Documentation
¶
Overview ¶
Package semaphore provides a weighted semaphore implementation.
Example (WorkerPool) ¶
Example_workerPool demonstrates how to use a semaphore to limit the number of goroutines working on parallel tasks.
This use of a semaphore mimics a typical “worker pool” pattern, but without the need to explicitly shut down idle workers when the work is done.
package main
import (
"context"
"fmt"
"log"
"runtime"
"golang.org/x/sync/semaphore"
)
// Example_workerPool demonstrates how to use a semaphore to limit the number of
// goroutines working on parallel tasks.
//
// This use of a semaphore mimics a typical “worker pool” pattern, but without
// the need to explicitly shut down idle workers when the work is done.
func main() {
ctx := context.TODO()
var (
maxWorkers = runtime.GOMAXPROCS(0)
sem = semaphore.NewWeighted(int64(maxWorkers))
out = make([]int, 32)
)
// Compute the output using up to maxWorkers goroutines at a time.
for i := range out {
// When maxWorkers goroutines are in flight, Acquire blocks until one of the
// workers finishes.
if err := sem.Acquire(ctx, 1); err != nil {
log.Printf("Failed to acquire semaphore: %v", err)
break
}
go func(i int) {
defer sem.Release(1)
out[i] = collatzSteps(i + 1)
}(i)
}
// Acquire all of the tokens to wait for any remaining workers to finish.
//
// If you are already waiting for the workers by some other means (such as an
// errgroup.Group), you can omit this final Acquire call.
if err := sem.Acquire(ctx, int64(maxWorkers)); err != nil {
log.Printf("Failed to acquire semaphore: %v", err)
}
fmt.Println(out)
}
// collatzSteps computes the number of steps to reach 1 under the Collatz
// conjecture. (See https://en.wikipedia.org/wiki/Collatz_conjecture.)
func collatzSteps(n int) (steps int) {
if n <= 0 {
panic("nonpositive input")
}
for ; n > 1; steps++ {
if steps < 0 {
panic("too many steps")
}
if n%2 == 0 {
n /= 2
continue
}
const maxInt = int(^uint(0) >> 1)
if n > (maxInt-1)/3 {
panic("overflow")
}
n = 3*n + 1
}
return steps
}
Output: [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]
Index ¶
Examples ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
This section is empty.
Types ¶
type Weighted ¶
type Weighted struct {
// contains filtered or unexported fields
}
Weighted provides a way to bound concurrent access to a resource. The callers can request access with a given weight.
func NewWeighted ¶
NewWeighted creates a new weighted semaphore with the given maximum combined weight for concurrent access.
func (*Weighted) Acquire ¶
Acquire acquires the semaphore with a weight of n, blocking until resources are available or ctx is done. On success, returns nil. On failure, returns ctx.Err() and leaves the semaphore unchanged.
If ctx is already done, Acquire may still succeed without blocking.
func (*Weighted) ForceAcquire ¶
ForceAcquire is used when external force can unconditionally acquire semaphore. For example if semaphore limits disk space used by disk queue, the app can start with different limit so that disk used is already over limit In this case we create semaphore with new limit and use ForceAcquire to unconditionally acquire current disk used semaphore will wait until enough disk space released before allowing Acquire to succeed
func (*Weighted) SetSize ¶
SetN is used when max value is controlled by config and config is reloaded with different value If new value is less than current number, semaphore will wait until enough clients release before allowing Acquire to succeed
func (*Weighted) TryAcquire ¶
TryAcquire acquires the semaphore with a weight of n without blocking. On success, returns true. On failure, returns false and leaves the semaphore unchanged.
Source Files