Two-Thread Process on 2-CPU-4-Thread Machine

JVM Tutorials - Herong's Tutorial Examples

∟CPU Impact of Multi-Thread Applications

∟Two-Thread Process on 2-CPU-4-Thread Machine

This section provides a tutorial example on how a two-threaded process behaves on a 2-CPU-4-Thread architecture. Windows will only allocate 2 primary CPU-Threads to the process.

Now, let's continue with a 2-thread process.

1. Close all applications on the system.

2. Launch MultithreadingCpuRunner.java with 2 threads. The performances is almost doubled comparing to the 1 thread execution in the previous tutorial.

C:\>"\Program Files\Java\jdk1.7.0_45\bin\java" 
   MultithreadingCpuRunner 1000 2

Thread parameter: 1000
Number of threads: 2
Seconds | Productivity per thread | Total | Average
...
146 | 833 833 | 1666 | 1665
147 | 832 833 | 1665 | 1665
148 | 839 838 | 1677 | 1665
149 | 830 832 | 1662 | 1665
150 | 839 838 | 1677 | 1665
...

3. View individual CPU-Thread usage on the Performance tab of Task Manager. You will see that the running process is being distributed between 2 CPU-Threads with 2 other CPU-Threads doing nothing:

CPU-Thread 1 - 100% used (approximately)
CPU-Thread 2 -   0% used
CPU-Thread 3 - 100% used (approximately)
CPU-Thread 4 -   0% used

4. View total CPU usage on the Performance tab of Task Manager. You will that the total CPU usage is about 50%. See the picture below:
2 Threads Running on 4 CPU-Threads

Question: Why the JVM is not making full use of all 4 CPU-Threads? I have no idea.

5. Force Windows to use only 1 CPU-Thread by going to the on the Processes tab of Task Manager. Then right-mouse click on the "java.exe" process, which represents the running MultithreadingCpuRunner.java program. When select "Set Affinity...", you will see the processor affinity options. Uncheck CPU 1, 2, and 3. Then click OK to force Windows to only use CPU-Thread 0 for the execution of MultithreadingCpuRunner.java.

6. Go back to view individual CPU-Thread usage on the Performance tab of Task Manager again. You will see that the running process is being distributed only to the first CPU-Thread:

CPU-Thread 1 - 100% used
CPU-Thread 2 -   0% used
CPU-Thread 3 -   0% used
CPU-Thread 4 -   0% used

This behavior is expected, because we asked the Windows to only use the first CPU-Thread. Of course, the total productivity is dropped almost by half and the average productivity is slowly reaching 885:

Seconds | Productivity per thread | Total | Average
...
321 | 421 472 | 890 | 900
322 | 416 467 | 883 | 900
323 | 453 432 | 886 | 900
324 | 470 417 | 881 | 900
325 | 427 466 | 885 | 900
...

7. Go back to the Process tab and force Windows to use the first 2 CPU-Threads. Windows will listen to us and allocate both CPU-Threads to MultithreadingCpuRunner.java:

CPU-Thread 1 - 100% used
CPU-Thread 2 - 100% used
CPU-Thread 3 -   0% used
CPU-Thread 4 -   0% used

However, the performance is not back to the same level when CPU-Thread 1 and 3 was 100% used. The total productivity is around 985 and the average productivity is slowly reaching the same number:

Seconds | Productivity per thread | Total | Average
...
711 | 493 495 | 985 | 993
712 | 492 494 | 986 | 993
713 | 493 491 | 984 | 993
714 | 491 494 | 985 | 993
715 | 494 495 | 985 | 993
...

The answer is in the design of the 2-CPU-4-Thread architecture. My guess is that CPU-Thread 1 and 3 represent 2 real CPUs. CPU-Thread 2 and 4 are just secondary processing threads added to each real CPUs.

If we use the average productivity number from the CPU-Thread 1 execution, 885, and from the CPU-Thread 1 and 2, 985, we can estimate the execution power of CPU-Thread 2 as (985-885)/885 = 11% of CPU-Thread 1. If CPU-Thread 3 and 4 are behave in the same way CPU-Thread 1 and 2, we can estimate that my 2-CPU-4-Thread architecture is giving me only about 2.22 CPUs in terms of execution power:

CPU-Thread #1 - 100% CPU
CPU-Thread #2 -  11% CPU, if used together with #1
CPU-Thread #3 - 100% CPU
CPU-Thread #4 -  11% CPU, if used together with #3

Note that this unequal execution power on each CPU-Thread will cause Windows total CPU usage report inaccurate. For example, when MultithreadingCpuRunner.java in running on CPU-Thread 1 and 3 with 100% usage on each of them, Windows reports 50% usage of total CPU. But in reality, 90% (200 out of 222) of the total CPU power is used.

Last update: 2014.

Table of Contents

About This Book

Downloading and Installing JDK 1.8.0 on Windows

Downloading and Installing JDK 1.7.0 on Windows

java.lang.Runtime Class - The JVM Instance

java.lang.System Class - The Operating System

ClassLoader Class - Class Loaders

Class Class - Class Reflections

Sun's JVM - Java HotSpot VM

JRockit JVM 28.2.7 by Oracle Corporation

JVM Runtime Data Areas