为什么在这个例子中LINQ更快 [英] Why is LINQ faster in this example
问题描述
我写了以下内容来测试使用 foreach
vs LINQ
的性能:
private class Widget
{
public string Name {get;组; }
static void Main(string [] args)
{
List< Widget> widgets = new List< Widget>();
int found = 0; ()(){$ i
(int i = 0; i <= 500000 - 1; i ++)
widgets.Add(new Widget(){Name = Guid.NewGuid()。ToString );
DateTime starttime = DateTime.Now;
$ b $ foreach(小工具中的小工具)
{
if(w.Name.StartsWith(4))
found + = 1;
Console.WriteLine(found + - + DateTime.Now.Subtract(starttime).Milliseconds +ms);
starttime = DateTime.Now;
found = widgets.Where(a => a.Name.StartsWith(4))。Count();
Console.WriteLine(found + - + DateTime.Now.Subtract(starttime).Milliseconds +ms);
Console.ReadLine();
}
我得到如下输出:
31160 - 116ms
31160 - 95 ms
LINQ的表现胜过20%左右。这是我的理解,LINQ扩展方法使用标准的C#下盖。
那么为什么LINQ在这种情况下更快?
所以我改变我的代码使用秒表而不是日期时间,仍然得到相同的结果。如果我先运行LINQ查询,那么我的结果显示LINQ比foreach慢20%左右。这必须是某种JIT暖身问题。我的问题是我如何弥补我的测试用例中的JIT热身? 解决方案
这是因为你没有热身。如果你扭转了你的情况,你会得到相反的结果:
31272 - 110ms
31272 - 80 ms
开始添加热身并使用秒表以获得更好的计时。
使用热身运行测试:
// WARM UP:
widgets.Where(a => ; a.Name.StartsWith(4))。Count();
$ b $ foreach(小工具中的小工具)
{
if(w.Name.StartsWith(4))
found + = 1;
}
//运行测试
秒表stopwatch1 =新的秒表();
stopwatch1.Start();
found = widgets.Where(a => a.Name.StartsWith(4))。Count();
stopwatch1.Stop();
Console.WriteLine(found + - + stopwatch1.Elapsed);
found = 0;
Stopwatch stopwatch2 =新的秒表();
stopwatch2.Start();
$ b $ foreach(小工具中的小工具)
{
if(w.Name.StartsWith(4))
found + = 1;
}
stopwatch2.Stop();
Console.WriteLine(found + - + stopwatch2.Elapsed);
结果:
31039 - 00:00:00.0783508
31039 - 00:00:00.0766299
I wrote the following to test the performance of using foreach
vs LINQ
:
private class Widget
{
public string Name { get; set; }
}
static void Main(string[] args)
{
List<Widget> widgets = new List<Widget>();
int found = 0;
for (int i = 0; i <= 500000 - 1; i++)
widgets.Add(new Widget() { Name = Guid.NewGuid().ToString() });
DateTime starttime = DateTime.Now;
foreach (Widget w in widgets)
{
if (w.Name.StartsWith("4"))
found += 1;
}
Console.WriteLine(found + " - " + DateTime.Now.Subtract(starttime).Milliseconds + " ms");
starttime = DateTime.Now;
found = widgets.Where(a => a.Name.StartsWith("4")).Count();
Console.WriteLine(found + " - " + DateTime.Now.Subtract(starttime).Milliseconds + " ms");
Console.ReadLine();
}
I get something like following output:
31160 - 116ms 31160 - 95 ms
In every run, LINQ outperforms foreach by around 20%. It was my understanding that the LINQ extension methods used standard c# under the covers.
So why is LINQ faster in this case?
EDIT:
So I changed my code to use stopwatch instead of datetime and still get the same results. If I run the LINQ query first then my results show LINQ to be about 20% slower then foreach. This has to be some sort of JIT warmnup issue. My question is how do I compensate for JIT warmup in my test case?
It's because you do not have a warmup. If you reverse your cases you will get excatly the opposit result:
31272 - 110ms
31272 - 80 ms
Start adding a warmup and use a stopwatch for better timing.
Running the test with warmup:
//WARM UP:
widgets.Where(a => a.Name.StartsWith("4")).Count();
foreach (Widget w in widgets)
{
if (w.Name.StartsWith("4"))
found += 1;
}
//RUN Test
Stopwatch stopwatch1 = new Stopwatch();
stopwatch1.Start();
found = widgets.Where(a => a.Name.StartsWith("4")).Count();
stopwatch1.Stop();
Console.WriteLine(found + " - " + stopwatch1.Elapsed);
found = 0;
Stopwatch stopwatch2 = new Stopwatch();
stopwatch2.Start();
foreach (Widget w in widgets)
{
if (w.Name.StartsWith("4"))
found += 1;
}
stopwatch2.Stop();
Console.WriteLine(found + " - " + stopwatch2.Elapsed);
result:
31039 - 00:00:00.0783508
31039 - 00:00:00.0766299
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