Question

I wanted to modify the linux kernel code that dynamically changes the process priority.

I have Ubuntu 10.10 and I downloaded linux 2.6.37. What I found was that in 2.6.37 version there is no concept of dynamically changing the priority of the process. [In CentOS there is a recalc_task_prio() function that dynamically changes the process priority].

I wanted to know if anyone is aware of any other ubuntu kernel version that supports the functionality of dynamically changing the process priority (based on penalizing the process priority if task is cpu bound and consuming lot of cpu)?

I mostly refering to this link – http://www.ibm.com/developerworks/linux/library/l-scheduler/

Thanks in Advance,

Answer 1

In Linux v2.6.23 a new CFS scheduler was integrated that relies less on the HZ and more on nanoseconds of execution time.

All processes are stored in a red-black tree that makes it cheap O(1) to find the left-most (and thus most-deserving) task to execute. Periodically (task_tick()) the system checks to see if another process might be “more deserving” and preempt a running task. (Of course, tasks can yield (yield_task()) on their own and newly-runnable tasks can preempt a running task (check_preempt_curr()). The time a process spent running is accounted to it and it is re-inserted into the tree O(log(N)) to find its new placement in the RB-tree.

Thus, tasks that wait on IO are accounted very little time and aren’t moved very far away in the tree — which corresponds to a priority boost. Tasks that don’t sleep on IO event will use comparatively more CPU time before they are preempted by a higher-priority task.

For more details, be sure to read the Documentation/scheduler/ documentation and the kernel/sched* implementation files.

If it were my homework project to fiddle with schedulers (as used to be common in the olden days) I’d look seriously at adding a new realtime scheduling class like SCHED_FIFO or SCHED_RR (depending upon which one was closer to the scheduler design being discussed) and fiddling with priorities of the tasks that way. It might be a little steep for an introductory OS course to do this — an older Linux or older BSD kernel might be easier to play with.