我有500000名隨機生成Tuple<long,long,string>
對象列表上我執行一個簡單的搜索「之間」時:爲什麼處理排序後的數組要比未排序的數組慢?
var data = new List<Tuple<long,long,string>>(500000);
...
var cnt = data.Count(t => t.Item1 <= x && t.Item2 >= x);
當我生成我隨機陣列和運行我爲x
100個隨機生成的值的搜索,所述大約四秒鐘完成搜索。然而,在知道great wonders that sorting does to searching後,我決定對我的數據進行排序 - 首先按Item1
,然後按Item2
,最後按Item3
- 在運行我的100次搜索之前。由於分支預測,我預計排序後的版本會執行得更快一些:我的想法是,一旦我們達到Item1 == x
的所有進一步檢查將正確預測分支爲「不帶」,加快尾部的搜索。令我驚訝的是,搜索花費了兩倍於排序陣列!
我試着開關周圍,我跑我的實驗,並用不同的種子隨機數發生器的順序,但效果一直不變:在排序的數組的搜索跑了近兩倍的速度在搜查相同的數組,但排序!
有沒有人有這個奇怪的效果很好的解釋?我的測試源代碼如下;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)
由於分支預測的:P –
@jalf我預期排序版本來執行,因爲分支預測的快一點。我的想法是,一旦我們到達了'Item1 == x'的位置,'t.Item1 <= x'的所有進一步檢查都會正確地預測分支爲「不帶」,從而加快了搜索的尾部。顯然,這種思維線已經被證明是錯誤的殘酷的現實:) – dasblinkenlight
有趣的是,'TotalCount'左右'10,000'以下,排序的版本並執行得更快(當然,平凡更快那些小數字)(僅供參考,你的代碼可能希望具有'var data List = new List>(500000)'的初始大小'而不是硬編碼容量''TotalCount'' –