POSIX

1. POSIX • POSIX: Portable OS Interface • IEEE standard • Mandatory + Optional parts • Mostly based on and adopted by Unix community • POSIX.4 = POSIX 1003.1b • Added Realtime functionality • POSIX.4a = POSIX 1003.1c • Threads Extensions Dr. Hugh Melvin, Dept. of IT, NUI,G

2. POSIX.4 = POSIX 1003.1b • Range of RT Features • Shared Memory / Memory Locking / Priority Scheduling / Signals / Semaphores / Clocks & Timers • Will examine many of these issues • Adopted by many RTOS : QNX, LynxOS, VxWorks • POSIX.4b = POSIX 1003.1d • More realtime extensions Dr. Hugh Melvin, Dept. of IT, NUI,G

3. POSIX.4 RT Scheduling • Defines two main scheduling policies • SCHED_FIFO and SCHED_RR • Each have attributes • Also have SCHED_OTHER • Currently a single attribute = priority struct sched_param{ int sched_priority; } • Eg. Could implement EDF by extending structure to include • struct timespec sched_deadline; • struct timespec sched_timerequired; Dr. Hugh Melvin, Dept. of IT, NUI,G

4. POSIX.4 RT Scheduling • SCHED_FIFO • Simple priority based preemptive scheduler • Most common in RTS • FIFO used to schedule processes within each priority level • If no other process exists at higher priority, process runs until complete • Next process at that priority (if present) then allocated CPU • Highest priority process guaranteed processor time Dr. Hugh Melvin, Dept. of IT, NUI,G

5. POSIX.4 RT Scheduling • SCHED_RR • Round robin used to timeslice among processes at same priority level • System provided timeslice • Use for lower priority tasks Dr. Hugh Melvin, Dept. of IT, NUI,G

6. POSIX.4 RT Scheduling • Setting scheduling policy and attribute #include <sched.h> struct sched_param scheduling_parameters; int scheduling_policy; int i; scheduling_parameters.sched_priority=17; i=sched_setscheduler(getpid( ),SCHED_FIFO, &scheduling_parameters); • getpid( ) used to determine process ID • Process set to FIFO, priority 17 Dr. Hugh Melvin, Dept. of IT, NUI,G

7. POSIX.4 RT Scheduling • Process priority ranges differ among OS • Need this info before setting priority level int sched_rr_min, sched_rr_max; int sched_fifo_min, sched_fifo_max; sched_rr_min=sched_get_priority_min(SCHED_RR); sched_rr_max=sched_get_priority_max(SCHED_RR); sched_fifo_min=sched_get_priority_min(SCHED_FIFO); sched_fifo_max=sched_get_priority_max(SCHED_FIFO); • Eg. 256 priority levels • FIFO range 128-255 • RR range 0-127 Dr. Hugh Melvin, Dept. of IT, NUI,G

8. POSIX.4 RT Scheduling Eg. #include<sched.h> int i; struct sched_param my_sched_params; // determine max FIFO priority level my_sched_params.sched_priority= sched_get_priority_max(SCHED_FIFO); // Set priority i=sched_setparam(getpid (),&my_sched_params); Dr. Hugh Melvin, Dept. of IT, NUI,G

9. POSIX.4 Clocks & Timers • Must be at least one clock • CLOCK_REALTIME • clock_gettime( ), clock_settime( ) • Replaces gettimeofday( ),settimeofday() • timespec structure (sec + nsec) • struct timespec{ time_t tv_sec; time_t tv_nsec; } • clock_getres(CLOCK_REALTIME,&realtime_res) • Returns clock resolution : must be at least 50 Hz • realtime_res is timespec structure • Realtime libraries reqd Dr. Hugh Melvin, Dept. of IT, NUI,G

10. POSIX #include<unistd.h> #include<time.h> int main(){ struct timespec clock_res; int stat; stat=clock_getres(CLOCK_REALTIME, &clock_res); printf("Clock resol is %d sec, %ld nseconds\n",clock_res.tv_sec,clock_res.tv_nsec); return 0; } Dr. Hugh Melvin, Dept. of IT, NUI,G

11. POSIX.4 Clocks & Timers • nanosleep(&nap,&time_left) • Can delay process (both nap & time_left are timespec) • Interval Timers • Useful to specify precise intervals • struct itimerspec{ struct timespec it_value; struct timespec it_interval; } it_value = 1st occasion of timer event it_interval = interval between subsequent events • System calls • timer_create( ) and timer_delete( ) • Can have multiple timers within any process Dr. Hugh Melvin, Dept. of IT, NUI,G

12. POSIX.4 Clocks & Timers • Interval Timer example timer_t created_timer; i = timer_create( _ , _ , &created_timer); struct itimerspec new,old; new.it_value.tv_sec=1; new.it_value.tv_nsec=0; new.it_interval.tv_sec=0; new.it_interval.tv_nsec=100000; i=timer_settime(created_timer, 0,&new, &old) .. i=timer_delete(created_timer) Dr. Hugh Melvin, Dept. of IT, NUI,G

13. POSIX.4 Clocks & Timers • Absolute Timer Events • Eg. Timer event reqd at time tabs • Determine interval and use interval timer • clock_gettime(CLOCK_REALTIME, &now); • Calculate interval • Interval = tabs - now • Create and set Interval timer as above • But • Process may be preempted between step 1 and 2 Use absolute times timer_settime(created_timer,TIMER_ABSTIME,&tabs ,NULL) Dr. Hugh Melvin, Dept. of IT, NUI,G

14. This function reads the hardware clock using the "intel" RDTSC assembly instruction. void get_timestamp(unsigned long* upper32bits, unsigned long* lower32bits) { unsigned long lower32, upper32, first_upper32; do { __asm RDTSC __asm mov upper32, EDX __asm mov lower32, EAX first_upper32 = upper32; __asm RDTSC __asm mov upper32, EDX } while (first_upper32 != upper32); // protect against wraparound *lower32bits = lower32; *upper32bits = upper32; } Dr. Hugh Melvin, Dept. of IT, NUI,G