Thread atributes

 Process and thread: processes own resources, threads run code.
(Default) thread attributes
  • PTHREAD_CREATE_JOINABLE: the thread is put into a zoombie state when it terminates, until you retrieve its exit status or detach it.
  • inherit-scheduling attribute
    • PTHREAD_INHERIT_SCHED: the thread inherits the scheduling policy of its parent thread.
    • another value: PTHREAD_EXPLICIT_SCHED
      • if we want to use different scheduling parameters for the new thread, this need to be changed to PTHREAD_EXPLICIT_SCHED.
  • cancellation state
    • PTHREAD_CANCEL_ENABLE: cancellation requests may be acted on accordingly to the cancelation type
    • another state: PTHREAD_CANCEL_DISABLE
  • cancellation type
    • PTHREAD_CANCEL_DEFERRED: cancellation requests are held pending until a cancellation point.
    • PTHREAD_CANCEL_A
  • 4K stack

Thread scheduling

  1. Scheduling policy: inherited from the parent process (PTHREAD_INHERT_SCHED), by default. In all cases, a higher-priority thread can preempts all lower-priority threads one it gets READY.
    • FIFO scheduling: apply only when two or more threads that share the same priority are READY.
    • Round-robin scheduling: a thread stop executing when it consumes its timeslice.
    • Sporadic scheduling
  2. Scheduling priority, independent of the scheduling policy. The scheduler selects the highest priority thread to run, from the READY threads.
    • range 0 ~ 255
      • 0 is for the special idle thread (in kernel), which is always ready to run.
      • 1 ~ 63 for user threads. (the range can be changed by pronto -P option).
      • 1 ~255 for root threads.

Data structure

  • pthread_attr_t: stack size, detachstate, schedparam, schedpolicy, etc
  • struct sched_param: contains sched_priority, sched_curpriority
  • int policy: FIFO(1), RR(2, default?), OTHER(3), SPORADIC(4)

 APIs

  1. process level
    • Get/set the priority of a process; if pid 0 used, it is for current process.
      • sched_setparam(pid|0, &sched_param): 
      • sched_getparam(pid|0, &sched_param);
    • get the priority range for the scheduling policy
      • sched_get_priority_max(int policy);
      • sched_get_priority_min(int policy);
    • calculate the allowable priority for the policy
      • allowable_priority = sched_get_priority_adjust(int priority, int policy, int adjust);
        • priority = -1, priority of current thread.
        • policy=SCHED_NOCHANGE, the policy of the called thread
        • adjust=+20, we want a priority of “priority + 20”.
  2. thread level (used after the thread is created)
    • Set/get scheduling parameters
      • pthread_getschedparam(tid, &policy, &param);
      • pthread_setschedparam(tid, policy, &param)
    • set the priority
      • pthread_setschedprio(tid, prio);
      • Note: the priority get should be achieved using pthread_getschedparam().
    • set cancellation state/type
      • pthread_setcancelstate(int newstate, &oldstate)
      • pthread_setcanceltype(int newtype, &oldtype);
        • not _get functions.
        • can only be set after thread creation.
  3. attribute level (only take effect before the thread is created)
    • initialize/destroy attr object
      • pthread_attr_init(&attr)
      • pthread_attr_destroy(&attr);  // the memory is not freed. but after destroying, reinit the object using pthread_attr_init()
    • set/get policy
      • pthread_attr_setschedpolicy(&attr, int policy)
      • pthread_attr_getschedpolicy(&attr, &policy)
    • set/get scheduling parameters
      • pthread_attr_setschedparam(&attr, &param);
        • used only after pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED);
      • pthread_attr_getschedparam(&attr, &param);
    • set/get inherit-scheduling attributes
      • pthread_attr_setinheritsched(&attr, interitsched);
      • pthread_attr_getinheritsched(&attr, &interitsched);
        • need to be set prior to the thread creation
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