semaphore: add a weighted semaphore implementation

This package provides a weighted semaphore that is context-aware.

The code is derived from a similar package inside Google.

Change-Id: Id1dad96d79e8ccfd289e4299e8265aa5bdad3a5b
Reviewed-on: https://go-review.googlesource.com/38298
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

semaphore/semaphore_bench_test.go

@@ -0,0 +1,130 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.7
+
+package semaphore
+
+import (
+	"fmt"
+	"testing"
+
+	"golang.org/x/net/context"
+)
+
+// weighted is an interface matching a subset of *Weighted.  It allows
+// alternate implementations for testing and benchmarking.
+type weighted interface {
+	Acquire(context.Context, int64) error
+	TryAcquire(int64) bool
+	Release(int64)
+}
+
+// semChan implements Weighted using a channel for
+// comparing against the condition variable-based implementation.
+type semChan chan struct{}
+
+func newSemChan(n int64) semChan {
+	return semChan(make(chan struct{}, n))
+}
+
+func (s semChan) Acquire(_ context.Context, n int64) error {
+	for i := int64(0); i < n; i++ {
+		s <- struct{}{}
+	}
+	return nil
+}
+
+func (s semChan) TryAcquire(n int64) bool {
+	if int64(len(s))+n > int64(cap(s)) {
+		return false
+	}
+
+	for i := int64(0); i < n; i++ {
+		s <- struct{}{}
+	}
+	return true
+}
+
+func (s semChan) Release(n int64) {
+	for i := int64(0); i < n; i++ {
+		<-s
+	}
+}
+
+// acquireN calls Acquire(size) on sem N times and then calls Release(size) N times.
+func acquireN(b *testing.B, sem weighted, size int64, N int) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < N; j++ {
+			sem.Acquire(context.Background(), size)
+		}
+		for j := 0; j < N; j++ {
+			sem.Release(size)
+		}
+	}
+}
+
+// tryAcquireN calls TryAcquire(size) on sem N times and then calls Release(size) N times.
+func tryAcquireN(b *testing.B, sem weighted, size int64, N int) {
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < N; j++ {
+			if !sem.TryAcquire(size) {
+				b.Fatalf("TryAcquire(%v) = false, want true", size)
+			}
+		}
+		for j := 0; j < N; j++ {
+			sem.Release(size)
+		}
+	}
+}
+
+func BenchmarkNewSeq(b *testing.B) {
+	for _, cap := range []int64{1, 128} {
+		b.Run(fmt.Sprintf("Weighted-%d", cap), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				_ = NewWeighted(cap)
+			}
+		})
+		b.Run(fmt.Sprintf("semChan-%d", cap), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				_ = newSemChan(cap)
+			}
+		})
+	}
+}
+
+func BenchmarkAcquireSeq(b *testing.B) {
+	for _, c := range []struct {
+		cap, size int64
+		N         int
+	}{
+		{1, 1, 1},
+		{2, 1, 1},
+		{16, 1, 1},
+		{128, 1, 1},
+		{2, 2, 1},
+		{16, 2, 8},
+		{128, 2, 64},
+		{2, 1, 2},
+		{16, 8, 2},
+		{128, 64, 2},
+	} {
+		for _, w := range []struct {
+			name string
+			w    weighted
+		}{
+			{"Weighted", NewWeighted(c.cap)},
+			{"semChan", newSemChan(c.cap)},
+		} {
+			b.Run(fmt.Sprintf("%s-acquire-%d-%d-%d", w.name, c.cap, c.size, c.N), func(b *testing.B) {
+				acquireN(b, w.w, c.size, c.N)
+			})
+			b.Run(fmt.Sprintf("%s-tryAcquire-%d-%d-%d", w.name, c.cap, c.size, c.N), func(b *testing.B) {
+				tryAcquireN(b, w.w, c.size, c.N)
+			})
+		}
+	}
+}