POSIX

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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

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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

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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

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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

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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

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POSIX.4 RT Scheduling

• Setting scheduling policy and attribute
• include ltsched.hgt
• struct sched_param scheduling_parameters
• int scheduling_policy
• int i
• scheduling_parameters.sched_priority17
• isched_setscheduler(getpid( ),SCHED_FIFO,
  scheduling_parameters)
• getpid( ) used to determine process ID
• Process set to FIFO, priority 17

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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_minsched_get_priority_min(SCHED_RR)
• sched_rr_maxsched_get_priority_max(SCHED_RR)
• sched_fifo_minsched_get_priority_min(SCHED_FIFO)
  
• sched_fifo_maxsched_get_priority_max(SCHED_FIFO)
  
• Eg. 256 priority levels
• FIFO range 128-255
• RR range 0-127

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POSIX.4 RT Scheduling

• Eg.
• includeltsched.hgt
• 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
• isched_setparam(getpid (),my_sched_params)

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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

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POSIX

• includeltunistd.hgt
• includelttime.hgt
• int main()
• struct timespec clock_res
• int stat
• statclock_getres(CLOCK_REALTIME, clock_res)
• printf("Clock resol is d sec, ld
  nseconds\n",clock_res.tv_sec,clock_res.tv_nsec)
  
• return 0

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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

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POSIX.4 Clocks Timers

• Interval Timer example
• timer_t created_timer
• i timer_create( _ , _ , created_timer)
• struct itimerspec new,old
• new.it_value.tv_sec1
• new.it_value.tv_nsec0
• new.it_interval.tv_sec0
• new.it_interval.tv_nsec100000
• itimer_settime(created_timer, 0,new, old)
• ..
• itimer_delete(created_timer)

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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)

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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