Add shared memory lecture
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298
lectures/05-concurrency/lectures.slide
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298
lectures/05-concurrency/lectures.slide
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Concurrency with Shared Memory
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Лекция 5
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Фёдор Короткий
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* Happens Before
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- Одно событие в программе может _произойти_раньше_ (_happens_before_) другого события.
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- Внутри одной горутины `a`:=`1` _happens_before_ `a`+=`1`.
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a := 1
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a += 1
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- Посылка значения из канал _happens_before_ получения этого значения из канала.
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- Два события A и B _происходят_одновременно_ (_are_concurrent_), если нельзя сказать что одно случилось раньше другого.
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* Race Condition
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- _Race_Condition_ - ситуация, когда код работает некорректно в зависимости от порядка выполнения нескольких горутин.
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* Example Bank
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// Package bank implements a bank with only one account.
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package bank
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var balance int
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func Deposit(amount int) { balance = balance + amount }
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func Balance() int { return balance }
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Работает корректно в одной горутине.
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// Alice:
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go func() {
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bank.Deposit(200)
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fmt.Println("=", bank.Balance())
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}()
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// Bob
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go bank.Deposit(100)
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Ошибка в этом примере называется _data_race_.
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* Data Race breaks memory safety
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var x []int
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go func() { x = make([]int, 10) }()
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go func() { x = make([]int, 1000000) }()
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x[999999] = 1 // NOTE: undefined behavior; memory corruption possible!
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Программа с _data_race_ перестаёт вести себя так, как написано в коде.
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Иногда люди говорят _"это_безопасный_рейс"_. *Не*бывыет*безопасных*data-race-ов*
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* Data Race
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*Data*race* случается, когда несколько горутин одновременно работают с переменной, и хотябы одна из них пишет в эту переменную.
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Как защититься от _data_race_?
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1. Не писать в переменную.
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2. Не работать с переменной из нескольких горутин.
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3. Не работать с переменной одновременно. (mutex)
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* Не писать в переменую
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var icons = make(map[string]image.Image)
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func loadIcon(name string) image.Image
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// NOTE: not concurrency-safe!
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func Icon(name string) image.Image {
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icon, ok := icons[name]
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if !ok {
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icon = loadIcon(name)
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icons[name] = icon
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}
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return icon
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}
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Правильно:
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var icons = map[string]image.Image{
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"spades.png": loadIcon("spades.png"),
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"hearts.png": loadIcon("hearts.png"),
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"diamonds.png": loadIcon("diamonds.png"),
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"clubs.png": loadIcon("clubs.png"),
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}
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func Icon(name string) image.Image { return icons[name] }
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* Не работать с переменной из нескольких горутин
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// Package bank provides a concurrency-safe bank with one account.
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package bank
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var deposits = make(chan int) // send amount to deposit
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var balances = make(chan int) // receive balance
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func Deposit(amount int) { deposits <- amount }
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func Balance() int { return <-balances }
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func teller() {
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var balance int // balance is confined to teller goroutine
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for {
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select {
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case amount := <-deposits:
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balance += amount
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case balances <- balance:
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}
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}
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}
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func init() {
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go teller() // start the monitor goroutine
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}
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* sync.Mutex
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import "sync"
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var (
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mu sync.Mutex // guards balance
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balance int
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)
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func Deposit(amount int) {
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mu.Lock()
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balance = balance + amount
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mu.Unlock()
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}
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func Balance() int {
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mu.Lock()
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defer mu.Unlock()
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b := balance
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return b
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}
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* sync.RWMutex
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var mu sync.RWMutex
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var balance int
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func Balance() int {
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mu.RLock() // readers lock
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defer mu.RUnlock()
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return balance
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}
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* Memory Order
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var x, y int
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go func() {
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x = 1 // A1
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fmt.Print("y:", y, " ") // A2
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}()
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go func() {
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y = 1 // B1
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fmt.Print("x:", x, " ") // B2
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}()
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Можем ожидать
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y:0 x:1
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x:0 y:1
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x:1 y:1
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y:1 x:1
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Но реально может произойти
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x:0 y:0
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y:0 x:0
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* Ленивая инициализация sync.Once.
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var icons map[string]image.Image
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func loadIcons() {
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icons = map[string]image.Image{
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"spades.png": loadIcon("spades.png")
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}
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}
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// NOTE: not concurrency-safe!
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func Icon(name string) image.Image {
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if icons == nil {
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loadIcons() // one-time initialization
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}
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return icons[name]
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}
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* Ленивая инициализация sync.Once.
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var mu sync.Mutex // guards icons
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var icons map[string]image.Image
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// Concurrency-safe.
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func Icon(name string) image.Image {
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mu.Lock()
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defer mu.Unlock()
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if icons == nil {
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loadIcons()
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}
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return icons[name]
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}
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* Ленивая инициализация sync.Once.
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var loadIconsOnce sync.Once
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var icons map[string]image.Image
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// Concurrency-safe.
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func Icon(name string) image.Image {
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loadIconsOnce.Do(loadIcons)
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return icons[name]
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}
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* Concurrent cache
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.play memo1/memo.go /type Memo/,/OMIT/
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* Concurrent cache
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.play memo2/memo.go /type Memo/,/OMIT/
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* Concurrent cache
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.play memo3/memo.go /func \(memo/,/OMIT/
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* Concurrent cache
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.play memo4/memo.go /type entry/,/OMIT/
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* sync.Map
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Что происходит, когда go совершает type cast из interface{}?
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- Приведение к конкретному типу требует одной проверки на `==`
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var v interface{}
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x, ok := v.(int)
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- Приведение к интерфейсу требует (в общем случае) сравнивать method set.
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var v interface{}
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x, ok := v.(io.Reader)
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// Конкретный тип v должен иметь метод Read([]byte) (int, error)
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- Для каждой пары `(concrete`type,`interface`type)` нужно знать, корректно ли такое приведение.
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var cache map[typeConversion]conversionResult
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* sync.Map
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var cache map[typeConversion]conversionResult
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Как защитить cache? `sync.Mutex`? Кеш из раннего примера?
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Какой паттерн нагрузки?
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- Запись в начале программы.
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- Потом только чтения.
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- Никогда не удаляем ключи.
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- Не страшно сделать вычисление несколько раз.
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Чего хотим?
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- Скорость как у `map` без лока.
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* sync.Map
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var cache sync.Map
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func convertType(from, to typ) *conversionResult {
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key := typeConversion{from: from, to: to}
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res, ok := cache.Load(key)
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if ok {
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return res.(*conversionResult)
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}
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res = doConversion(from, to)
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cache.Store(key, res)
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return res.(*conversionResult)
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}
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- `sync.Map` хранит две `map` внутри. `clean` и `dirty`.
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- Обращение к `clean` всегда происходит без лока.
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- Обращение к `dirty` требует лока.
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- Периодически `dirty` повышается до `clean`.
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* Как сделать sync.Map без двойного вычисления?
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.play synconce/map.go /type result/,/OMIT/
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36
lectures/05-concurrency/memo1/memo.go
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36
lectures/05-concurrency/memo1/memo.go
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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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// See page 272.
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//!+
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// Package memo provides a concurrency-unsafe
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// memoization of a function of type Func.
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package memo
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// A Memo caches the results of calling a Func.
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type Memo struct {
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f Func
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cache map[string]result
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}
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// Func is the type of the function to memoize.
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type Func func(key string) (interface{}, error)
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type result struct {
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value interface{}
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err error
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}
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// NOTE: not concurrency-safe!
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func (memo *Memo) Get(key string) (interface{}, error) {
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res, ok := memo.cache[key]
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if !ok {
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res.value, res.err = memo.f(key)
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memo.cache[key] = res
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}
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return res.value, res.err
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}
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// OMIT
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67
lectures/05-concurrency/memo1/memo_test.go
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67
lectures/05-concurrency/memo1/memo_test.go
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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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package memo_test
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import (
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"testing"
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"gopl.io/ch9/memo1"
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"gopl.io/ch9/memotest"
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)
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var httpGetBody = memotest.HTTPGetBody
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func Test(t *testing.T) {
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m := memo.New(httpGetBody)
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memotest.Sequential(t, m)
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}
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// NOTE: not concurrency-safe! Test fails.
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func TestConcurrent(t *testing.T) {
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m := memo.New(httpGetBody)
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memotest.Concurrent(t, m)
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}
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/*
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//!+output
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$ go test -v gopl.io/ch9/memo1
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=== RUN Test
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https://golang.org, 175.026418ms, 7537 bytes
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https://godoc.org, 172.686825ms, 6878 bytes
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https://play.golang.org, 115.762377ms, 5767 bytes
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http://gopl.io, 749.887242ms, 2856 bytes
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https://golang.org, 721ns, 7537 bytes
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https://godoc.org, 152ns, 6878 bytes
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https://play.golang.org, 205ns, 5767 bytes
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http://gopl.io, 326ns, 2856 bytes
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--- PASS: Test (1.21s)
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PASS
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ok gopl.io/ch9/memo1 1.257s
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//!-output
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*/
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/*
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//!+race
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$ go test -run=TestConcurrent -race -v gopl.io/ch9/memo1
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=== RUN TestConcurrent
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...
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WARNING: DATA RACE
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Write by goroutine 36:
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runtime.mapassign1()
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~/go/src/runtime/hashmap.go:411 +0x0
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gopl.io/ch9/memo1.(*Memo).Get()
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~/gobook2/src/gopl.io/ch9/memo1/memo.go:32 +0x205
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...
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Previous write by goroutine 35:
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runtime.mapassign1()
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~/go/src/runtime/hashmap.go:411 +0x0
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gopl.io/ch9/memo1.(*Memo).Get()
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~/gobook2/src/gopl.io/ch9/memo1/memo.go:32 +0x205
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...
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Found 1 data race(s)
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FAIL gopl.io/ch9/memo1 2.393s
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//!-race
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*/
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44
lectures/05-concurrency/memo2/memo.go
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44
lectures/05-concurrency/memo2/memo.go
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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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// See page 275.
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// Package memo provides a concurrency-safe memoization a function of
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// type Func. Concurrent requests are serialized by a Mutex.
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package memo
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import "sync"
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// Func is the type of the function to memoize.
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type Func func(string) (interface{}, error)
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type result struct {
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value interface{}
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err error
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}
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func New(f Func) *Memo {
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return &Memo{f: f, cache: make(map[string]result)}
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}
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//!+
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type Memo struct {
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f Func
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mu sync.Mutex // guards cache
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cache map[string]result
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}
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// Get is concurrency-safe.
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func (memo *Memo) Get(key string) (value interface{}, err error) {
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memo.mu.Lock()
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res, ok := memo.cache[key]
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if !ok {
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res.value, res.err = memo.f(key)
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memo.cache[key] = res
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}
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memo.mu.Unlock()
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return res.value, res.err
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}
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// OMIT
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23
lectures/05-concurrency/memo2/memo_test.go
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23
lectures/05-concurrency/memo2/memo_test.go
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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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package memo_test
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import (
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"testing"
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"gopl.io/ch9/memo2"
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"gopl.io/ch9/memotest"
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)
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var httpGetBody = memotest.HTTPGetBody
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func Test(t *testing.T) {
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m := memo.New(httpGetBody)
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memotest.Sequential(t, m)
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}
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func TestConcurrent(t *testing.T) {
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m := memo.New(httpGetBody)
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memotest.Concurrent(t, m)
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}
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46
lectures/05-concurrency/memo3/memo.go
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46
lectures/05-concurrency/memo3/memo.go
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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
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// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
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// See page 276.
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// Package memo provides a concurrency-safe memoization a function of
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// type Func. Requests for different keys run concurrently.
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// Concurrent requests for the same key result in duplicate work.
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package memo
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import "sync"
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type Memo struct {
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f Func
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mu sync.Mutex // guards cache
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cache map[string]result
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}
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type Func func(string) (interface{}, error)
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type result struct {
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value interface{}
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err error
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}
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func New(f Func) *Memo {
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return &Memo{f: f, cache: make(map[string]result)}
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}
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func (memo *Memo) Get(key string) (value interface{}, err error) {
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memo.mu.Lock()
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res, ok := memo.cache[key]
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memo.mu.Unlock()
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if !ok {
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res.value, res.err = memo.f(key)
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|
||||
// 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
|
||||
}
|
||||
|
||||
// OMIT
|
23
lectures/05-concurrency/memo3/memo_test.go
Normal file
23
lectures/05-concurrency/memo3/memo_test.go
Normal file
|
@ -0,0 +1,23 @@
|
|||
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
|
||||
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
|
||||
|
||||
package memo_test
|
||||
|
||||
import (
|
||||
"testing"
|
||||
|
||||
"gopl.io/ch9/memo3"
|
||||
"gopl.io/ch9/memotest"
|
||||
)
|
||||
|
||||
var httpGetBody = memotest.HTTPGetBody
|
||||
|
||||
func Test(t *testing.T) {
|
||||
m := memo.New(httpGetBody)
|
||||
memotest.Sequential(t, m)
|
||||
}
|
||||
|
||||
func TestConcurrent(t *testing.T) {
|
||||
m := memo.New(httpGetBody)
|
||||
memotest.Concurrent(t, m)
|
||||
}
|
59
lectures/05-concurrency/memo4/memo.go
Normal file
59
lectures/05-concurrency/memo4/memo.go
Normal file
|
@ -0,0 +1,59 @@
|
|||
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
|
||||
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
|
||||
|
||||
// See page 276.
|
||||
|
||||
// Package memo provides a concurrency-safe memoization a function of
|
||||
// a function. Requests for different keys proceed in parallel.
|
||||
// Concurrent requests for the same key block until the first completes.
|
||||
// This implementation uses a Mutex.
|
||||
package memo
|
||||
|
||||
import "sync"
|
||||
|
||||
// Func is the type of the function to memoize.
|
||||
type Func func(string) (interface{}, error)
|
||||
|
||||
type result struct {
|
||||
value interface{}
|
||||
err error
|
||||
}
|
||||
|
||||
//!+
|
||||
|
||||
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
|
||||
}
|
||||
|
||||
type entry struct {
|
||||
res result
|
||||
ready chan struct{} // closed when res is ready
|
||||
}
|
||||
|
||||
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
|
||||
}
|
||||
|
||||
// OMIT
|
23
lectures/05-concurrency/memo4/memo_test.go
Normal file
23
lectures/05-concurrency/memo4/memo_test.go
Normal file
|
@ -0,0 +1,23 @@
|
|||
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
|
||||
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
|
||||
|
||||
package memo_test
|
||||
|
||||
import (
|
||||
"testing"
|
||||
|
||||
"gopl.io/ch9/memo4"
|
||||
"gopl.io/ch9/memotest"
|
||||
)
|
||||
|
||||
var httpGetBody = memotest.HTTPGetBody
|
||||
|
||||
func Test(t *testing.T) {
|
||||
m := memo.New(httpGetBody)
|
||||
memotest.Sequential(t, m)
|
||||
}
|
||||
|
||||
func TestConcurrent(t *testing.T) {
|
||||
m := memo.New(httpGetBody)
|
||||
memotest.Concurrent(t, m)
|
||||
}
|
31
lectures/05-concurrency/synconce/map.go
Normal file
31
lectures/05-concurrency/synconce/map.go
Normal file
|
@ -0,0 +1,31 @@
|
|||
package synconce
|
||||
|
||||
import "sync"
|
||||
|
||||
var cache sync.Map
|
||||
|
||||
type result struct{}
|
||||
|
||||
func do(key string) *result { return new(result) }
|
||||
|
||||
type entry struct {
|
||||
res *result
|
||||
sync.Once
|
||||
}
|
||||
|
||||
func get(key string) *result {
|
||||
myEntry := &entry{}
|
||||
|
||||
old, loaded := cache.LoadOrStore(key, myEntry)
|
||||
if loaded {
|
||||
myEntry = old.(*entry)
|
||||
}
|
||||
|
||||
myEntry.Do(func() {
|
||||
myEntry.res = do(key)
|
||||
})
|
||||
|
||||
return myEntry.res
|
||||
}
|
||||
|
||||
// OMIT
|
Loading…
Reference in a new issue