Realtime API¶

Scheduling Policies ¶

Scheduling policies determine the scheduler’s way of assigning CPUs …

SCHED_OTHER: the fair world (runnable processes are only added to pool of all runnables)
SCHED_FIFO (First-in first-out)
- Process gets CPU immediately assigned
- Remains on CPU until he relinquishes
- … or a higher prio process wants CPU
SCHED_RR (Round Robin)
- Like SCHED_FIFO
- Equal prio processes: short timeslices in round robin order

Scheduling Priorities ¶

0 … Reserved for good old fair processes (SCHED_OTHER)
1 … lowest realtime priority
…
99 … highest realtime priority

What’s Runnning On A System?¶

Scheduling: System Calls ¶

Manipulating scheduling attributes of a process:

man 2 sched_setscheduler

int sched_setscheduler(pid_t pid, int policy, const struct sched_param *param);
int sched_getscheduler(pid_t pid);

struct sched_param {
    int sched_priority;
};

Scheduling: Threads (1)¶

Manipulating scheduling attributes of an existing thread:

man 3 pthread_setschedparam

pthread_setschedparam(
    pthread_t thread, int policy,
    const struct sched_param *param);
pthread_getschedparam(
    pthread_t thread, int *policy,
    struct sched_param *param);

Scheduling: Threads (2)¶

Start a new thread with predefinied scheduling attributes:

man 3 pthread_attr_setschedparam

int pthread_attr_setschedparam(
    pthread_attr_t *attr,
    const struct sched_param *param);

man 3 pthread_attr_setschedpolicy

int pthread_attr_setschedpolicy(
    pthread_attr_t *attr, int policy);

Priority Inversion ¶

Priority Inversion: Mutex Protocols (1)¶

Solution, in spoken words: at the time that C wants the mutex, A has to carry on ⟶ protocol between both, communicated via the mutex

⟶ Mutex Attribute

man 3 pthread_mutexattr_setprotocol

int pthread_mutexattr_setprotocol(
    pthread_mutexattr_t *attr,
    int protocol);

Protocols

PTHREAD_PRIO_INHERIT: A’s priority is temporarily* (until mutex is acquired) boosted to B’s
PTHREAD_PRIO_PROTECT: A’s priority is temporarily risen to a fixed limit ⟶ man 3 pthread_mutexattr_setprioceiling