O 专题目标

理解profile基本原理
熟悉go常用性能分析工具pprof
快速对线上服务的cpu、内存、goroutine的问题进行分析和排查

对性能分析，golang是采取采样分析的方式，语言原生支持对运行的程序进行采样，收集采样数据通过累加计算并通过提供相应的工具来分析堆栈信息。【对比java：java通过发布jdk周边工具jps jmap jstack等，在问题分析前先将内存中的数据进行dump的方式进行分析】

一 profile原理

A Profile is a collection of stack traces showing the call sequences that led to instances of a particular event, such as allocation. Packages can create and maintain their own profiles; the most common use is for tracking resources that must be explicitly closed, such as files or network connections.

go默认会初始化六种profile, 每种profile存储的实际内容被抽象为 countProfile,存储栈帧地址，堆栈地址通过调用runtime.CallersFrames(stk)可以获取堆栈信息，下面主要讲最常用的三种方式：内存、CPU和协程

type countProfile interface {  Len() int Stack(i int) []uintptr}// A countProfile is a set of stack traces to be printed as counts grouped by stack trace. There are multiple implementations:all that matters is that we can find out how many traces there are and obtain each trace in turn.

内存采样

go程序启动后，runtime会按照一定频率对内存的分配进行采样记录，当内存分配每达到一定值(默认是512KB，参数由runtime.MemProfileRate设定), runtime就会记录下当前这次内存分配的大小、stack等信息到profile

type MemProfileRecord struct { AllocBytes, FreeBytes int64 // number of bytes allocated, freed AllocObjects, FreeObjects int64 // number of objects allocated, freed Stack0 [32]uintptr // stack trace for this record; ends at first 0 entry}

CPU采样

cpu的采样是通过调用函数StartCPUProfile来启动采样的，结束调用StopCPUProfile调用链如下StartCPUProfile->runtime.SetCPUProfileRate->sighandler；采样频率是100hz

// The runtime routines allow a variable profiling rate,// but in practice operating systems cannot trigger signals// at more than about 500 Hz, and our processing of the// signal is not cheap (mostly getting the stack trace).// 100 Hz is a reasonable choice: it is frequent enough to// produce useful data, rare enough not to bog down the// system, and a nice round number to make it easy to// convert sample counts to seconds. Instead of requiring// each client to specify the frequency, we hard code it.const hz = 100// readProfile, provided by the runtime, returns the next chunk of// binary CPU profiling stack trace data, blocking until data is available.// If profiling is turned off and all the profile data accumulated while it was// on has been returned, readProfile returns eof=true.// The caller must save the returned data and tags before calling readProfile again.func readProfile() (data []uint64, tags []unsafe.Pointer, eof bool)