Seminar on Linux Process Management Presentation by Manoj Dhage MTech 1 st Year, SIT, IIT Kharagpur.

Outline  Process -process descriptor, process state, process switch.  The Linux Scheduler -Runqueues, priority arrays -Calcucalation of priorities and timeslices, -schedule() function -Scheduler tuning

Process  Process  Lightweight Process  Process Descriptor

Process Descriptor Ref: Understanding the Linux Kernel

Process State  TASK_RUNNING  TASK_INTERRUPTIBLE  TASK_UNINTERRUPTIBLE  TASK_STOPPED  TASK_ZOMBIE

Process Descriptor Handling  The Process List Ref: Understanding the Linux Kernel

continued…  List of Data Structure Ref: Understanding the Linux Kernel

Process Switch  Hardware Context  Happens only in schedule() function  Steps -switch page global directory -switch kernel mode stack and H/W context

The Linux CPU Scheduler  Runqueues  The O(1) Scheduling Algorithm - one Runqueue per CPU - active priority array - expired priority array

Use Priority Arrays  active priority array  bitmap[BITMAP_SIZE]

bitmap[BITMAP_SIZE]

Calculating priority  static priority -nice() system call -specified by user -never modified by scheduler - used to calculate timeslice

continued…  dynamic priority -subtracts –5 to +5 from static priority depending on the time spend in running or waiting -used for scheduling purpose

The schedule() function.  steps involved -find process with maximum dynamic priority -prev and next pointers to process descriptors -switch to next switch page global directory switch kernel mode stack

continued…  Called in prev, return in next  Why this is O(1)  Invoked by -scheduler_tick() -system call

scheduler_tick()  Invoked every 1 ms by timer interrupt  Do not always invoke schedule()  Whenever a task runs out of time slice, it is given new time slice and dynamic priority and put in expired array  Reinsert interactive task on active array  If there is no active task swap the pointers of active and expired arrays

System calls  Insert current task in a waitqueue  Change state to TASK_INTERRUPTABLE or TASK_UNINTERRUPTABLE  Invoke schedule()  If resource available remove from waitqueue

Handling real time tasks  Task priority range -20 to 19  SCHED_FIFO Scheduling  SCHED_RR Scheduling

Scheduler tuning  MIN_TIMESLICE AND MAXI_TIMESLICE  PRIO_BONUS_RATIO  STARVATION_LIMIT

Future scope  Scheduler modes -server mode, bigger timeslice -desktop mode, more responsiveness  Swappable schedulers -different scheduler for different users -basic kernel scheduler would be round robin

3. References Josh Aas, “Understanding the Linux CPU Scheduler”, February Abraham Silberschatz, Greg Gagne, Peter Galvin, “Operating System Concepts”, Wiley. Daniel P. Bovet, Marco Cesati, “Understanding the Linux Kernel”, 2ndEdition. O’Reilly, July

Thank You Thank You.