.

syspread_test.go

+package main
+
+import (
+	"crypto/rand"
+	"fmt"
+	"os"
+	"testing"
+)
+
+
+// xpread is pread(2) but aborts on an error or when fewer bytes was read
+func xpread(f *os.File, buf []byte, offset int64) {
+	n, err := f.ReadAt(buf, offset)
+	if err != nil {
+		panic(err)
+	}
+	if n != len(buf) {
+		panic(fmt.Errorf("pread(%v) -> %v", len(buf), n))
+	}
+}
+
+// BenchmarkPread1 benchmarks pread syscall when reading 1 byte from a file on tmpfs
+func BenchmarkPread1(b *testing.B) {
+	// setup 4K file on a tmpfs
+	f, err := os.Create("/dev/shm/x.dat")
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	buf4K := make([]byte, 4096)
+	_, err = rand.Read(buf4K)
+	if err != nil {
+		// rand promises n = len(buf) <=> err == nil
+		b.Fatal(err)
+	}
+
+	_, err = f.Write(buf4K)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	buf1B := make([]byte, 1)
+
+	// warm up
+	xpread(f, buf1B, 0)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		xpread(f, buf1B, 0)
+	}
+	b.StopTimer()
+
+	err = f.Close()
+	if err != nil {
+		b.Fatal(f)
+	}
+}