1 files changed, 88 insertions, 0 deletions
diff --git a/src/math/big/calibrate_test.go b/src/math/big/calibrate_test.go
new file mode 100644
index 000000000..f69ffbf5c
--- /dev/null
+++ b/src/math/big/calibrate_test.go
@@ -0,0 +1,88 @@
+// Copyright 2009 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.
+
+// This file prints execution times for the Mul benchmark
+// given different Karatsuba thresholds. The result may be
+// used to manually fine-tune the threshold constant. The
+// results are somewhat fragile; use repeated runs to get
+// a clear picture.
+
+// Usage: go test -run=TestCalibrate -calibrate
+
+package big
+
+import (
+	"flag"
+	"fmt"
+	"testing"
+	"time"
+)
+
+var calibrate = flag.Bool("calibrate", false, "run calibration test")
+
+func karatsubaLoad(b *testing.B) {
+	BenchmarkMul(b)
+}
+
+// measureKaratsuba returns the time to run a Karatsuba-relevant benchmark
+// given Karatsuba threshold th.
+func measureKaratsuba(th int) time.Duration {
+	th, karatsubaThreshold = karatsubaThreshold, th
+	res := testing.Benchmark(karatsubaLoad)
+	karatsubaThreshold = th
+	return time.Duration(res.NsPerOp())
+}
+
+func computeThresholds() {
+	fmt.Printf("Multiplication times for varying Karatsuba thresholds\n")
+	fmt.Printf("(run repeatedly for good results)\n")
+
+	// determine Tk, the work load execution time using basic multiplication
+	Tb := measureKaratsuba(1e9) // th == 1e9 => Karatsuba multiplication disabled
+	fmt.Printf("Tb = %10s\n", Tb)
+
+	// thresholds
+	th := 4
+	th1 := -1
+	th2 := -1
+
+	var deltaOld time.Duration
+	for count := -1; count != 0 && th < 128; count-- {
+		// determine Tk, the work load execution time using Karatsuba multiplication
+		Tk := measureKaratsuba(th)
+
+		// improvement over Tb
+		delta := (Tb - Tk) * 100 / Tb
+
+		fmt.Printf("th = %3d  Tk = %10s  %4d%%", th, Tk, delta)
+
+		// determine break-even point
+		if Tk < Tb && th1 < 0 {
+			th1 = th
+			fmt.Print("  break-even point")
+		}
+
+		// determine diminishing return
+		if 0 < delta && delta < deltaOld && th2 < 0 {
+			th2 = th
+			fmt.Print("  diminishing return")
+		}
+		deltaOld = delta
+
+		fmt.Println()
+
+		// trigger counter
+		if th1 >= 0 && th2 >= 0 && count < 0 {
+			count = 10 // this many extra measurements after we got both thresholds
+		}
+
+		th++
+	}
+}
+
+func TestCalibrate(t *testing.T) {
+	if *calibrate {
+		computeThresholds()
+	}
+}