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The Linux Scheduler 2.4 vs 2.6

The Linux Scheduler 2.4 vs 2.6. Michael McCabe mccabemt@clarkson.edu. Scheduling Basics. Tasks are divided into three groups, real time processes, IO bound, and CPU bound

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The Linux Scheduler 2.4 vs 2.6

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  1. The Linux Scheduler 2.4 vs 2.6 Michael McCabe mccabemt@clarkson.edu

  2. Scheduling Basics • Tasks are divided into three groups, real time processes, IO bound, and CPU bound • Real Time - Extremely high scheduling requirements, needs a guarantee on how often they will run, usually the highest priority process in a system • IO bound - processes that spend most of their time waiting for data going to or coming from the disk

  3. Scheduling Basics cont. • CPU bound - Processes that consume large amounts of cpu • Time slice - amount of time that a process can run on the CPU • Preemption - When the execution of the currently running process is interrupted in order to run a different, higher priority process

  4. The schedule function • Schedule() is the function in the linux kernel that does the actual scheduling • Has multiple ways of being run • Runs when a new process needs to be selected for scheduling • Is called when the currently running process is blocked, waiting for a resource • Each processor can call schedule on its own • Many device drivers will call schedule

  5. 2.4 Basics • 1400 Lines of code • Three basic data structures • Basic data structure is schedule_data. This data structure contains a pointer to the currently running process and the timestamp of the last time the schedule function ran • There is one run queue, and it’s a linked list

  6. Schedule_data struct schedule_data { struct task_struct * curr; cycles_t last_schedule; } schedule_data; char __pad [SMP_CACHE_BYTES];

  7. Schedule_data explained • Remarkably simple data structure • Defined in sched.c • Contains a time stamp of the last process switch • Also contains a pointer to the process that is currently running

  8. 2.4 SMP • Reschedule_idle checks to see if the process that just moved out of the running state should be moved to a different cpu • It doesn’t use the counter and nice values directly, it uses the goodness function to check priorities • Goodness takes into account the cost of moving a process across cpus

  9. 2.6 Basics • 5700 Lines of code • Run queue and priority arrays are the basic data structures • One run queue per processor • Two priority arrays per run queue

  10. Run Queue spinlock_t lock; unsigned long nr_running; #ifdef CONFIG_SMP unsigned long prio_bias; unsigned long cpu_load[3]; #endif unsigned long long nr_switches; unsigned long nr_uninterruptible; unsigned long expired_timestamp; unsigned long long timestamp_last_tick; task_t *curr, *idle; struct mm_struct *prev_mm; prio_array_t *active, *expired, arrays[2]; int best_expired_prio; atomic_t nr_iowait; #ifdef CONFIG_SMP struct sched_domain *sd; int active_balance; int push_cpu; task_t *migration_thread; struct list_head migration_queue; #endif

  11. Run queue explained • Primary scheduling data structure • Defined in sched.c • Needs to be locked before its modified • Locks are obtained on multiple run queues in ascending order

  12. Priority Array struct prio_array { unsigned int nr_active; unsigned long bitmap[BITMAP_SIZE]; struct list_head queue[MAX_PRIO]; };

  13. Priority Arrays explained • Defined in sched.c • Provides constant running time for the scheduling algorithm • Contains lists of runnable processes at each priority level • A bitmap is used to efficiently discover the highest priority process • When a task with priority 10 becomes runnable bit 10 in the bitmap gets set to 1

  14. 2.6 SMP • Load_balance is the function that makes sure each processor has a relatively equal number of processes on it • Only is run on multi processor systems • Runs every millisecond when the system is idle or every 200 milliseconds • Only tasks that are not running are moved

  15. The 2.6 kernel has a constant running time O(1) Leads to better scalability than the 2.4 kernel Also has a much more complex implementation Time slices are calculated when a process’s timeslice is used, before it moves to the expired array 2.4 kernel has a linear running time in the worst case Loops over the process list at the end of each time quantum Recalculates each processes’s time slice Big O running times

  16. 2.6 provides soft real time support Real time processes will preempt regular processes Real time priorities are set statically Not all versions of the 2.4 kernel can offer any real time guarantees Not all versions of the 2.4 kernel offer preemption Priority inversion occurs frequently in the 2.4 kernel Real time differences

  17. References • Understanding the Linux Kernel 2nd Edition • Kernel newbies • Linux Kernel Cross Reference • Linux Kernel Development • Linux Device Drivers 3rd Edition

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