Transcript POSIX - Welcome to the Department of Information Technology

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
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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
Dr. Hugh Melvin, Dept. of IT, NUI,G
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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;
Dr. Hugh Melvin, Dept. of IT, NUI,G
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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
Dr. Hugh Melvin, Dept. of IT, NUI,G
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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
Dr. Hugh Melvin, Dept. of IT, NUI,G
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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
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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_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
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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
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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
Dr. Hugh Melvin, Dept. of IT, NUI,G
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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
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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
Dr. Hugh Melvin, Dept. of IT, NUI,G
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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_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
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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)
Dr. Hugh Melvin, Dept. of IT, NUI,G
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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;
}
Dr. Hugh Melvin, Dept. of IT, NUI,G
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