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

Advertisements