Meshes/SkeletalMesh,Niagara,MediaStreaming,Meshes/InstancedMesh,Physics/PhysXLandscape,Audio/AudioSynthesis,
。。。 。。。
35.32,6.21,0.02,0.90,21.65,5.05,9.52,0.62,9.55,3.08,105.19,13.72,0.54,19.11,0.65,12.84,42.70,18.71,67.28,31.85,34.72,40.85,0.80,8.79,7.12,0.31,0.15,0.78,36.59,2.80,0.00,6.32,6.71,0.13,0.00,0.00,0.39,0.71,3.78,157.18,3.61,55.31,789.20,830.58,41.44,0.44,28.88,0.03,13.38,12.33,0.00,0.00,0.13,0.00,0.00,
35.32,6.21,0.02,0.90,21.52,5.05,9.52,0.62,9.55,3.08,105.22,13.72,0.54,19.11,0.65,12.84,42.71,18.71,67.28,31.78,34.25,40.85,0.80,8.79,7.12,0.31,0.15,0.78,36.59,2.80,0.00,6.33,6.71,0.13,0.00,0.00,0.39,0.71,3.78,157.18,3.61,56.45,789.74,842.68,52.88,0.44,28.91,0.03,13.38,12.27,0.00,0.00,0.13,0.00,0.00,
35.32,6.21,0.02,0.90,21.65,5.05,9.52,0.62,9.55,3.08,105.22,13.72,0.54,19.11,0.65,12.84,42.74,18.71,67.28,31.89,34.99,40.85,0.80,8.79,7.12,0.31,0.15,0.78,36.59,2.80,0.00,6.34,6.73,0.13,0.00,0.00,0.39,0.71,3.79,157.18,3.61,55.68,790.05,828.77,38.50,0.44,28.94,0.03,13.38,12.37,0.00,0.00,0.13,0.00,0.00,
35.32,6.21,0.02,0.90,21.59,5.05,9.52,0.62,9.55,3.08,105.25,13.72,0.54,19.11,0.65,12.84,42.72,18.71,67.28,31.86,35.13,40.85,0.80,8.79,7.12,0.31,0.15,0.78,36.59,2.80,0.00,6.32,6.71,0.13,0.00,0.00,0.39,0.71,3.81,157.18,3.61,55.56,790.05,841.99,52.15,0.44,28.96,0.03,13.44,12.34,0.00,0.00,0.13,0.00,0.00,
35.32,6.21,0.02,0.90,21.40,5.05,9.52,0.65,9.55,3.08,105.18,13.72,0.54,19.11,0.65,12.84,42.72,18.71,67.28,31.32,35.28,40.85,0.80,8.79,7.12,0.31,0.15,0.78,36.59,2.80,0.00,6.34,6.73,0.13,0.00,0.00,0.39,0.71,3.77,157.18,3.61,54.50,788.20,838.28,50.08,0.44,28.99,0.03,13.26,11.80,0.00,0.00,0.13,0.00,0.00,
35.32,6.21,0.02,0.90,21.84,5.05,9.52,0.65,9.55,3.08,105.18,13.72,0.54,19.11,0.65,12.84,42.74,18.71,67.28,31.59,35.64,40.85,0.80,8.79,7.12,0.31,0.15,0.78,36.59,2.80,0.00,6.34,6.73,0.13,0.00,0.00,0.39,0.71,3.77,157.18,3.61,55.04,789.89,837.26,46.91,0.44,29.02,0.03,13.26,12.07,0.00,0.00,0.13,0.00,0.00,
35.32,6.21,0.02,0.90,22.03,5.05,9.52,0.65,9.55,3.08,105.17,13.72,0.54,19.11,0.65,12.84,42.75,18.71,67.28,31.52,35.90,40.85,0.80,8.79,7.12,0.31,0.15,0.78,36.59,2.80,0.00,6.34,6.73,0.13,0.00,0.00,0.39,0.71,3.74,157.18,3.61,54.95,790.17,831.60,40.49,0.44,29.05,0.03,13.26,11.96,0.00,0.00,0.13,0.00,0.00,
35.32,6.21,0.02,0.90,21.46,5.05,9.52,0.65,9.55,3.08,105.19,13.72,0.54,19.11,0.65,12.84,42.73,18.71,67.28,31.74,35.36,40.85,0.80,8.79,7.12,0.31,0.15,0.78,36.59,2.80,0.00,6.34,6.73,0.13,0.00,0.00,0.39,0.71,3.75,157.18,3.61,55.90,790.32,843.07,53.02,0.44,29.07,0.03,13.31,12.22,0.00,0.00,0.13,0.00,0.00,
35.32,6.21,0.02,0.90,21.46,5.05,9.52,0.65,9.55,3.08,105.22,13.72,0.54,19.11,0.65,12.84,42.74,18.71,67.28,31.82,35.44,40.85,0.80,8.79,7.12,0.31,0.15,0.78,36.59,2.80,0.00,6.34,6.73,0.13,0.00,0.00,0.39,0.71,3.77,157.18,3.61,56.08,790.86,843.15,52.41,0.44,29.23,0.03,13.32,12.30,0.00,0.00,0.13,0.00,0.00,
35.32,6.21,0.02,0.90,21.78,5.05,9.52,0.65,9.55,3.08,105.23,13.72,0.54,19.11,0.65,12.84,42.75,18.71,67.28,31.71,36.19,40.85,0.80,8.79,7.12,0.31,0.15,0.78,36.59,2.80,0.00,6.33,6.72,0.13,0.00,0.00,0.39,0.71,3.77,157.18,3.61,55.50,791.27,842.89,51.62,0.44,29.26,0.03,13.32,12.18,0.00,0.00,0.13,0.00,0.00,
LLMPlatform_Pid23989_2021.03.30-14.14.55.csv内容如下：
Untagged,FMalloc,Graphics,ProgramSize,GenericPlatformMallocCrash,TrackedTotal,Total,Untracked,LLMOverhead,OSAvailable,
。。。 。。。
0.00,511.05,115.27,157.18,3.61,918.22,961.69,43.47,131.11,3389.45,
0.00,511.61,115.27,157.18,3.61,907.32,962.20,54.88,119.64,3388.55,
0.00,511.86,115.28,157.18,3.61,920.53,961.36,40.84,132.58,3387.90,
0.00,511.86,115.27,157.18,3.61,907.62,961.63,54.02,119.69,3386.14,
0.00,510.05,115.27,157.18,3.61,905.81,957.96,52.16,119.69,3389.23,
0.00,511.74,115.27,157.18,3.61,907.49,956.95,49.46,119.69,3387.49,
0.00,511.99,115.31,157.18,3.61,919.30,962.82,43.52,131.22,3385.75,
0.00,512.18,115.27,157.18,3.61,907.96,962.80,54.84,119.72,3388.29,
0.00,512.68,115.27,157.18,3.61,908.46,962.87,54.41,119.72,3386.87,
0.00,513.11,115.27,157.18,3.61,908.93,962.64,53.72,119.75,3386.62,
以第1条采集的值为例（单位MB），说明各统计项含义：
Untagged // 未分类的内存，即分配内存时候不在 LLM_SCOPE 宏范围内
ELLMTag::Untagged（Default Tracker）： 35.32
ELLMTag::Untagged（Platform Tracker）： 0
LLMOverhead // LLM本身消耗的内存
（Default Tracker）： 无
ELLMTag::PlatformOverhead（Platform Tracker）: 119.64
TrackedTotal // 通过调用 LLM OnLowLevelAlloc/OnLowLevelFree 统计到的所有内存
ELLMTag::TrackedTotal（Default Tracker）：789.74
ELLMTag::PlatformTrackedTotal（Platform Tracker）：907.32
注：统计到的内存值ELLMTag::PlatformTrackedTotal比ELLMTag::TrackedTotal要大
Total // 总内存
ELLMTag::PlatformTotal（Platform Tracker）：962.2
在 Android 上为 RSS，在 iOS 上为 resident_size，在其它平台上为系统所有进程消耗的内存(不明白这里为什么是这样)，所以这个值目前只有在移动端有参考意义
ELLMTag::Total（Default Tracker）：842.68
PlatformTotal - LLMOverhead。即为上面的内存减去 LLM 本身消耗的内存，这个内存值可以认为是在不打开 LLM 功能下的进程内存值，同上，这个值目前只在移动端上有参考意义
Untracked // 未知的内存分配，即 LLM 无法追踪的内存，比如第三方库里的内存分配
ELLMTag::PlatformUntracked（Platform Tracker）： 54.88 // PlatformTotal - PlatformTrackedTotal
ELLMTag::Untracked（Default Tracker）：52.88 // Total - TrackedTotal
FMalloc // 通过FMemory内存分配器分配的内存，也就是一般意义上引擎分配的内存，比如各种UObject，纹理，动画等占用的内存
（Default Tracker）： 无
ELLMTag::FMalloc（Platform Tracker）: 511.61
FMallocUnused // ELLMTag::FMalloc内存分配器的内存 - 通过FMemory得到的内存。即内存分配器中未使用的内存
ELLMTag::FMallocUnused（Default Tracker）： 56.45
（Platform Tracker）: 无
注：LLM追踪的内存有2种ELLMAllocType::FMalloc和ELLMAllocType::System
ProgramSize // 程序刚起来时占用的内存，即刚进 Main 函数时，在 Android 上为 RSS，在 iOS 上为 resident_size
ELLMTag::ProgramSize（Default Tracker）：157.18
ELLMTag::ProgramSizePlatform（Platform Tracker）：157.18
Graphics // 显存
（Default Tracker）： 无
ELLMTag::GraphicsPlatform（Platform Tracker）: 115.27
GenericPlatformMallocCrash // 预分配的 crash 相关的内存，当 crash 发生时用来替换GMalloc
ELLMTag::GenericPlatformMallocCrash（Default Tracker）：3.61
ELLMTag::GenericPlatformMallocCrashPlatform（Platform Tracker）：3.61
OSAvailable // 系统当前可用的物理内存
（Default Tracker）： 无
ELLMTag::PlatformOSAvailable（Platform Tracker）: 3388.55
OOMBackupPool // Out of memory 备用内存池
ELLMTag::BackupOOMMemoryPool（Default Tracker）：未统计
ELLMTag::BackupOOMMemoryPoolPlatform（Platform Tracker）: 未统计
Default Tracker详细的统计项
ELLMTag::EngineMisc 3.78 // 在以下函数执行过程中分配的内存
AActor::ProcessEvent
UPackageMapClient::SerializeNewActor
FEngineLoop::Tick
ELLMTag::TaskGraphTasksMisc 0 // FTaskThreadAnyThread::ProcessTasks 函数执行过程中分配的内存
Audio 9.52 // 音频相关的内存，包括音频资源，音频线程中分配的内存，具体还包含以下子类别
ELLMTag::AudioMisc 0.02
ELLMTag::AudioSoundWaves
ELLMTag::AudioMixer 9.52
ELLMTag::AudioPrecache
ELLMTag::AudioDecompress
ELLMTag::AudioRealtimePrecache
ELLMTag::AudioFullDecompress
ELLMTag::AudioVoiceChat
ELLMTag::FName 7.12 // FName占用的内存
ELLMTag::Networking // 网络相关的内存，包含网络包处理，连接对象内存等
ELLMTag::Meshes 31.85 // Mesh 的顶点缓存，顶点索引缓存占用的内存，还有以下子类别
ELLMTag::StaticMesh 8.79 // StaticMesh 相关的内存，包含 StaticMesh 资源，StaticMeshComponent
ELLMTag::SkeletalMesh 12.33 // SkeletalMeshComponent, SkinnedMeshComponent, SkeletalMesh内存
ELLMTag::InstancedMesh 0.13 // InstancedStaticMeshComponent, HierarchicalInstancedStaticMeshComponent内存
ELLMTag::Landscape // LandscapeComponent, LandscapeProxy, LandscapeComponentSceneProxy内存
ELLMTag::Stats 21.65 // stat 统计相关的内存占用
ELLMTag::Shaders 19.11 // 各种类型 Shander 的内存占用
ELLMTag::PSO 0.15 // Pipeline State Object 缓存的内存占用
ELLMTag::Textures 67.28 // 纹理相关的内存占用
ELLMTag::TextureMetaData 0.65 // UTexture2D::Serialize 函数执行过程中分配的内存
ELLMTag::RenderTargets 40.85 // RT 相关的内存，包含在 Viewport 中渲染使用的内存
ELLMTag::SceneRender 34.72 // 场景渲染相关的内存占用，包含 Slate RHI Render 的窗口渲染
ELLMTag::RHIMisc 0.8 // 其它渲染相关的内存占用，如各种渲染状态，RHI Thread 运行过程中的内存
ELLMTag::AsyncLoading 3.08 // 异步资源加载过程中的内存占用，包含 AsyncLoadingThread, Event Driven Loader
ELLMTag::UObject 105.19 // UObject 占用的内存，包含 UClass 等
ELLMTag::Animation 13.38 // 动画相关的内存，包含 AnimInstance, AnimSequence, AnimationAsset, AnimBlueprint, MorphTarget
ELLMTag::Materials 0.54 // 材质相关的内存，包含 MaterialInterface, MaterialFunction
ELLMTag::Particles 0.78 // 特效相关的内存，包含 ParticleSystemComponent
ELLMTag::GC 0.62 // GC 过程中的内存，分别为以下两个函数
CollectGarbageInternal
PerformReachabilityAnalysis
ELLMTag::UI 42.7 // Slate 相关的内存，包含字体，TextureAtlas
ELLMTag::PhysX // PhysX 物理相关的内存
ELLMTag::EnginePreInitMemory 18.71 // FEngineLoop::PreInit 函数执行过程中的内存
ELLMTag::EngineInitMemory 0.9 // FEngineLoop::Init 函数执行过程中的内存
ELLMTag::RenderingThreadMemory 6.21 // 渲染线程(RenderingThreadMain)执行过程中的内存
ELLMTag::LoadMapMisc 5.05 // 地图加载过程中的内存，分别为以下两个函数
UEngine::LoadMap
UWorld::UpdateLevelStreaming
ELLMTag::StreamingManager 0.44 // StreamableManager 相关的内存，主要是资源 streaming 函数执行过程中的内存
ELLMTag::FileSystem 13.72 // 文件系统相关的内存，主要是文件读取时的缓冲区，比如Pak 文件
ELLMTag::Localization 36.59 // 本地化相关的内存
ELLMTag::AssetRegistry 0.13 // AssetRegistryModule 内存
ELLMTag::ConfigSystem 2.8 // 配置文件相关的内存
ELLMTag::InitUObject 12.84 // InitUObject函数执行过程中的内存
ELLMTag::MaterialInstance 0.31 // MaterialInstance, MaterialInstanceDynamic包括材质参数内存占用
ELLMTag::Lua 28.88 // Lua 内存
Windows下特有统计项
ELLMTag::WorkingSetSize // 进程物理内存，可用这个值替代上面的PlatformTotal
ELLMTag::PagefileUsed // 进程虚拟内存
ThreadStack // Windows 线程栈内存
ELLMTag::ThreadStack（Platform Tracker）
ELLMTag::ThreadStackPlatform（Default Tracker）
llm stat控制台命令
stat llm // 显示 Default Tracker 分组的简要内存统计信息
stat llmfull // 显示 Default Tracker 分组的所有内存统计信息
注1：默认一个Group最多显示的行数为25，可通过执行控制台变量stats.MaxPerGroup xx来指定新的最大行数为xx
注2：为了能查看显示不下的内容，自定义开发了stat scroll控制台命令
stat scroll 1 // 向下滚动1行
stat scroll -1 // 向上滚动1行
stat scroll 999 // 滚动到底部
stat scroll -999 // 滚动到顶部
stat scroll restore //恢复初始
Stat LLMPlatform // 显示 Platform Tracker 分组的内存统计信息
Stat LLMOverhead // 显示 LLM 本身使用的内存信息
参考