Question

What is the best Timed Wait algorithm for Semaphores?

Asked by: ShawnCurry

I am working on a cross platform concurrency library.  I would like to have timed polling of semaphores.  I am considering a few different approaches.

My first idea was to set up a timer signal, and do something like this:

int xpsrv_sem_wait( sem_t* sem )
{
      int result;
      while( (result = sem_wait( sem ) == -1) && errno == EINTR );
      return (result);
}

int xpsrv_sem_trywait( sem_t* sem, unsigned millis )
{
      int result;
      unsigned long start = System::CurrentTimeMillis();
      start_timer();
      while( (result = sem_wait( sem ) == -1) && errno == EINTR )
      {
            if( System::CurrentTimeMillis() - start >= millis )
                  break;
      }
      stop_timer();
      return (result);
}

But it occurred to me that I have probably have no guarantee that the thread waiting for the semaphore will ever catch the timer signal.  If I set up the timer signal to just fire at regular intervals - say once per millisecond - would that be a strong enough guarantee that the waiting thread would be awakened within a few milliseconds of the desired time?

Assuming that is a strong enough guarantee, should I activate/deactivate the timer each time, or can I get away with leaving it running and just blocking/unblocking the signal?  Does it introduce extra overhead to leave it running?

While researching the problem online, I came across another approach which uses System V semaphores and the select function: http://www-128.ibm.com/developerworks/eserver/library/es-win32linux-sem.html (listing 28)

Which of these approaches are better or safer?  Are there any other methods?

Thanks in advance,
Shawn.

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Asked On
2006-12-10 at 15:44:48ID22088634
Tags

wait

,

timed

Topic

Linux Programming

Participating Experts
1
Points
500
Comments
7

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Answers

 

by: duncan_roePosted on 2006-12-10 at 22:30:58ID: 18113209

pthread_cond_timedwait() is a reasonably standard posix implementation of what you're seeking to achieve.
Under linux at least, a timer process can use the Real Time Clock - very accurate as you get told about missed interrupts.

 

by: ShawnCurryPosted on 2006-12-11 at 08:42:02ID: 18115787

Is there a way to use pthread_cond_timedwait() with semaphores?  I would like to use regular POSIX or System V Semaphores, rather than simulate them using other synchronization objects.  Should I decide to simulate them, I'll probably go with a raw assembly command like CMPXCHG (which would probably be more "atomically correct" for cases when the semaphore is incremented/decremented by more than 1).  However, my impression was that using a test-and-set algorithm will require additional testing to determine how long to spin when the test-and-set fails.

After thinking about this some more, what I'd really like to do is to be able to send an asynchronous signal directly to the thread that is blocked in sem_wait().  Signaling a specific thread will also be useful for general thread-interruption purposes.  Is there a way to do this using the thread id or something?

 

by: duncan_roePosted on 2006-12-11 at 12:55:19ID: 18117830

If you want to use semaphores in a timed fashion, you pretty-much need to have a timer thread which can signal them. And the threads have to cancel the timer associated with a semaphore before waiting on it. The way I have seen it done is to use a highher-level structure which we called a maibox. The thread waits for a mail message then actions it. The received message can be an action request from another thread or "Timer Expired". That was actually a native interface under one OS and we ported it to Linux. I didnm't do the low-level port but IIRC it's just done with semaphores and mutexes. I guess you could do it with System V IPC too.

 

by: ShawnCurryPosted on 2006-12-11 at 19:55:46ID: 18120169

BINGO: http://opengroup.org/onlinepubs/007908799/xsh/pthread_kill.html

I think pthread_kill() is exactly what I'm looking for.  I just want to be able to send a signal directly to the thread blocked in sem_wait(), hopefully causing sem_wait() to fail with EINTR.

I think I like your mailbox idea.  I would still need to have a timer for each thread blocked in xpsrv_sem_trywait(), possibly with different durations, starting at different times.  So instead of a timer for each thread, it's probably more efficient to have a "timekeeper" thread that accepts timing requests, and signals the thread when the time has elapsed. (That's essentially what cron is right?)

I suppose you'll get at least partial credit for that...  I'd like to leave the question open for now, in case I run into something else in the process.

Thanks,
Shawn.

 

by: duncan_roePosted on 2006-12-30 at 14:54:05ID: 18219972

There's negligible overhead to multiple timer threads, as long as you don't kill / recreate them every time you use them. Our calling interface specified a function for the timer process to call on timer expiry - a little more general than giving it a mailbox to post to (though that's what the functions usually did). A timekeeper is a lot more work - I've done it in a non-threaded environment but I think when you have threads it's just not worth doing.
The trouble with signals is, it may not be the one you were expecting.

20120131-EE-VQP-002

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