85 lines
2 KiB
Go
85 lines
2 KiB
Go
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// Copyright 2017 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package semaphore_test
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import (
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"context"
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"fmt"
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"log"
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"runtime"
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"golang.org/x/sync/semaphore"
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)
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// Example_workerPool demonstrates how to use a semaphore to limit the number of
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// goroutines working on parallel tasks.
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//
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// This use of a semaphore mimics a typical “worker pool” pattern, but without
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// the need to explicitly shut down idle workers when the work is done.
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func Example_workerPool() {
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ctx := context.TODO()
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var (
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maxWorkers = runtime.GOMAXPROCS(0)
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sem = semaphore.NewWeighted(int64(maxWorkers))
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out = make([]int, 32)
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)
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// Compute the output using up to maxWorkers goroutines at a time.
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for i := range out {
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// When maxWorkers goroutines are in flight, Acquire blocks until one of the
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// workers finishes.
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if err := sem.Acquire(ctx, 1); err != nil {
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log.Printf("Failed to acquire semaphore: %v", err)
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break
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}
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go func(i int) {
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defer sem.Release(1)
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out[i] = collatzSteps(i + 1)
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}(i)
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}
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// Acquire all of the tokens to wait for any remaining workers to finish.
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//
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// If you are already waiting for the workers by some other means (such as an
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// errgroup.Group), you can omit this final Acquire call.
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if err := sem.Acquire(ctx, int64(maxWorkers)); err != nil {
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log.Printf("Failed to acquire semaphore: %v", err)
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}
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fmt.Println(out)
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// Output:
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// [0 1 7 2 5 8 16 3 19 6 14 9 9 17 17 4 12 20 20 7 7 15 15 10 23 10 111 18 18 18 106 5]
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}
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// collatzSteps computes the number of steps to reach 1 under the Collatz
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// conjecture. (See https://en.wikipedia.org/wiki/Collatz_conjecture.)
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func collatzSteps(n int) (steps int) {
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if n <= 0 {
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panic("nonpositive input")
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}
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for ; n > 1; steps++ {
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if steps < 0 {
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panic("too many steps")
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}
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if n%2 == 0 {
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n /= 2
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continue
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}
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const maxInt = int(^uint(0) >> 1)
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if n > (maxInt-1)/3 {
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panic("overflow")
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}
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n = 3*n + 1
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}
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return steps
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}
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