CPU high usage of the usleep on Cent OS 6.3

Question

I compile the code below on cent os 5.3 and cent os 6.3:

#include 
#include 
#include 
#include 
usi         


        

Answer 1

You're correct, it has to do with the kernel config. usleep(1) will try to sleep for one microsecond. Before high resolution timers, it was not possible to sleep for less than a jiffy (in your case HZ=1000 so 1 jiffy == 1 millisecond).

On CentOS 5.3 which does not have these high resolution timers, you would sleep between 1ms and 2ms[1]. On CentOS 6.3 which has these timers, you're sleeping for close to one microsecond. That's why you're using more cpu on this platform: you're simply polling your task list 500-1000 times more.

If you change the code to usleep(1000), CentOS 5.3 will behave the same. CentOS 6.3 cpu time will decrease and be in the same ballpark as the program running on CentOS 5.3

There is a full discussion of this in the Linux manual: run man 7 time.

Note that your code should use condition variables instead of polling your task list at a certain time interval. That's a more efficient and clean way to do what you're doing.

Also, your main should really join the threads instead of just sleeping for 14 seconds.

[1] There is one exception. If your application was running under a realtime scheduling policy (SCHED_FIFO or SCHED_RR), it would busy-wait instead of sleeping to sleep close to the right amount. But by default you need root privileges