技术志 | Tony Bai

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理解Golang语句中的求值顺序

八月 27, 2015
2 条评论

Golang在变量声明、初始化以及赋值语句上照比C语言有了许多改进：

a) 支持在同一行声明多个变量

var a, b, c int

b) 支持在同一行初始化多个变量（不同类型也可以）

var a, b, c = 5, "hello", 3.45
a, b, c := 5, "hello", 3.45 (short variable declaration)

c) 支持在同一行对多个变量进行赋值（在声明后且不同类型也可以）

a, b, c = 5, "hello", 3.45

这种语法糖我们是笑纳的，毕竟人生苦短，少写一行是一行啊^_^。

但这种语法糖却给我们带来了一些令人困惑的问题！比如下面这个就是Rob Pike在一个talk中slide(Go Course Day2)中的一个问题：

n0, n1 = n0 + n1, n0

or：

n0, n1 = op(n0,n1), n0

n0, n1的值在上述语句执行完毕后到底为多少呢？

显然这个问题涉及到Go语言的语句求值顺序(evaluation order)。求值序在任何一门编程语言中都是比较难缠的，很多情形下，语言规范给出的答案都是“undefined（未定义）” or "not specified" or “依赖实现”，尤其是对于哪些模棱两可的写法，就如Rob Pike给出的那个问题。

我们要想搞清楚Go中的求值顺序，我们需要求助于Go language specification，Go spec与Go发行版一起发布，你可以启动一个godoc web server(godoc -http=:6060，然后访问localhost:6060/ref/spec)查看go language specification。Go language specification专门有一个小节/ref/spec#Order_of_evaluation对求值顺序做了说明。

在Go specs中，有这样三点陈述：

1、变量声明(variable declaration)中的初始化表达式(initialization expressions)的求值顺序(evaluation order)由初始化依赖(initialization dependencies)决定；但对于初始化表达式内部的操作数的求值需要按照2中的顺序：从左到右；
2、在非变量初始化语句中，对表达式、赋值语句或返回语句中的操作数进行求值时，操作数中包含的函数(function)调用、方法(method)调用和通信操作(主要针对channel)将按语法从左到右的顺序求值。
3、赋值语句求值分为两个阶段，第一阶段是等号左边的index expressions、pointer indirections和等号右边的表达式中的操作数的求值顺序按照2中从左到右的顺序；第二阶段按从左到右的顺序对变量赋值。

下面我们就分别理解一下这三点。

一、变量声明中初始化表达式的求值顺序

带初始化表达式的变量声明的形式如下：

var a, b, c = expr1, expr2, expr3 //包级别或函数/方法内部

a, b, c := expr1, expr2, expr3 //仅函数/方法内部

根据lang specs说明，求值顺序是由初始化依赖（initialization dependencies）规则决定的。那初始化依赖规则是什么呢？在Golang specs中也有专门章节说明：ref/spec#Package_initialization。

初始化依赖规则总结一下，大致有如下几条：

1、包中，包级别变量的初始化顺序按照声明先后的顺序，但如果某个变量(比如a)的初始化表达式中依赖其他变量(比如b)，那么变量a的初始化顺序在变量b后面。
2、对于未初始化的，且不含有对应初始化表达式或其初始化表达式不依赖任何未初始化变量的变量，我们称之为"ready for initialization"变量。初始化就是按照声明顺序重复执行对下一个变量的初始化过程，直到没有"ready for initialization"变量为止。
3、如果初始化过程完毕后依然有变量处于未初始化状态，那程序有语法错误。
4、多个处于不同文件中的变量的声明顺序依赖编译器处理文件的顺序，先处理的文件中的变量的声明顺序先于后处理的文件中的所有变量。
5、依赖分析以包为单位执行，只有位于同一个包中的被依赖的变量、函数、方法才会被考虑。

规则是抽象难懂的，例子更直观易理解，我们看一个golang spec中的例子，并使用上述规则进行分析。实验环境：go 1.5, amd64,Darwin Kernel Version 13.1.0。

//golang-statements-evaluating-order/example1.go
package main

import "fmt"

var (
    a = c + b
    b = f()
    c = f()
    d = 3
)

func f() int {
d++
return d
}

func main() {
fmt.Println(a, b, c, d)
}

我们来分析一下程序执行后的a, b, c, d四个变量的结果值，不过不同的初始化顺序会导致结果值不同，因此分析四个变量的初始化顺序是至关重要的。

变量a, b, c, d的初始化过程如下：

1、根据规则，初始化按照变量声明先后顺序进行，因此先来分析变量a，a初始化表达式依赖b 和c；因此变量a的初始化次序排在b、c的后面；
2、按照a的初始化右值表达式，c、b在右值表达式中的出现顺序是c先于b；
3、c是否是一个ready for initialization变量呢？我们看到c依赖f这个函数，而f这个函数则依赖变量d的初始化，因此d排在c之前；
4、我们来看变量d，"d = 3"，d未初始化且不含有初始化表达式，因此d是一个ready for initialization变量，我们可以从d开始初始化了。至此四个变量的初始化顺序排定 d -> c -> b -> a；(这块儿与spec中分析有差异，但从运行结果来看，应该是这个顺序;关于这个spec的issue参见#12369)
5、d初始化为3，此时已初始化变量集合[d=3]；
6、接着初始化c：c = f()，因此c = 4（此时d=4），此时已初始化变量集合[c=4,d=4]；
7、接下来轮到b：b = f()，因此b = 5 （此时d = 5)，此时已初始化变量集合[b=5,c=4,d=5]；
8、最后初始化a： a = c + b，在已初始化变量集合中我们可以找到b和c，因此a= 9，这样四个变量到此均已初始化；
9、经过分析：程序执行的结果应该是9，5，4，5。

我们来执行一下这个程序，验证一下我们的分析结果是否正确：

$go run example1.go
9 5 4 5

我们再来看一个例子，也是golang specs中的例子，我们稍作改造，并把它设定为example2：

//golang-statements-evaluating-order/example2.go
package main

import "fmt"

var a, b, c = f() + v(), g(), sqr(u()) + v()

func f() int {
fmt.Println("calling f")
return c
}

func g() int {
fmt.Println("calling g")
return a
}

func sqr(x int) int {
fmt.Println("calling sqr")
return x * x
}

func v() int {
fmt.Println("calling v")
return 1
}

func u() int {
fmt.Println("calling u")
return 2
}

func main() {
fmt.Println(a, b, c)
}

同样根据变量初始化依赖规则对这个例子进行分析：

1、按照变量声明顺序，先初始化a：a= f() + v()，f()依赖变量c；v不依赖任何变量，因此变量c的初始化顺序应该在a变量前：c -> a。
2、分析c：c = sqr(u()) + v()；u、sqr、v三个函数不依赖任何变量，因此c处于ready for initialization，于是对c进行初始化，函数执行顺序(从左到右)为：u() -> sqr() -> v(); 此时已初始化变量集合：[c = 5]；
3、回到a：a = f() + v()，c初始化后，a也处理ready for initialization，于是对a初始化，函数执行顺序为：f() -> v()，此时已初始化变量集合：[c=5, a= 6]；
4、按照变量声明次序，接下来轮到变量b：b= g()，而g()依赖a，a已经初始化完毕了，因此b也是ready for initialization，于是对b初始化，函数执行次序为：g()，至此已初始化变量集合：[c=5, a=6, b=6]。
5、经过分析：程序执行的结果应该是6，6，5。

我们来执行一下这个程序，验证一下我们的分析结果是否正确：

$go run example2.go
calling u
calling sqr
calling v
calling f
calling v
calling g
6 6 5

二、非变量初始化语句中的求值顺序

前面提到过：在非变量初始化语句中，对表达式、赋值语句或返回语句中的操作数进行求值时，操作数中包含的函数(function)调用、方法(method)调用和通信操作(主要针对channel)将按语法从左到右的顺序求值。

我们同样来看一个例子：example3.go。

//golang-statements-evaluating-order/example3.go
package main

import "fmt"

func f() int {
fmt.Println("calling f")
return 1
}

func g(a, b, c int) int {
fmt.Println("calling g")
return 2
}

func h() int {
fmt.Println("calling h")
return 3
}

func i() int {
fmt.Println("calling i")
return 1
}

func j() int {
fmt.Println("calling j")
return 1
}

func k() bool {
fmt.Println("calling k")
return true
}

func main() {
var y = []int{11, 12, 13}
var x = []int{21, 22, 23}

    var c chan int = make(chan int)
    go func() {
        c <- 1
    }()

y[f()], _ = g(h(), i()+x[j()], <-c), k()

fmt.Println(y)
}

y[f()], _ = g(h(), i()+x[j()], <-c), k() 这行语句是赋值语句，但赋值语句的操作数中包含函数调用、channel操作，按照规则，这些函数调用、channel操作按从左到右顺序估值。

1、按照从左到右顺序，第一个是y[f()]中的f()；
2、接下来是g()，g()的参数列表依然是一个赋值操作，因此其涉及到的函数调用顺序为h()， i()，j()，<-c，因此实际上的顺序为h() –> i()–> j() –> c操作 -> g()；
3、最后是k(),因此完整的调用顺序是：f()-> h() –> i()–> j() –> c操作 -> g() –> k()。

实际运行情况如下：

$go run example3.go
calling f
calling h
calling i
calling j
calling g
calling k
[11 2 13]

三、赋值语句的求值顺序

我们再回到前面Rob Pike那个问题：

n0, n1 = n0 + n1, n0

or：

n0, n1 = op(n0, n1), n0

这是一个赋值语句，根据规则3，我们对等号两端的表达式的操作数采用从左到右的求值顺序。

我们假定初值：
n0, n1 = 1, 2

1、第一阶段：等号两端表达式求值，上述问题中，只有右端有n0+n1和n0两个表达式，但表达式的操作数(n0，n1)都是初始化过后的了，因此直接将值带入，得到求值结果。求值后，语句可以看成：n0, n1 = 3, 1；
2、第二阶段：赋值。n0 =3， n1 = 1

//golang-statements-evaluating-order/example4.go
package main

import "fmt"

func example1() {
    n0, n1 := 1, 2
    n0, n1 = n0+n1, n0
    fmt.Println(n0, n1)
}

func op(a, b int) int {
return a + b
}

func example2() {
    n0, n1 := 1, 2
    n0, n1 = op(n0, n1), n0
    fmt.Println(n0, n1)
}

func main() {
example1()
example2()
}

$go run example4.go
3 1
3 1

四、小结

虽说理解了规则，但实际工作中我们还是尽量不要写出像："var a, b, c = f() + v(), g(), sqr(u()) + v()"这样复杂、难以让人理解的语句。必要的话，拆分成多行就好了，还可以增加些代码量（如果你的公司是以代码量为评价绩效指标之一的），得饶人处且饶人啊，烧脑的语句还是尽量避免为好。

以上实验代码在这里可以下载到。

五、参考资料

The Go Programming Language Specification (Version of August 5, 2015)

Go程序调试、分析与优化

八月 25, 2015
18 条评论

Brad Fitzpatrick在YAPC Asia 2015（Yet Another Perl Conference）上做了一次技术分享，题为："Go Debugging, Profiling, and Optimization"。个人感觉这篇分享中价值最大的是BradFitz现场演示的一个有关如何对Go程序进行调试、分析和优化的 Demo，Brad将demo上传到了他个人在github.com的repo中，但不知为何，repo中的代码似乎与repo里talk.md中的说明不甚一致(btw，我并没有看video)。于是打算在这里按照Brad的思路重新走一遍demo的演示流程(所有演示代码在这里可以下载到)。

一、实验环境

$uname -a
Linux pc-tony 3.13.0-61-generic #100~precise1-Ubuntu SMP Wed Jul 29 12:06:40 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

注意:在Darwin或Windows下，profile的结果可能与这里有很大不同(甚至完全不一样的输出和瓶颈热点)。

$go version
go version go1.5 linux/amd64

$ go env
GOARCH="amd64"
GOBIN="/home1/tonybai/.bin/go15/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home1/tonybai/proj/GoProjects"
GORACE=""
GOROOT="/home1/tonybai/.bin/go15"
GOTOOLDIR="/home1/tonybai/.bin/go15/pkg/tool/linux_amd64"
GO15VENDOREXPERIMENT="1"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
CXX="g++"
CGO_ENABLED="1"

代码基于Brad的github.com/bradfitz/talk-yapc-asia-2015。

二、待优化程序(step0)

待优化程序，也就是原始程序，我们放在step0中：

//go-debug-profile-optimization/step0/demo.go

package main

import (
    "fmt"
    "log"
    "net/http"
    "regexp"
)

var visitors int

func handleHi(w http.ResponseWriter, r *http.Request) {
    if match, _ := regexp.MatchString(`^\w*$`, r.FormValue("color")); !match {
        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
        return
    }
    visitors++
    w.Header().Set("Content-Type", "text/html; charset=utf-8")
    w.Write([]byte("<h1 style='color: " + r.FormValue("color") +
        "'>Welcome!</h1>You are visitor number " + fmt.Sprint(visitors) + "!"))
}

func main() {
    log.Printf("Starting on port 8080")
    http.HandleFunc("/hi", handleHi)
    log.Fatal(http.ListenAndServe("127.0.0.1:8080", nil))
}

$go run demo.go
2015/08/25 09:42:35 Starting on port 8080

在浏览器输入：http://localhost:8080/hi

一切顺利的话，页面会显示：

Welcome!

You are visitor number 1!

三、添加测试代码

按照talk.md中的说明，brad repo中demo中根本没有测试代码(commit 2427d0faa12ed1fb05f1e6a1e69307c11259c2b2)。

于是我根据作者的意图，新增了demo_test.go，采用TestHandleHi_Recorder和TestHandleHi_TestServer对HandleHi进行测试：

//go-debug-profile-optimization/step0/demo_test.go
package main

import (
    "bufio"
    "net/http"
    "net/http/httptest"
    "strings"
    "testing"
)

func TestHandleHi_Recorder(t *testing.T) {
    rw := httptest.NewRecorder()
    handleHi(rw, req(t, "GET / HTTP/1.0\r\n\r\n"))
    if !strings.Contains(rw.Body.String(), "visitor number") {
        t.Errorf("Unexpected output: %s", rw.Body)
    }
}

func req(t *testing.T, v string) *http.Request {
    req, err := http.ReadRequest(bufio.NewReader(strings.NewReader(v)))
    if err != nil {
        t.Fatal(err)
    }
    return req
}

func TestHandleHi_TestServer(t *testing.T) {
    ts := httptest.NewServer(http.HandlerFunc(handleHi))
    defer ts.Close()
    res, err := http.Get(ts.URL)
    if err != nil {
        t.Error(err)
        return
    }
    if g, w := res.Header.Get("Content-Type"), "text/html; charset=utf-8"; g != w {
        t.Errorf("Content-Type = %q; want %q", g, w)
    }
    slurp, err := ioutil.ReadAll(res.Body)
    defer res.Body.Close()
    if err != nil {
        t.Error(err)
        return
    }
    t.Logf("Got: %s", slurp)
}

$ go test -v
=== RUN   TestHandleHi_Recorder
— PASS: TestHandleHi_Recorder (0.00s)
=== RUN   TestHandleHi_TestServer
— PASS: TestHandleHi_TestServer (0.00s)
    demo_test.go:45: Got: <h1 style='color: '>Welcome!</h1>You are visitor number 2!
PASS
ok     _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step0    0.007s

测试通过！

至此，step0使命结束。

四、Race Detector(竞态分析）

并发设计使得程序可以更好更有效的利用现代处理器的多核心。但并发设计很容易引入竞态，导致严重bug。Go程序中竞态就是当多个goroutine并发访问某共享数据且未使用同步机制时，且至少一个goroutine进行了写操作。不过go工具自带race分析功能。在分析优化step0中demo代码前，我们先要保证demo代码中不存在竞态。

工具的使用方法就是在go test后加上-race标志，在step0目录下：

$ go test -v -race
=== RUN   TestHandleHi_Recorder
— PASS: TestHandleHi_Recorder (0.00s)
=== RUN   TestHandleHi_TestServer
— PASS: TestHandleHi_TestServer (0.00s)
    demo_test.go:45: Got: <h1 style='color: '>Welcome!</h1>You are visitor number 2!
PASS
ok     _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step0    1.012s

-race通过做运行时分析做竞态分析，虽然不存在误报，但却存在实际有竞态，但工具没发现的情况。接下来我们改造一下测试代码，让test并发起来：

向step1(copy自step0)中demo_test.go中添加一个test method:

//go-debug-profile-optimization/step1/demo_test.go
… …
func TestHandleHi_TestServer_Parallel(t *testing.T) {
    ts := httptest.NewServer(http.HandlerFunc(handleHi))
    defer ts.Close()
    var wg sync.WaitGroup
    for i := 0; i < 2; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
            res, err := http.Get(ts.URL)
            if err != nil {
                t.Error(err)
                return
            }
            if g, w := res.Header.Get("Content-Type"), "text/html; charset=utf-8"; g != w {
                t.Errorf("Content-Type = %q; want %q", g, w)
            }
            slurp, err := ioutil.ReadAll(res.Body)
            defer res.Body.Close()
            if err != nil {
                t.Error(err)
                return
            }
            t.Logf("Got: %s", slurp)
        }()
    }
    wg.Wait()
}
… …

执行竞态test：

$ go test -v -race
=== RUN   TestHandleHi_Recorder
— PASS: TestHandleHi_Recorder (0.00s)
=== RUN   TestHandleHi_TestServer
— PASS: TestHandleHi_TestServer (0.00s)
    demo_test.go:46: Got: <h1 style='color: '>Welcome!</h1>You are visitor number 2!
=== RUN   TestHandleHi_TestServer_Parallel
==================
WARNING: DATA RACE
Read by goroutine 22:
_/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step1.handleHi()
      /home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step1/demo.go:17 +0xf5
net/http.HandlerFunc.ServeHTTP()
      /tmp/workdir/go/src/net/http/server.go:1422 +0×47
net/http/httptest.(*waitGroupHandler).ServeHTTP()
      /tmp/workdir/go/src/net/http/httptest/server.go:200 +0xfe
net/http.serverHandler.ServeHTTP()
      /tmp/workdir/go/src/net/http/server.go:1862 +0×206
net/http.(*conn).serve()
      /tmp/workdir/go/src/net/http/server.go:1361 +0x117c

Previous write by goroutine 25:
_/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step1.handleHi()
      /home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step1/demo.go:17 +0×111
net/http.HandlerFunc.ServeHTTP()
      /tmp/workdir/go/src/net/http/server.go:1422 +0×47
net/http/httptest.(*waitGroupHandler).ServeHTTP()
      /tmp/workdir/go/src/net/http/httptest/server.go:200 +0xfe
net/http.serverHandler.ServeHTTP()
      /tmp/workdir/go/src/net/http/server.go:1862 +0×206
net/http.(*conn).serve()
      /tmp/workdir/go/src/net/http/server.go:1361 +0x117c

Goroutine 22 (running) created at:
net/http.(*Server).Serve()
/tmp/workdir/go/src/net/http/server.go:1910 +0×464

Goroutine 25 (running) created at:
net/http.(*Server).Serve()
      /tmp/workdir/go/src/net/http/server.go:1910 +0×464
==================
— PASS: TestHandleHi_TestServer_Parallel (0.00s)
    demo_test.go:71: Got: <h1 style='color: '>Welcome!</h1>You are visitor number 3!
    demo_test.go:71: Got: <h1 style='color: '>Welcome!</h1>You are visitor number 4!
PASS
Found 1 data race(s)
exit status 66
FAIL    _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step1    1.023s

工具发现demo.go第17行：
visitors++
是一处潜在的竞态条件。

visitors被多个goroutine访问但未采用同步机制。

既然发现了竞态条件，我们就需要fix it。有多种fix方法可选：

1、使用channel
2、使用Mutex
3、使用atomic

Brad使用了atomic：

//go-debug-profile-optimization/step1/demo.go
… …
var visitors int64 // must be accessed atomically

func handleHi(w http.ResponseWriter, r *http.Request) {
    if match, _ := regexp.MatchString(`^\w*$`, r.FormValue("color")); !match {
        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
        return
    }
    visitNum := atomic.AddInt64(&visitors, 1)
    w.Header().Set("Content-Type", "text/html; charset=utf-8")
    w.Write([]byte("<h1 style='color: " + r.FormValue("color") +
        "'>Welcome!</h1>You are visitor number " + fmt.Sprint(visitNum) + "!"))
}
… …

再做一次测试：

竞态条件被消除了！

至此，step1结束了使命！

五、CPU Profiling

要做CPU Profilling，我们需要benchmark数据，Go test提供benchmark test功能，我们只要写对应的Benchmark测试方法即可：

//go-debug-profile-optimization/step2/demo_test.go
… …
func BenchmarkHi(b *testing.B) {
b.ReportAllocs()

    req, err := http.ReadRequest(bufio.NewReader(strings.NewReader("GET / HTTP/1.0\r\n\r\n")))
    if err != nil {
        b.Fatal(err)
    }

    for i := 0; i < b.N; i++ {
        rw := httptest.NewRecorder()
        handleHi(rw, req)
    }
}
… …

$ go test -v -run=^$ -bench=.
PASS
BenchmarkHi-4 100000 14808 ns/op 4961 B/op 81 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step2 1.648s

开始CPU Profiling：

$ go test -v -run=^$ -bench=^BenchmarkHi$ -benchtime=2s -cpuprofile=prof.cpu
PASS
BenchmarkHi-4 200000 14679 ns/op 4961 B/op 81 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step2 3.096s

执行完benchmark test后，step2目录下出现两个新文件prof.cpu和step2.test，这两个文件将作为后续go tool pprof的输入：
$ls
demo.go demo_test.go prof.cpu step2.test*

使用go profile viewer工具：

$ go tool pprof step2.test prof.cpu
Entering interactive mode (type "help" for commands)
(pprof) top
1830ms of 3560ms total (51.40%)
Dropped 53 nodes (cum <= 17.80ms)
Showing top 10 nodes out of 133 (cum >= 1290ms)
      flat flat%   sum%        cum   cum%
     480ms 13.48% 13.48%      980ms 27.53% runtime.growslice
     360ms 10.11% 23.60%      700ms 19.66% runtime.mallocgc
     170ms 4.78% 28.37%      170ms 4.78% runtime.heapBitsSetType
     170ms 4.78% 33.15%      200ms 5.62% runtime.scanblock
     120ms 3.37% 36.52%     1100ms 30.90% regexp.makeOnePass.func2
     120ms 3.37% 39.89%      550ms 15.45% runtime.newarray
     110ms 3.09% 42.98%      300ms 8.43% runtime.makeslice
     110ms 3.09% 46.07%      220ms 6.18% runtime.mapassign1
     100ms 2.81% 48.88%      100ms 2.81% runtime.futex
      90ms 2.53% 51.40%     1290ms 36.24% regexp.makeOnePass

(pprof) top –cum
0.18s of 3.56s total ( 5.06%)
Dropped 53 nodes (cum <= 0.02s)
Showing top 10 nodes out of 133 (cum >= 1.29s)
      flat flat%   sum%        cum   cum%
         0     0%     0%      3.26s 91.57% runtime.goexit
     0.02s 0.56% 0.56%      2.87s 80.62% BenchmarkHi
         0     0% 0.56%      2.87s 80.62% testing.(*B).launch
         0     0% 0.56%      2.87s 80.62% testing.(*B).runN
     0.03s 0.84% 1.40%      2.80s 78.65% step2.handleHi
     0.01s 0.28% 1.69%      2.46s 69.10% regexp.MatchString
         0     0% 1.69%      2.24s 62.92% regexp.Compile
         0     0% 1.69%      2.24s 62.92% regexp.compile
     0.03s 0.84% 2.53%      1.56s 43.82% regexp.compileOnePass
     0.09s 2.53% 5.06%      1.29s 36.24% regexp.makeOnePass

(pprof) list handleHi
Total: 3.56s
ROUTINE ======================== handleHi in go-debug-profile-optimization/step2/demo.go
      30ms      2.80s (flat, cum) 78.65% of Total
         .          .      9:)
         .          .     10:
         .          .     11:var visitors int64 // must be accessed atomically
         .          .     12:
         .          .     13:func handleHi(w http.ResponseWriter, r *http.Request) {
         .      2.47s     14:    if match, _ := regexp.MatchString(`^\w*$`, r.FormValue("color")); !match {
         .          .     15:        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
         .          .     16:        return
         .          .     17:    }
      10ms       20ms     18:    visitNum := atomic.AddInt64(&visitors, 1)
      10ms       90ms     19:    w.Header().Set("Content-Type", "text/html; charset=utf-8")
      10ms       20ms     20:    w.Write([]byte("<h1 style='color: " + r.FormValue("color") +
         .      200ms     21:        "'>Welcome!</h1>You are visitor number " + fmt.Sprint(visitNum) + "!"))
         .          .     22:}
         .          .     23:
         .          .     24:func main() {
         .          .     25:    log.Printf("Starting on port 8080")
         .          .     26:    http.HandleFunc("/hi", handleHi)
(pprof)

从top –cum来看，handleHi消耗cpu较大，而handleHi中，又是MatchString耗时最长。

六、第一次优化

前面已经发现MatchString较为耗时，优化手段：让正则式仅编译一次(step3)：

// go-debug-profile-optimization/step3/demo.go

… …
var visitors int64 // must be accessed atomically

var rxOptionalID = regexp.MustCompile(`^\d*$`)

func handleHi(w http.ResponseWriter, r *http.Request) {
    if !rxOptionalID.MatchString(r.FormValue("color")) {
        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
        return
    }

    visitNum := atomic.AddInt64(&visitors, 1)
    w.Header().Set("Content-Type", "text/html; charset=utf-8")
    w.Write([]byte("<h1 style='color: " + r.FormValue("color") +
        "'>Welcome!</h1>You are visitor number " + fmt.Sprint(visitNum) + "!"))
}
… …

运行一下bench：

$ go test -bench=.
PASS
BenchmarkHi-4 1000000 1678 ns/op 720 B/op 9 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step3 1.710s

对比之前在step2中运行的bench结果：

$ go test -v -run=^$ -bench=.
PASS
BenchmarkHi-4 100000 14808 ns/op 4961 B/op 81 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step2 1.648s

耗时相同，但优化后的bench运行了100w次，而之前的Bench运行10w次，相当于性能提高10倍。

再看看cpu prof结果：

$ go test -v -run=^$ -bench=^BenchmarkHi$ -benchtime=3s -cpuprofile=prof.cpu
PASS
BenchmarkHi-4 3000000 1640 ns/op 720 B/op 9 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step3 6.540s

$ go tool pprof step3.test prof.cpu
Entering interactive mode (type "help" for commands)
(pprof) top –cum 30
2.74s of 8.07s total (33.95%)
Dropped 72 nodes (cum <= 0.04s)
Showing top 30 nodes out of 103 (cum >= 0.56s)
      flat flat%   sum%        cum   cum%
         0     0%     0%      7.17s 88.85% runtime.goexit
     0.05s 0.62% 0.62%      6.21s 76.95% step3.BenchmarkHi
         0     0% 0.62%      6.21s 76.95% testing.(*B).launch
         0     0% 0.62%      6.21s 76.95% testing.(*B).runN
     0.06s 0.74% 1.36%      4.96s 61.46% step3.handleHi
     1.15s 14.25% 15.61%      2.35s 29.12% runtime.mallocgc
     0.02s 0.25% 15.86%      1.63s 20.20% runtime.systemstack
         0     0% 15.86%      1.53s 18.96% net/http.Header.Set
     0.06s 0.74% 16.60%      1.53s 18.96% net/textproto.MIMEHeader.Set
     0.09s 1.12% 17.72%      1.22s 15.12% runtime.newobject
     0.05s 0.62% 18.34%      1.09s 13.51% fmt.Sprint
     0.20s 2.48% 20.82%         1s 12.39% runtime.mapassign1
         0     0% 20.82%      0.81s 10.04% runtime.mcall
     0.01s 0.12% 20.94%      0.79s 9.79% runtime.schedule
     0.05s 0.62% 21.56%      0.76s 9.42% regexp.(*Regexp).MatchString
     0.09s 1.12% 22.68%      0.71s 8.80% regexp.(*Regexp).doExecute
     0.01s 0.12% 22.80%      0.71s 8.80% runtime.concatstring5
     0.20s 2.48% 25.28%      0.70s 8.67% runtime.concatstrings
         0     0% 25.28%      0.69s 8.55% runtime.gosweepone
     0.05s 0.62% 25.90%      0.69s 8.55% runtime.mSpan_Sweep
         0     0% 25.90%      0.68s 8.43% runtime.bgsweep
     0.04s   0.5% 26.39%      0.68s 8.43% runtime.newarray
     0.01s 0.12% 26.52%      0.67s 8.30% runtime.goschedImpl
     0.01s 0.12% 26.64%      0.65s 8.05% runtime.gosched_m
         0     0% 26.64%      0.65s 8.05% runtime.gosweepone.func1
     0.01s 0.12% 26.77%      0.65s 8.05% runtime.sweepone
     0.28s 3.47% 30.24%      0.62s 7.68% runtime.makemap
     0.17s 2.11% 32.34%      0.59s 7.31% runtime.heapBitsSweepSpan
     0.02s 0.25% 32.59%      0.58s 7.19% fmt.(*pp).doPrint
     0.11s 1.36% 33.95%      0.56s 6.94% fmt.(*pp).printArg

handleHi耗时有一定下降。

七、Mem Profiling

在step3目录下执行bench，获取mem分配数据：

$ go test -v -run=^$ -bench=^BenchmarkHi$ -benchtime=2s -memprofile=prof.mem
PASS
BenchmarkHi-4 2000000 1657 ns/op 720 B/op 9 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step3 5.002s

使用pprof工具分析mem：

$ go tool pprof –alloc_space step3.test prof.mem
Entering interactive mode (type "help" for commands)
(pprof) top
2065.91MB of 2067.41MB total (99.93%)
Dropped 14 nodes (cum <= 10.34MB)
      flat flat%   sum%        cum   cum%
1076.35MB 52.06% 52.06% 1076.35MB 52.06% net/textproto.MIMEHeader.Set
535.54MB 25.90% 77.97% 2066.91MB   100% step3.BenchmarkHi
406.52MB 19.66% 97.63% 1531.37MB 74.07% step3.handleHi
   47.50MB 2.30% 99.93%    48.50MB 2.35% fmt.Sprint
         0     0% 99.93% 1076.35MB 52.06% net/http.Header.Set
         0     0% 99.93% 2066.91MB   100% runtime.goexit
         0     0% 99.93% 2066.91MB   100% testing.(*B).launch
         0     0% 99.93% 2066.91MB   100% testing.(*B).runN

(pprof) top -cum
2065.91MB of 2067.41MB total (99.93%)
Dropped 14 nodes (cum <= 10.34MB)
      flat flat%   sum%        cum   cum%
535.54MB 25.90% 25.90% 2066.91MB   100% step3.BenchmarkHi
         0     0% 25.90% 2066.91MB   100% runtime.goexit
         0     0% 25.90% 2066.91MB   100% testing.(*B).launch
         0     0% 25.90% 2066.91MB   100% testing.(*B).runN
406.52MB 19.66% 45.57% 1531.37MB 74.07% step3.handleHi
         0     0% 45.57% 1076.35MB 52.06% net/http.Header.Set
1076.35MB 52.06% 97.63% 1076.35MB 52.06% net/textproto.MIMEHeader.Set
   47.50MB 2.30% 99.93%    48.50MB 2.35% fmt.Sprint

(pprof) list handleHi
Total: 2.02GB
     ROUTINE =========step3.handleHi in step3/demo.go
406.52MB     1.50GB (flat, cum) 74.07% of Total
         .          .     17:        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
         .          .     18:        return
         .          .     19:    }
         .          .     20:
         .          .     21:    visitNum := atomic.AddInt64(&visitors, 1)
         .     1.05GB     22:    w.Header().Set("Content-Type", "text/html; charset=utf-8")
         .          .     23:    w.Write([]byte("<h1 style='color: " + r.FormValue("color") +
406.52MB   455.02MB     24:        "'>Welcome!</h1>You are visitor number " + fmt.Sprint(visitNum) + "!"))
         .          .     25:}
         .          .     26:
         .          .     27:func main() {
         .          .     28:    log.Printf("Starting on port 8080")
         .          .     29:    http.HandleFunc("/hi", handleHi)
(pprof)

可以看到handleHi22、23两行占用了较多内存。

八、第二次优化

第二次优化的方法：
1、删除w.Header().Set这行
2、用fmt.Fprintf替代w.Write

第二次优化的代码在step4目录中：

// go-debug-profile-optimization/step4/demo.go
… …
func handleHi(w http.ResponseWriter, r *http.Request) {
    if !rxOptionalID.MatchString(r.FormValue("color")) {
        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
        return
    }

visitNum := atomic.AddInt64(&visitors, 1)
fmt.Fprintf(w, "<html><h1 stype='color: \"%s\"'>Welcome!</h1>You are visitor number %d!", r.FormValue("color"), visitNum)
}
… …

执行一遍pprof:

$ go test -v -run=^$ -bench=^BenchmarkHi$ -benchtime=2s -memprofile=prof.mem
PASS
BenchmarkHi-4 2000000 1428 ns/op 304 B/op 6 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step4 4.343s

$ go tool pprof –alloc_space step4.test prof.mem
Entering interactive mode (type "help" for commands)
(pprof) top
868.06MB of 868.56MB total (99.94%)
Dropped 5 nodes (cum <= 4.34MB)
      flat flat%   sum%        cum   cum%
559.54MB 64.42% 64.42%   868.06MB 99.94% step4.BenchmarkHi
219.52MB 25.27% 89.70%   219.52MB 25.27% bytes.makeSlice
      89MB 10.25% 99.94%   308.52MB 35.52% step4.handleHi
         0     0% 99.94%   219.52MB 25.27% bytes.(*Buffer).Write
         0     0% 99.94%   219.52MB 25.27% bytes.(*Buffer).grow
         0     0% 99.94%   219.52MB 25.27% fmt.Fprintf
         0     0% 99.94%   219.52MB 25.27% net/http/httptest.(*ResponseRecorder).Write
         0     0% 99.94%   868.06MB 99.94% runtime.goexit
         0     0% 99.94%   868.06MB 99.94% testing.(*B).launch
         0     0% 99.94%   868.06MB 99.94% testing.(*B).runN
(pprof) top –cum
868.06MB of 868.56MB total (99.94%)
Dropped 5 nodes (cum <= 4.34MB)
      flat flat%   sum%        cum   cum%
559.54MB 64.42% 64.42%   868.06MB 99.94% step4.BenchmarkHi
         0     0% 64.42%   868.06MB 99.94% runtime.goexit
         0     0% 64.42%   868.06MB 99.94% testing.(*B).launch
         0     0% 64.42%   868.06MB 99.94% testing.(*B).runN
      89MB 10.25% 74.67%   308.52MB 35.52% step4.handleHi
         0     0% 74.67%   219.52MB 25.27% bytes.(*Buffer).Write
         0     0% 74.67%   219.52MB 25.27% bytes.(*Buffer).grow
219.52MB 25.27% 99.94%   219.52MB 25.27% bytes.makeSlice
         0     0% 99.94%   219.52MB 25.27% fmt.Fprintf
         0     0% 99.94%   219.52MB 25.27% net/http/httptest.(*ResponseRecorder).Write
(pprof) list handleHi
Total: 868.56MB
ROUTINE ============ step4.handleHi in step4/demo.go
      89MB   308.52MB (flat, cum) 35.52% of Total
         .          .     17:        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
         .          .     18:        return
         .          .     19:    }
         .          .     20:
         .          .     21:    visitNum := atomic.AddInt64(&visitors, 1)
      89MB   308.52MB     22:    fmt.Fprintf(w, "<html><h1 stype='color: \"%s\"'>Welcome!</h1>You are visitor number %d!", r.FormValue("color"), visitNum)
         .          .     23:}
         .          .     24:
         .          .     25:func main() {
         .          .     26:    log.Printf("Starting on port 8080")
         .          .     27:    http.HandleFunc("/hi", handleHi)
(pprof)

可以看出内存占用大幅减少。

九、Benchcmp

golang.org/x/tools中有一个工具：benchcmp，可以给出两次bench的结果对比。

github.com/golang/tools是golang.org/x/tools的一个镜像。安装benchcmp步骤：

1、go get -u github.com/golang/tools
2、mkdir -p $GOPATH/src/golang.org/x
3、mv $GOPATH/src/github.com/golang/tools $GOPATH/src/golang.org/x
4、go install golang.org/x/tools/cmd/benchcmp

我们分别在step2、step3和step4下执行如下命令：

$ go-debug-profile-optimization/step2$ go test -bench=. -memprofile=prof.mem | tee mem.2
PASS
BenchmarkHi-4 100000 14786 ns/op 4961 B/op 81 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step2 1.644s

go-debug-profile-optimization/step3$ go test -bench=. -memprofile=prof.mem | tee mem.3
PASS
BenchmarkHi-4 1000000 1662 ns/op 720 B/op 9 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step3 1.694s

go-debug-profile-optimization/step4$ go test -bench=. -memprofile=prof.mem | tee mem.4
PASS
BenchmarkHi-4 1000000 1428 ns/op 304 B/op 6 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step4 1.456s

利用benchcmp工具对比结果（benchcmp old new）：

$ benchcmp step3/mem.3 step4/mem.4
benchmark old ns/op new ns/op delta
BenchmarkHi-4 1662 1428 -14.08%

benchmark old allocs new allocs delta
BenchmarkHi-4 9 6 -33.33%

benchmark old bytes new bytes delta
BenchmarkHi-4 720 304 -57.78%

$ benchcmp step2/mem.2 step4/mem.4
benchmark old ns/op new ns/op delta
BenchmarkHi-4 14786 1428 -90.34%

benchmark old allocs new allocs delta
BenchmarkHi-4 81 6 -92.59%

benchmark old bytes new bytes delta
BenchmarkHi-4 4961 304 -93.87%

可以看出优化后，内存分配大幅减少，gc的时间也随之减少。

十、内存来自哪

我们在BenchmarkHi中清理每次handleHi执行后的内存：

//step5/demo_test.go
… …
func BenchmarkHi(b *testing.B) {
b.ReportAllocs()

    req, err := http.ReadRequest(bufio.NewReader(strings.NewReader("GET / HTTP/1.0\r\n\r\n")))
    if err != nil {
        b.Fatal(err)
    }

    for i := 0; i < b.N; i++ {
        rw := httptest.NewRecorder()
        handleHi(rw, req)
        reset(rw)
    }
}

func reset(rw *httptest.ResponseRecorder) {
    m := rw.HeaderMap
    for k := range m {
        delete(m, k)
    }
    body := rw.Body
    body.Reset()
    *rw = httptest.ResponseRecorder{
        Body:      body,
        HeaderMap: m,
    }
}

… …
$ go test -v -run=^$ -bench=^BenchmarkHi$ -benchtime=2s -memprofile=prof.mem
PASS
BenchmarkHi-4 2000000 1518 ns/op 304 B/op 6 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step5 4.577s

$ go tool pprof –alloc_space step5.test prof.mem
Entering interactive mode (type "help" for commands)
(pprof) top –cum 10
290.52MB of 291.52MB total (99.66%)
Dropped 14 nodes (cum <= 1.46MB)
      flat flat%   sum%        cum   cum%
         0     0%     0%   291.02MB 99.83% runtime.goexit
179.01MB 61.41% 61.41%   290.52MB 99.66% step5.BenchmarkHi
         0     0% 61.41%   290.52MB 99.66% testing.(*B).launch
         0     0% 61.41%   290.52MB 99.66% testing.(*B).runN
   26.50MB 9.09% 70.50%   111.51MB 38.25% step5.handleHi
         0     0% 70.50%    85.01MB 29.16% bytes.(*Buffer).Write
         0     0% 70.50%    85.01MB 29.16% bytes.(*Buffer).grow
   85.01MB 29.16% 99.66%    85.01MB 29.16% bytes.makeSlice
         0     0% 99.66%    85.01MB 29.16% fmt.Fprintf
         0     0% 99.66%    85.01MB 29.16% net/http/httptest.(*ResponseRecorder).Write
(pprof) list handleHi
Total: 291.52MB
ROUTINE ======================== _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step5.handleHi in /home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step5/demo.go
   26.50MB   111.51MB (flat, cum) 38.25% of Total
         .          .     17:        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
         .          .     18:        return
         .          .     19:    }
         .          .     20:
         .          .     21:    visitNum := atomic.AddInt64(&visitors, 1)
   26.50MB   111.51MB     22:    fmt.Fprintf(w, "<html><h1 stype='color: \"%s\"'>Welcome!</h1>You are visitor number %d!", r.FormValue("color"), visitNum)
         .          .     23:}
         .          .     24:
         .          .     25:func main() {
         .          .     26:    log.Printf("Starting on port 8080")
         .          .     27:    http.HandleFunc("/hi", handleHi)
(pprof)

内存从300MB降到111MB。内存来自哪？看到list handleHi，fmt.Fprintf分配了111.51MB。

我们来看这一行代码：
fmt.Fprintf(w, "<h1 style='color: %s'>Welcome!</h1>You are visitor number %d!",
r.FormValue("color"), num)

fmt.Fprintf的manual：

$ go doc fmt.Fprintf
func Fprintf(w io.Writer, format string, a …interface{}) (n int, err error)

Fprintf formats according to a format specifier and writes to w. It returns
the number of bytes written and any write error encountered.

这里回顾一下Go type在runtime中的内存占用：

A Go interface is 2 words of memory: (type, pointer).
A Go string is 2 words of memory: (base pointer, length)
A Go slice is 3 words of memory: (base pointer, length, capacity)

每次调用fmt.Fprintf，参数以value值形式传入函数时，程序就要为每个变参分配一个占用16bytes的empty interface，然后用传入的类型初始化该interface value。这就是这块累计分配内存较多的原因。

十一、消除所有内存分配

下面的优化代码可能在实际中并不需要，但一旦真的成为瓶颈，可以这么做：

//go-debug-profile-optimization/step6/demo.go
… …
var bufPool = sync.Pool{
    New: func() interface{} {
        return new(bytes.Buffer)
    },
}

    visitNum := atomic.AddInt64(&visitors, 1)
    buf := bufPool.Get().(*bytes.Buffer)
    defer bufPool.Put(buf)
    buf.Reset()
    buf.WriteString("<h1 style='color: ")
    buf.WriteString(r.FormValue("color"))
    buf.WriteString("'>Welcome!</h1>You are visitor number ")
    b := strconv.AppendInt(buf.Bytes(), int64(visitNum), 10)
    b = append(b, '!')
    w.Write(b)
}
… …

$ go test -v -run=^$ -bench=^BenchmarkHi$ -benchtime=2s -memprofile=prof.mem
PASS
BenchmarkHi-4 5000000 780 ns/op 192 B/op 3 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step6 4.709s

go tool pprof –alloc_space step6.test prof.mem
Entering interactive mode (type "help" for commands)
(pprof) top –cum 10
1.07GB of 1.07GB total ( 100%)
Dropped 5 nodes (cum <= 0.01GB)
      flat flat%   sum%        cum   cum%
    1.07GB   100%   100%     1.07GB   100% step6.BenchmarkHi
         0     0%   100%     1.07GB   100% runtime.goexit
         0     0%   100%     1.07GB   100% testing.(*B).launch
         0     0%   100%     1.07GB   100% testing.(*B).runN

$ go test -bench=. -memprofile=prof.mem | tee mem.6
PASS
BenchmarkHi-4 2000000 790 ns/op 192 B/op 3 allocs/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step6 2.401s

$ benchcmp step5/mem.5 step6/mem.6
benchmark old ns/op new ns/op delta
BenchmarkHi-4 1513 790 -47.79%

benchmark old allocs new allocs delta
BenchmarkHi-4 6 3 -50.00%

benchmark old bytes new bytes delta
BenchmarkHi-4 304 192 -36.84%

可以看到handleHi已经不在top列表中了。benchcmp结果也显示内存分配又有大幅下降！

十二、竞争(Contention)优化

为handleHi编写一个Parallel benchmark test:

//go-debug-profile-optimization/step7/demo_test.go
… …
func BenchmarkHiParallel(b *testing.B) {
    r, err := http.ReadRequest(bufio.NewReader(strings.NewReader("GET / HTTP/1.0\r\n\r\n")))
    if err != nil {
        b.Fatal(err)
    }

    b.RunParallel(func(pb *testing.PB) {
        rw := httptest.NewRecorder()
        for pb.Next() {
            handleHi(rw, r)
            reset(rw)
        }
    })
}
… …

执行测试，并分析结果:

$ go test -bench=Parallel -blockprofile=prof.block
PASS
BenchmarkHiParallel-4 5000000 305 ns/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step7 1.947s

$ go tool pprof step7.test prof.block
Entering interactive mode (type "help" for commands)
(pprof) top –cum 10
3.68s of 3.72s total (98.82%)
Dropped 29 nodes (cum <= 0.02s)
Showing top 10 nodes out of 20 (cum >= 1.84s)
      flat flat%   sum%        cum   cum%
         0     0%     0%      3.72s   100% runtime.goexit
     1.84s 49.46% 49.46%      1.84s 49.46% runtime.chanrecv1
         0     0% 49.46%      1.84s 49.46% main.main
         0     0% 49.46%      1.84s 49.46% runtime.main
         0     0% 49.46%      1.84s 49.46% testing.(*M).Run
         0     0% 49.46%      1.84s 49.43% testing.(*B).run
         0     0% 49.46%      1.84s 49.43% testing.RunBenchmarks
         0     0% 49.46%      1.84s 49.36% step7.BenchmarkHiParallel
     1.84s 49.36% 98.82%      1.84s 49.36% sync.(*WaitGroup).Wait
         0     0% 98.82%      1.84s 49.36% testing.(*B).RunParallel
(pprof) list BenchmarkHiParallel
Total: 3.72s
ROUTINE ====== step7.BenchmarkHiParallel in step7/demo_test.go
         0      1.84s (flat, cum) 49.36% of Total
         .          .    113:        rw := httptest.NewRecorder()
         .          .    114:        for pb.Next() {
         .          .    115:            handleHi(rw, r)
         .          .    116:            reset(rw)
         .          .    117:        }
         .      1.84s    118:    })
         .          .    119:}
ROUTINE ==== step7.BenchmarkHiParallel.func1 in step7/demo_test.go
         0    43.02ms (flat, cum) 1.16% of Total
         .          .    110:    }
         .          .    111:
         .          .    112:    b.RunParallel(func(pb *testing.PB) {
         .          .    113:        rw := httptest.NewRecorder()
         .          .    114:        for pb.Next() {
         .    43.02ms    115:            handleHi(rw, r)
         .          .    116:            reset(rw)
         .          .    117:        }
         .          .    118:    })
         .          .    119:}
(pprof) list handleHi
Total: 3.72s
ROUTINE =====step7.handleHi in step7/demo.go
         0    43.02ms (flat, cum) 1.16% of Total
         .          .     18:        return new(bytes.Buffer)
         .          .     19:    },
         .          .     20:}
         .          .     21:
         .          .     22:func handleHi(w http.ResponseWriter, r *http.Request) {
         .    43.01ms     23:    if !rxOptionalID.MatchString(r.FormValue("color")) {
         .          .     24:        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
         .          .     25:        return
         .          .     26:    }
         .          .     27:
         .          .     28:    visitNum := atomic.AddInt64(&visitors, 1)
         .     2.50us     29:    buf := bufPool.Get().(*bytes.Buffer)
         .          .     30:    defer bufPool.Put(buf)
         .          .     31:    buf.Reset()
         .          .     32:    buf.WriteString("<h1 style='color: ")
         .          .     33:    buf.WriteString(r.FormValue("color"))
         .          .     34:    buf.WriteString("'>Welcome!</h1>You are visitor number ")
(pprof)

handleHi中MatchString这块是一个焦点，这里耗时较多。

优化方法（step8）：

//go-debug-profile-optimization/step8/demo.go
… …
var colorRxPool = sync.Pool{
New: func() interface{} { return regexp.MustCompile(`\w*$`) },
}

func handleHi(w http.ResponseWriter, r *http.Request) {
    if !colorRxPool.Get().(*regexp.Regexp).MatchString(r.FormValue("color")) {
        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
        return
    }

测试执行与分析：

$ go test -bench=Parallel -blockprofile=prof.block
PASS
BenchmarkHiParallel-4 100000 19190 ns/op
ok _/home1/tonybai/proj/opensource/github/experiments/go-debug-profile-optimization/step8 2.219s

$ go tool pprof step8.test prof.block
Entering interactive mode (type "help" for commands)
(pprof) top –cum 10
4.22s of 4.23s total (99.69%)
Dropped 28 nodes (cum <= 0.02s)
Showing top 10 nodes out of 12 (cum >= 2.11s)
      flat flat%   sum%        cum   cum%
         0     0%     0%      4.23s   100% runtime.goexit
     2.11s 49.90% 49.90%      2.11s 49.90% runtime.chanrecv1
         0     0% 49.90%      2.11s 49.89% main.main
         0     0% 49.90%      2.11s 49.89% runtime.main
         0     0% 49.90%      2.11s 49.89% testing.(*M).Run
         0     0% 49.90%      2.11s 49.86% testing.(*B).run
         0     0% 49.90%      2.11s 49.86% testing.RunBenchmarks
         0     0% 49.90%      2.11s 49.79% step8.BenchmarkHiParallel
     2.11s 49.79% 99.69%      2.11s 49.79% sync.(*WaitGroup).Wait
         0     0% 99.69%      2.11s 49.79% testing.(*B).RunParallel
(pprof) list BenchmarkHiParallel
Total: 4.23s
ROUTINE ======step8.BenchmarkHiParallel in step8/demo_test.go
         0      2.11s (flat, cum) 49.79% of Total
         .          .    113:        rw := httptest.NewRecorder()
         .          .    114:        for pb.Next() {
         .          .    115:            handleHi(rw, r)
         .          .    116:            reset(rw)
         .          .    117:        }
         .      2.11s    118:    })
         .          .    119:}
ROUTINE ======step8.BenchmarkHiParallel.func1 in step8/demo_test.go
         0    11.68ms (flat, cum) 0.28% of Total
         .          .    110:    }
         .          .    111:
         .          .    112:    b.RunParallel(func(pb *testing.PB) {
         .          .    113:        rw := httptest.NewRecorder()
         .          .    114:        for pb.Next() {
         .    11.68ms    115:            handleHi(rw, r)
         .          .    116:            reset(rw)
         .          .    117:        }
         .          .    118:    })
         .          .    119:}
(pprof) list handleHi
Total: 4.23s
ROUTINE ======step8.handleHi in step8/demo.go
         0    11.68ms (flat, cum) 0.28% of Total
         .          .     21:var colorRxPool = sync.Pool{
         .          .     22:    New: func() interface{} { return regexp.MustCompile(`\w*$`) },
         .          .     23:}
         .          .     24:
         .          .     25:func handleHi(w http.ResponseWriter, r *http.Request) {
         .     5.66ms     26:    if !colorRxPool.Get().(*regexp.Regexp).MatchString(r.FormValue("color")) {
         .          .     27:        http.Error(w, "Optional color is invalid", http.StatusBadRequest)
         .          .     28:        return
         .          .     29:    }
         .          .     30:
         .          .     31:    visitNum := atomic.AddInt64(&visitors, 1)
         .     6.02ms     32:    buf := bufPool.Get().(*bytes.Buffer)
         .          .     33:    defer bufPool.Put(buf)
         .          .     34:    buf.Reset()
         .          .     35:    buf.WriteString("<h1 style='color: ")
         .          .     36:    buf.WriteString(r.FormValue("color"))
         .          .     37:    buf.WriteString("'>Welcome!</h1>You are visitor number ")
(pprof)