When to use PLINQ vs LINQ

Introduction

Be careful when you are using PLINQ. What I found is if the computation is very simple and non-expensive, it is better to use LINQ than PLINQ.

Using the code

Following code is tested in multicore machine (8 CPU). First I timed a CPU intensive code with LINQ and then with PLINQ. Then I timed NON CPU intensive code with LINQ and PLINQ.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;
using System.Threading;

namespace PLinqSample
{
    public class Employee
    {
        public string Name { get; set; }
        public double Salary { get; set; }

        public bool ExpensiveComputation()
        {
            Thread.Sleep(10);
            return (Salary > 2000 && Salary < 3000);
        }

        public bool NonExpensiveComputation()
        {
            return (Salary > 2000 && Salary < 3000);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            List<Employee> employeeList = GetData();

            Stopwatch stopWatch = new Stopwatch();


            ///////////////////////////////// Testing Old LINQ With Expensive Computation///////////////
            stopWatch.Start();

            var linqResult = employeeList.Where<Employee>(e=> e.ExpensiveComputation());

            int empCount = linqResult.Count();

            stopWatch.Stop();

            Console.WriteLine(string.Format("Time taken by old LINQ in Expensive Computation is {0} to get {1} Employees", stopWatch.Elapsed.TotalMilliseconds, empCount));

            stopWatch.Reset();
            //////////////////////////////////////////////////////////

            ///////////////////////////////// Testing NEW PLINQ With Expensive Computation/////////////
            stopWatch.Start();

            linqResult = employeeList.AsParallel<Employee>().Where<Employee>(e=> e.ExpensiveComputation());

            empCount = linqResult.Count();

            stopWatch.Stop();

            Console.WriteLine(string.Format("Time taken by new PLINQ in Expensive Computation is {0} to get {1} Employees", stopWatch.Elapsed.TotalMilliseconds, empCount));

            stopWatch.Reset();
            /////////////////////////////////////////////////

            Console.WriteLine();


            ///////////////////////////////// Testing Old LINQ With NON Expensive Computation///////////////
            stopWatch.Start();

            linqResult = employeeList.Where<Employee>(e => e.NonExpensiveComputation());

            empCount = linqResult.Count();

            stopWatch.Stop();

            Console.WriteLine(string.Format("Time taken by old LINQ in Non Expensive Computation is {0} to get {1} Employees", stopWatch.Elapsed.TotalMilliseconds, empCount));

            stopWatch.Reset();
            //////////////////////////////////////////////////////////

            ///////////////////////////////// Testing NEW PLINQ With NON Expensive Computation/////////////
            stopWatch.Start();

            linqResult = employeeList.AsParallel<Employee>().Where<Employee>(e => e.NonExpensiveComputation());

            empCount = linqResult.Count();

            stopWatch.Stop();

            Console.WriteLine(string.Format("Time taken by new PLINQ in Non Expensive Computation is {0} to get {1} Employees", stopWatch.Elapsed.TotalMilliseconds, empCount));

            stopWatch.Reset();
            /////////////////////////////////////////////////

            Console.ReadKey();

        }

        static List<Employee> GetData()
        {
            List<Employee> employeeList = new List<Employee>();
            Random random = new Random(1000);

            for (int i = 0; i < 1000; i++)
            {
                employeeList.Add(new Employee() { Name = "Employee" + i, Salary = GetRandomNumber(random, 1000, 5000)});
            }

            return employeeList;
        }
        
        static double GetRandomNumber(Random random, double minimum, double maximum)
        {
            return random.NextDouble() * (maximum - minimum) + minimum;
        }
    }
}

Here is the result of above code