为什么我的程序在单核上运行得更快？
为什么我的程序在单核上运行得更快？
我正在使用TPL学习C#中的多核编程。遇到了一个问题，我觉得这个问题应该很容易处理，而且可以利用多核来提高速度。然而在运行程序时，我发现单核实现比多核更快。该死！我错过了什么吗？
请注意我正在使用同一台电脑。
问题：给定一个数字，找出从0到该数字的所有阶乘。
我的解决方案：由于我有一个4核CPU，我将我的实现硬编码为使用4个任务。这些任务均匀地分配了数字。如果有100个数字，每个任务处理大约25个数字（任务1处理0、4、8、12...，任务2处理1、4、9、13...）。
以下是我的解决方案（似乎最大输入数字为9999）：
public class Factorial
{
public int TotalNoOfElements { get; set; }
public BigInteger GetFactorial(BigInteger number)
{
if (number == 0)
return 0;
if (number == 1)
return 1;
return number * GetFactorial(number - 1);
}
public BigInteger[] GetFactorialsUsingSingleCore(int number)
{
BigInteger[] factorials = new BigInteger[number + 1];
factorials[0] = 0;
for (int index = 1; index <= number; index++)
{
Factorial f = new Factorial(); //TODO: remove. Throws stackoverflow exception @ GetFactorial() if local method is used directly.
factorials[index] = f.GetFactorial(index);
}
return factorials;
}
public BigInteger[] GetFactorialsForNumbers(int[] numbers)
{
BigInteger[] factorials = new BigInteger[TotalNoOfElements + 1];
foreach (int item in numbers)
{
Factorial f = new Factorial(); //TODO: remove. Throws stackoverflow exception @ GetFactorial() if local method is used directly.
factorials[item] = f.GetFactorial(item);
}
return factorials;
}
public BigInteger[] GetFactorialsUsingMultiCores(int number)
{
//int noOfCores = 4;
StopWatchHelper stopWatchHelper = new StopWatchHelper();
stopWatchHelper.StartWatch();
BigInteger[] factorials = new BigInteger[TotalNoOfElements + 1];
List splitNumbersList = SplitNumbersByTasks(number);
stopWatchHelper.StopWatch();
stopWatchHelper.ShowElapsedTimeInConsole("Creating list...");
Task t1 = Task.Factory.StartNew(
() =>
GetFactorialsForNumbers(splitNumbersList[0])
);
Task t2 = Task.Factory.StartNew(
() =>
GetFactorialsForNumbers(splitNumbersList[1])
);
Task t3 = Task.Factory.StartNew(
() =>
GetFactorialsForNumbers(splitNumbersList[2])
);
Task t4 = Task.Factory.StartNew(
() =>
GetFactorialsForNumbers(splitNumbersList[3])
);
Task[] tasks = { t1, t2, t3, t4 };
Task.WaitAll(tasks);
stopWatchHelper.StartWatch();
//Consolidate result arrays into 1 single array
foreach (Task task in tasks)
{
BigInteger[] result = task.Result;
for (int index = 0; index < result.Length; index++)
{
if(result[index] != 0)
factorials[index] = result[index];
}
}
stopWatchHelper.StartWatch();
stopWatchHelper.ShowElapsedTimeInConsole("Consolidating results...");
return factorials;
}
//Hardcoded for now
//Returns #ofArrays = InputNumber/TotalTasks such that the work is evenly divided to the Tasks
//Given an input number, we shall split it evenly across different Tasks.
//With input as 25, Task 1 gets -> 0, 4, 8, 12, ... and Task 2 gets 1, 5, 9, 13, ...
List SplitNumbersByTasks(int number)
{
int noOfCores = 4;
List splitNumbersList = new List();
int reminder = number % noOfCores;
for (int i = 0; i < noOfCores; i++)
{
if (reminder == 0)
{
splitNumbersList.Add(new int[(number / noOfCores)]);
}
else
{
splitNumbersList.Add(new int[(number / noOfCores) + 1]);
reminder--;
}
}
int arrayIndex = 0;
for (int index = 1; index <= number; index++)
{
splitNumbersList[0][arrayIndex] = index++;
if (index > number) break;
splitNumbersList[1][arrayIndex] = index++;
if (index > number) break;
splitNumbersList[2][arrayIndex] = index++;
if (index > number) break;
splitNumbersList[3][arrayIndex] = index;
++arrayIndex;
}
return splitNumbersList;
}
}
class StopWatchHelper
{
Stopwatch stopWatch = new Stopwatch();
public void StartWatch()
{
stopWatch.Reset();
stopWatch.Start();
}
public void StopWatch()
{
stopWatch.Stop();
}
public void ShowElapsedTimeInConsole(string message)
{
Console.WriteLine(message);
Console.WriteLine(new string('-', message.Length));
Console.WriteLine("Minutes: " + stopWatch.Elapsed.Minutes);
Console.WriteLine("Seconds: " + stopWatch.Elapsed.Seconds);
Console.WriteLine("Milliseconds: " + stopWatch.Elapsed.Milliseconds);
Console.WriteLine();
}
}
class Program
{
static void Main(string[] args)
{
int input;
string strInput;
bool isNumber;
StopWatchHelper helper = new StopWatchHelper();
Factorial factorial = new Factorial();
//Get input recursively until input is not a number
do
{
strInput = Console.ReadLine();
Console.WriteLine();
isNumber = Int32.TryParse(strInput, out input);
factorial.TotalNoOfElements = input;
if (isNumber)
{
helper.StartWatch();
BigInteger[] factorialsUsingMultiCore = factorial.GetFactorialsUsingMultiCores(input);
helper.StopWatch();
helper.ShowElapsedTimeInConsole("Factorials computed using Multicore");
helper.StartWatch();
BigInteger[] factorialsUsingSingleCore = factorial.GetFactorialsUsingSingleCore(input);
helper.StopWatch();
helper.ShowElapsedTimeInConsole("Factorials computed using Singlecore");
}
Console.WriteLine();
} while (isNumber);
}
}
助手类：
class StopWatchHelper
{
Stopwatch stopWatch = new Stopwatch();
public void StartWatch()
{
stopWatch.Reset();
stopWatch.Start();
}
public void StopWatch()
{
stopWatch.Stop();
}
public void ShowElapsedTimeInConsole(string message)
{
Console.WriteLine(message);
Console.WriteLine(new string('-', message.Length));
Console.WriteLine("Minutes: " + stopWatch.Elapsed.Minutes);
Console.WriteLine("Seconds: " + stopWatch.Elapsed.Seconds);
Console.WriteLine("Milliseconds: " + stopWatch.Elapsed.Milliseconds);
Console.WriteLine();
}
}
主类：
class Program
{
static void Main(string[] args)
{
int input;
string strInput;
bool isNumber;
StopWatchHelper helper = new StopWatchHelper();
Factorial factorial = new Factorial();
//Get input recursively until input is not a number
do
{
strInput = Console.ReadLine();
Console.WriteLine();
isNumber = Int32.TryParse(strInput, out input);
factorial.TotalNoOfElements = input;
if (isNumber)
{
helper.StartWatch();
BigInteger[] factorialsUsingMultiCore = factorial.GetFactorialsUsingMultiCores(input);
helper.StopWatch();
helper.ShowElapsedTimeInConsole("Factorials computed using Multicore");
helper.StartWatch();
BigInteger[] factorialsUsingSingleCore = factorial.GetFactorialsUsingSingleCore(input);
helper.StopWatch();
helper.ShowElapsedTimeInConsole("Factorials computed using Singlecore");
}
Console.WriteLine();
} while (isNumber);
}
}