Multi-Thread Programs on Multi-CPU Systems

C# Tutorials - Herong's Tutorial Examples

∟Multi-Thread Programs on Multi-CPU Systems

This section provides a tutorial example to see how multi-thread programs behave on multi-CPU systems and how to change processor affinity to control which processors to use for each application.

Since I have 2-CPU system, I want to see how multithreading application behave on a multi-CPU system.

1. Modify MultiThread.cs to set count = 2. Then compile and run MultiThread.exe. You will notice that your computer performance gets reduced after MultiThread.exe started. And you will this output:

22:09:58.4675000: Launched thread 1
22:09:58.5143750: Launched thread 2

2. Run "Task Manager", go to "Processes" tab, and click "CPU" column to see the process with highest CPU usage first. As you can see from the picture below, MultiThread.exe is on the top of the list and uses about 98% CPU resource:
2 CPUs with Multi Threads

3. Right-mouse click on MultiThread.exe and select "Set Affinity". You will see the "Processor Affinity" dialog box with this help text: "The Processor Affinity setting controls which CPUs the process will be allowed to execute on."

4. Uncheck "CPU 1" and keep "CPU 0" checked as shown below:
Processor Affinity Setting

5. Click "OK" to save the change. You will notice that your computer speed gets improved. And the CPU usage of MultiThread.exe is changed to about 50% as shown below:
1 CPU with Multi Threads

I guess what happened here was:

By default, MultiThread.exe was started to allow to 2 CPUs. Because MultiThread.exe has 2 sub threads running, Windows decided to use both CPUs: 1 sub thread on each CPU. The result was that MultiThread.exe uses 98% of the total CPU resources.
When the Processor Affinity is changed to stop using CPU 1, both sub threads were forced to run on CPU 0. The result was that MultiThread.exe uses 50% of the total CPU resources.