Making C++ run at full speed

Question

To compare C++ and Java on certain tasks I have made two similar programs, one in Java and one in C++. When I run the Java one it takes 25% CPU without fluctuation, which you would expect as I'm using a quad core. However, the C++ version only uses about 8% and fluctuates havily. I run both programs on the same computer, on the same OS, with the same programs active in the background. How do I make C++ use one full core? These are 2 programs both not interrupted by anything. They both ask for some info and then enter an infinite loop until you exit the program, giving feedback on how many calculations per second.

The code:

http://pastebin.com/5rNuR9wA

http://pastebin.com/gzSwgBC1

http://pastebin.com/60wpcqtn

To answer some questions:

I'm basically looping a bunch of code and seeing how often per second it loops. The problem is: it doesn't use all the CPU it can use. The whole point is to have the same processor do the same task in Java and C++ and compare the amount of loops per second. But if one is using irregular amounts of CPU time and the other one is looping stable at a certain percentage they are hard to compare. By the way, if I ask it to execute this:

while(true){}

it takes 25%, why doesn't it do that with my code?

----edit:----

After some experimenting it seems that my code starts to use less than 25% if I use a cout statement. It isn't clear to me why a cout would cause the program to use less cpu (I guess it pauses until the statement is written which appearantly takes a while?

With this knowledge I will reprogram both programs (to keep them comparable) and just let it report the results after 60 seconds instead of every time it completed a loop.

Thanks for all the help, some of the tips were really helpful. After I discovered the answer someone also turned out to give this as an answer, so even if I wouldn't have found it myself I would have gotten the answer. Thanks!

(though I would like to know why a std::cout takes such an amount of time)

Answer 1

Your main loop has a cout in it, which will call out to the OS to write the accumulated output at some point. Either OS time is not counted against your app, or it causes some disk IO or other activity that forces your program to wait.

Answer 2

It's probably not accurate to compare both of these running at the same time without considering the fact that they will compete for cpu time. The OS will automatically choose the scheduling for these two tasks which can be affected by which one started first and a multitude of other criteria.

Running them both at the same time would require some type of configuration of the scheduling so that each one is confined to run to one (or two) cpus and each application uses different cpus. This can be done by having each main function execute a separate thread that performs all the work and then setting the cpu where this thread will run. In c++11 this can be done using a std::thread and then setting the underlying cpu affinity by getting the native_handle and setting it there.

I'm not sure how to do this in Java but I'm sure the process is similar.