April 26, 2006

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POSIX:TMR Interval Timers

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• alternative to POSIX:XSI Extension
  • each process has access to three timers
• here, each process can create several independent timers for a small number of clocks, e.g., CLOCK_REALTIME
• struct itimerspec (we have seen this before)
  • struct timespec it_value: time remaining before expiration
  • struct timespec it_interval: value to reload into timer
• struct timespec offers better resolution than struct timeval
  • uses (seconds, nanoseconds) pair
• these timers are not inherited by fork
• timer_create: used to create specific timers
  • clock_id
  • timerid
  • evp
    • specifies the asynchronous notification to occur on expiration
    • has type struct sigevent (akin to siginfo_t)
    • specifies which signal should be sent to indicate expiration
    • NULL implies default (SIGALRM for CLOCK_REALTIME)
    • sigev_notify field specifies the action to be taken
      • SIGEV_SIGNAL vs. SIGEV_NONE
      • field sigev_value used to distinguish which timer generated the signal
• Example 9.16

  timer_t timerid;
  
  if (timer_create (CLOCK_REALTIME, NULL, &timerid) == -1)
     perror ("Failed to create a new timer);
  

• functions for manipulating these interval timers
  • timer_delete: deletes timer with ID timerid
    • what about pending signals?
  • timer_settime: starts and stops a timer created with timer_create
  • timer_gettime: analog of getitimer
  • timer_getoverrun: returns the number of timer overruns
    • timer overrun: when a timer expires while a signal from a prior expiration is still pending
• examples:
  • periodicmessage.c: displays a message every two seconds
  • tmrtimer.c: yet another way to measure the running time of a function

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Dealing with Timer Drift: Absolute Time

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• timer drift is the cumulative effect of the differences between when a timer expires and when it is reset
• Example 9.20: consider a timer with a 22 ms interval and a 10 ms resolution
  • the interval will be rounded up to 30 ms, yielding a drift of 8 ms on every expiration
  • 80 ms cumulative drift over 10 continuous expirations
• solution involves using absolute time
• Example 9.21:
  • record desired expiration time as (current time + 22 ms) ms
  • set the timer to expire in 22 ms
  • in signal handler, record new current time and set the timer to expire in (current time - new current time + 22 ms) ms
    • presumes that the next interval is not negative
      • would be another type of timer overrun
  • cumulative drift will never exceed the clock resolution
• POSIX:TMR timers
  • provide nice support for absolute time
    • set flags in timer_settime function to TIMER_ABSOLUTE to specify that it_value represents real time rather than an interval
  • can keep track of timer overruns
• code up the solution sketch given in Exercise 9.22
• experiment with the abstime program

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Summary: Clocks and Timers

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• basic timing concepts and POSIX times
• POSIX:XSI vs. POSIX:TMR interval timers
• realtime signals
• multiple ways to measure the wall-clock time it takes to execute a function
  • use the time function (second resolution); see simpletiming.c
  • use the gettimeofday function (microsecond resolution); see gettimeofdaytiming.c
  • use the clock_gettime function (a POSIX:TMR clock); see clockrealtimetiming.c
• multiple ways to measure the virtual time it takes to execute of function
  • use the times function; see cpufraction.c
  • use a POSIX:XSI interval timer; see xsitimer.c
  • use a POSIX:TMR interval timer; see tmrtimer.c
• multiple ways to perform some action (e.g., write a message) every n seconds
  • use a POSIX:XSI interval timer; see periodicasterick.c
  • use a POSIX:TMR interval timer; see periodicmessage.c
• sleep vs. nanosleep

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References

[USP] K. A. Robbins and S. Robbins. UNIX Systems Programming: Concurrency, Communication, and Threads. Prentice Hall, 2003