time taken by forked child process

Question

This is a sequel to my previous question . I am using fork to create child process. Inside child, I am giving command to run a process as follows:

if((childpid=fork())==0)
{
system("./runBinary ");
exit(1)
}

My runBinary has the functionality of measuring how much time it takes from start to finish.

What amazes me is that when I run runBinary directly on command-line, it takes ~60 seconds. However, when I run it as a child process, it takes more, like ~75 or more. Is there something which I can do or am currently doing wrong, which is leading to this?

Thanks for the help in advance. MORE DETAILS: I am running on linux RHEL server, with 24 cores. I am measuring CPU time. At a time, I only fork 8 child (sequentially), each of which is bound to different core, using taskset (not shown in code). The system is not loaded except for my own program.

Answer 1

The system() function is to invoke the shell. You can do anything inside it, including running a script. This gives you a lot of flexibility, but it comes with a price: you're loading a shell, and then runBinary inside it. Although I don't think loading the shell would be responsible to so much time difference (15 seconds is a lot, after all), since it doesn't seem you need that - just to run the app - try using something from the exec() family instead.

Answer 2

Without profiling the application, if the parent process which forks has a large memory space, you might find that there is time spent attempting to fork the process itself, and attempts to duplicate the memory space.

This isn't a problem in Red Hat Enterprise Linux 6, but was in earlier versions of Red Hat Enterprise Linux 5.