multi-thread CPU usage in C#

Question

My Program uses predetermined number of threads that each do independent work. I use i7-2600 CPU but I shut down the hyper-thread module so it runs 4 threads on 4 cores. When I run the program with 1 thread the CPU usage is 25% which is perfect since 1 thread is fully used, but when I run 4 or 3 thread I only get 60% CPU, why?

Like I mentioned before the threads are totally independent (there are no locks and no contentions) also when I run the program 4 times with 1 thread I get 100% CPU usage (ie when its 4 processes of 1 thread for each process I get proper CPU usage)

Any ideas?

some more info:

1. I'm not using I/O while processing all the needed data is loaded into to the memory, each thread loads its own data prior to processing.
2. I do load the data from a database (using oleDb) but the issue I'm describing happens after the loading while all threads are processing (no loading is done).
3. The numbers are:
   • 2 threads around 40% instead of 50 (85% for each thread).
   • 3 threads around 50% instead of 75 (65% for each thread).
   • 4 threads around 60% instead of 100 (60% for each thread).
4. I use i7 2600 with 16GB memory but the memory usage on that process doesn't come close to that.
5. I ran windows performance monitor to look for contentions there are none.

Answer 1

You may try to shift your program from workstation to server garbage collection mode. Currently you may use just one garbage collection thread.
The setup is explained here .
See this thread for an explanation/further details.
Do not expect 100 % CPU load after the change, but you will get closer to 100% and increase the spead of it.

Answer 2

The concurrency visualizer will help you here. You will see when each thread is executing, and if all threads are not executing you can see exactly why a specific thread is waiting. If you generate a lot of garbage, the GC may keep threads blocked leading to less than linear scaling. But you will only find out by making a real investigation.

Answer 3

I had the same issue while working with .NET Framework v2.0 then i changed the projects target framework to v4.0 it started with 100 % CPU load

Edit: You can watch the CPU usage per threads, simply with

Sysinternals Process Explorer

or

Process Hacker