我有一个包含500000个随机生成的Tuple<long,long,string>对象的列表，我正在对其执行简单的“between”搜索：
var data = new List<Tuple<long,long,string>>(500000);
...
var cnt = data.Count(t => t.Item1 <= x && t.Item2 >= x);
当我生成随机数组并对100个随机生成的
x 值运行搜索时，搜索大约需要四秒钟。然而，我知道排序对搜索的巨大帮助, 因此在运行我的100次搜索之前，我决定先按 Item1, 然后按 Item2, 最后按 Item3 排序我的数据。我预计排序后的版本会因为分支预测而表现得更快：我的想法是一旦我们到达 Item1 == x 的点，所有后续的 t.Item1 <= x 检查将正确地预测分支为“不取”，从而加速搜索的尾部部分。但令我惊讶的是，在排序后的数组上搜索所需时间是未排序数组的两倍！
我尝试改变实验运行的顺序，并使用了不同的随机数种子，但效果是相同的：未排序数组中的搜索几乎比排序后的数组中的搜索快了近两倍！
有人能解释一下这个奇怪的效果吗？以下是我的测试源代码；我正在使用.NET 4.0。
private const int TotalCount = 500000;
private const int TotalQueries = 100;
private static long NextLong(Random r) {
var data = new byte[8];
r.NextBytes(data);
return BitConverter.ToInt64(data, 0);
}
private class TupleComparer : IComparer<Tuple<long,long,string>> {
public int Compare(Tuple<long,long,string> x, Tuple<long,long,string> y) {
var res = x.Item1.CompareTo(y.Item1);
if (res != 0) return res;
res = x.Item2.CompareTo(y.Item2);
return (res != 0) ? res : String.CompareOrdinal(x.Item3, y.Item3);
}
}
static void Test(bool doSort) {
var data = new List<Tuple<long,long,string>>(TotalCount);
var random = new Random(1000000007);
var sw = new Stopwatch();
sw.Start();
for (var i = 0 ; i != TotalCount ; i++) {
var a = NextLong(random);
var b = NextLong(random);
if (a > b) {
var tmp = a;
a = b;
b = tmp;
}
var s = string.Format("{0}-{1}", a, b);
data.Add(Tuple.Create(a, b, s));
}
sw.Stop();
if (doSort) {
data.Sort(new TupleComparer());
}
Console.WriteLine("Populated in {0}", sw.Elapsed);
sw.Reset();
var total = 0L;
sw.Start();
for (var i = 0 ; i != TotalQueries ; i++) {
var x = NextLong(random);
var cnt = data.Count(t => t.Item1 <= x && t.Item2 >= x);
total += cnt;
}
sw.Stop();
Console.WriteLine("Found {0} matches in {1} ({2})", total, sw.Elapsed, doSort ? "Sorted" : "Unsorted");
}
static void Main() {
Test(false);
Test(true);
Test(false);
Test(true);
}
Populated in 00:00:01.3176257
Found 15614281 matches in 00:00:04.2463478 (Unsorted)
Populated in 00:00:01.3345087
Found 15614281 matches in 00:00:08.5393730 (Sorted)
Populated in 00:00:01.3665681
Found 15614281 matches in 00:00:04.1796578 (Unsorted)
Populated in 00:00:01.3326378
Found 15614281 matches in 00:00:08.6027886 (Sorted)