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2023-06-26 17:30:52 +00:00
< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface-zh.html" > 译者序< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< /
2022-08-04 06:59:33 +00:00
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2024-06-12 07:20:07 +00:00
< li > < em > Go语言圣经读书笔记(挖坑中...): < a href = "https://github.com/chai2010/gopl-notes-zh" > https://github.com/chai2010/gopl-notes-zh< / a > < / em > < / li >
< li > < em > 凹语言(Go实现, 面向WASM设计): < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2023-06-26 17:46:27 +00:00
< / ul >
2022-08-04 06:59:33 +00:00
< hr >
< h2 id = "97-示例-并发的非阻塞缓存" > < a class = "header" href = "#97-示例-并发的非阻塞缓存" > 9.7. 示例: 并发的非阻塞缓存< / a > < / h2 >
< p > 本节中我们会做一个无阻塞的缓存， （ ） （ ： ： ） ， ， ， ， ， < / p >
< p > 我们将使用下面的httpGetBody函数作为我们需要缓存的函数的一个样例。这个函数会去进行HTTP GET请求并且获取http响应body。对这个函数的调用本身开销是比较大的， < / p >
< pre > < code class = "language-go" > func httpGetBody(url string) (interface{}, error) {
resp, err := http.Get(url)
if err != nil {
return nil, err
}
defer resp.Body.Close()
return ioutil.ReadAll(resp.Body)
}
< / code > < / pre >
< p > 最后一行稍微隐藏了一些细节。ReadAll会返回两个结果， ， ， < / p >
< p > 下面是我们要设计的cache的第一个“草稿”： < / p >
< p > < u > < i > gopl.io/ch9/memo1< / i > < / u > < / p >
< pre > < code class = "language-go" > // Package memo provides a concurrency-unsafe
// memoization of a function of type Func.
package memo
// A Memo caches the results of calling a Func.
type Memo struct {
f Func
cache map[string]result
}
// Func is the type of the function to memoize.
type Func func(key string) (interface{}, error)
type result struct {
value interface{}
err error
}
func New(f Func) *Memo {
return & Memo{f: f, cache: make(map[string]result)}
}
// NOTE: not concurrency-safe!
func (memo *Memo) Get(key string) (interface{}, error) {
res, ok := memo.cache[key]
if !ok {
res.value, res.err = memo.f(key)
memo.cache[key] = res
}
return res.value, res.err
}
< / code > < / pre >
< p > Memo实例会记录需要缓存的函数f（ ） ， （ ） ， < / p >
< p > 下面是一个使用Memo的例子。对于流入的URL的每一个元素我们都会调用Get， ： < / p >
< pre > < code class = "language-go" > m := memo.New(httpGetBody)
for url := range incomingURLs() {
start := time.Now()
value, err := m.Get(url)
if err != nil {
log.Print(err)
}
fmt.Printf(" %s, %s, %d bytes\n" ,
url, time.Since(start), len(value.([]byte)))
}
< / code > < / pre >
< p > 我们可以使用测试包（ ） ， ， < code > (*Memo).Get< / code > 的调用都会花上几百毫秒， < / p >
< pre > < code > $ go test -v gopl.io/ch9/memo1
=== RUN Test
https://golang.org, 175.026418ms, 7537 bytes
https://godoc.org, 172.686825ms, 6878 bytes
https://play.golang.org, 115.762377ms, 5767 bytes
http://gopl.io, 749.887242ms, 2856 bytes
https://golang.org, 721ns, 7537 bytes
https://godoc.org, 152ns, 6878 bytes
https://play.golang.org, 205ns, 5767 bytes
http://gopl.io, 326ns, 2856 bytes
--- PASS: Test (1.21s)
PASS
ok gopl.io/ch9/memo1 1.257s
< / code > < / pre >
< p > 这个测试是顺序地去做所有的调用的。< / p >
< p > 由于这种彼此独立的HTTP请求可以很好地并发， < / p >
< pre > < code class = "language-go" > m := memo.New(httpGetBody)
var n sync.WaitGroup
for url := range incomingURLs() {
n.Add(1)
go func(url string) {
start := time.Now()
value, err := m.Get(url)
if err != nil {
log.Print(err)
}
fmt.Printf(" %s, %s, %d bytes\n" ,
url, time.Since(start), len(value.([]byte)))
n.Done()
}(url)
}
n.Wait()
< / code > < / pre >
< p > 这次测试跑起来更快了， （ ） ， ， < / p >
< p > 但更糟糕的是， （ ： ） ， ， （ < / p >
< pre > < code > $ go test -run=TestConcurrent -race -v gopl.io/ch9/memo1
=== RUN TestConcurrent
...
WARNING: DATA RACE
Write by goroutine 36:
runtime.mapassign1()
~/go/src/runtime/hashmap.go:411 +0x0
gopl.io/ch9/memo1.(*Memo).Get()
~/gobook2/src/gopl.io/ch9/memo1/memo.go:32 +0x205
...
Previous write by goroutine 35:
runtime.mapassign1()
~/go/src/runtime/hashmap.go:411 +0x0
gopl.io/ch9/memo1.(*Memo).Get()
~/gobook2/src/gopl.io/ch9/memo1/memo.go:32 +0x205
...
Found 1 data race(s)
FAIL gopl.io/ch9/memo1 2.393s
< / code > < / pre >
< p > memo.go的32行出现了两次， ， < / p >
< pre > < code class = "language-go" > 28 func (memo *Memo) Get(key string) (interface{}, error) {
29 res, ok := memo.cache(key)
30 if !ok {
31 res.value, res.err = memo.f(key)
32 memo.cache[key] = res
33 }
34 return res.value, res.err
35 }
< / code > < / pre >
< p > 最简单的使cache并发安全的方式是使用基于监控的同步。只要给Memo加上一个mutex， ， ， ： < / p >
< p > < u > < i > gopl.io/ch9/memo2< / i > < / u > < / p >
< pre > < code class = "language-go" > type Memo struct {
f Func
mu sync.Mutex // guards cache
cache map[string]result
}
// Get is concurrency-safe.
func (memo *Memo) Get(key string) (value interface{}, err error) {
memo.mu.Lock()
res, ok := memo.cache[key]
if !ok {
res.value, res.err = memo.f(key)
memo.cache[key] = res
}
memo.mu.Unlock()
return res.value, res.err
}
< / code > < / pre >
< p > 测试依然并发进行， ， ， < / p >
< p > 下一个Get的实现， ： ， ， ， < / p >
< p > < u > < i > gopl.io/ch9/memo3< / i > < / u > < / p >
< pre > < code class = "language-go" > func (memo *Memo) Get(key string) (value interface{}, err error) {
memo.mu.Lock()
res, ok := memo.cache[key]
memo.mu.Unlock()
if !ok {
res.value, res.err = memo.f(key)
// Between the two critical sections, several goroutines
// may race to compute f(key) and update the map.
memo.mu.Lock()
memo.cache[key] = res
memo.mu.Unlock()
}
return res.value, res.err
}
< / code > < / pre >
< p > 这些修改使性能再次得到了提升， ， ， ， < / p >
2022-10-23 23:50:32 +00:00
< p > 理想情况下是应该避免掉多余的工作的。而这种“避免”工作一般被称为duplicate suppression（ ） ， ， （ ） < / p >
2022-08-04 06:59:33 +00:00
< p > < u > < i > gopl.io/ch9/memo4< / i > < / u > < / p >
< pre > < code class = "language-go" > type entry struct {
res result
ready chan struct{} // closed when res is ready
}
func New(f Func) *Memo {
return & Memo{f: f, cache: make(map[string]*entry)}
}
type Memo struct {
f Func
mu sync.Mutex // guards cache
cache map[string]*entry
}
func (memo *Memo) Get(key string) (value interface{}, err error) {
memo.mu.Lock()
e := memo.cache[key]
if e == nil {
// This is the first request for this key.
// This goroutine becomes responsible for computing
// the value and broadcasting the ready condition.
e = & entry{ready: make(chan struct{})}
memo.cache[key] = e
memo.mu.Unlock()
e.res.value, e.res.err = memo.f(key)
close(e.ready) // broadcast ready condition
} else {
// This is a repeat request for this key.
memo.mu.Unlock()
< -e.ready // wait for ready condition
}
return e.res.value, e.res.err
}
< / code > < / pre >
< p > 现在Get函数包括下面这些步骤了： ， ， ， （ ） ， ， ， < / p >
< p > 如果没有条目的话， ， < / p >
< p > 条目中的e.res.value和e.res.err变量是在多个goroutine之间共享的。创建条目的goroutine同时也会设置条目的值， " ready" 的广播消息之后立刻会去读取条目的值。尽管会被多个goroutine同时访问， ， < / p >
< p > 这样并发、不重复、无阻塞的cache就完成了。< / p >
< p > 上面这样Memo的实现使用了一个互斥量来保护多个goroutine调用Get时的共享map变量。不妨把这种设计和前面提到的把map变量限制在一个单独的monitor goroutine的方案做一些对比， < / p >
< p > Func、result和entry的声明和之前保持一致： < / p >
< pre > < code class = "language-go" > // Func is the type of the function to memoize.
type Func func(key string) (interface{}, error)
// A result is the result of calling a Func.
type result struct {
value interface{}
err error
}
type entry struct {
res result
ready chan struct{} // closed when res is ready
}
< / code > < / pre >
< p > 然而Memo类型现在包含了一个叫做requests的channel， ， < / p >
< p > < u > < i > gopl.io/ch9/memo5< / i > < / u > < / p >
< pre > < code class = "language-go" > // A request is a message requesting that the Func be applied to key.
type request struct {
key string
response chan< - result // the client wants a single result
}
type Memo struct{ requests chan request }
// New returns a memoization of f. Clients must subsequently call Close.
func New(f Func) *Memo {
memo := & Memo{requests: make(chan request)}
go memo.server(f)
return memo
}
func (memo *Memo) Get(key string) (interface{}, error) {
response := make(chan result)
memo.requests < - request{key, response}
res := < -response
return res.value, res.err
}
func (memo *Memo) Close() { close(memo.requests) }
< / code > < / pre >
< p > 上面的Get方法， ， ， ， < / p >
< p > cache变量被限制在了monitor goroutine ``(*Memo).server`中， ， ， ， < / p >
< pre > < code class = "language-go" > func (memo *Memo) server(f Func) {
cache := make(map[string]*entry)
for req := range memo.requests {
e := cache[req.key]
if e == nil {
// This is the first request for this key.
e = & entry{ready: make(chan struct{})}
cache[req.key] = e
go e.call(f, req.key) // call f(key)
}
go e.deliver(req.response)
}
}
func (e *entry) call(f Func, key string) {
// Evaluate the function.
e.res.value, e.res.err = f(key)
// Broadcast the ready condition.
close(e.ready)
}
func (e *entry) deliver(response chan< - result) {
// Wait for the ready condition.
< -e.ready
// Send the result to the client.
response < - e.res
}
< / code > < / pre >
< p > 和基于互斥量的版本类似， ， ， < code > (*entry).call< / code > 来完成上述工作。< / p >
2022-10-23 23:50:32 +00:00
< p > 紧接着对同一个key的请求会发现map中已经有了存在的条目， ， < code > (*entry).deliver< / code > 来完成的。对call和deliver方法的调用必须让它们在自己的goroutine中进行以确保monitor goroutines不会因此而被阻塞住而没法处理新的请求。< / p >
2022-08-04 06:59:33 +00:00
< p > 这个例子说明我们无论用上锁，还是通信来建立并发程序都是可行的。< / p >
< p > 上面的两种方案并不好说特定情境下哪种更好， （ ： ） < / p >
< p > < strong > 练习 9.3： < / strong > 扩展Func类型和< code > (*Memo).Get< / code > 方法， ， （ ） < / p >
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