Using hardware timer in C

Question

Hi guys,

Okay, so I've got some C code to perform a mathematical operation which could, pretty much, take any length of time (depending on the operands supplied to it, of course). I was wondering if there is a way to register some kind of method which will be called every n seconds which can analyse the state of the operation, i.e. what iteration it is currently at, possibly using a hardware timer interrupt or something?

The reason I ask this is because I know the common way to implement this is to be keeping track of the current iteration in a variable; say, an integer called progress and have an IF statement like this in the code:

if ((progress % 10000) == 0)
    printf("Currently at iteration %d\n", progress);

but I believe that a mod operation takes a relatively long time to execute, so the idea of having it inside a loop which will be ran many, many times scares me, from an optimisation point of view.

So I get the feeling that having an external way of signalling a progress print is nice and efficient. Are there any great ways to perform this, or is the simple 'mod check' the best (in terms of optimising)?

Answer 1

While mod operations are usually slow, the compiler should be able to optimize and predict this really well and only mis-predict once ever 10'000 ifs, burning one mod operation and ~20 cycles (for the mis-prediction) on it, which is fine. So you are trying to optimize one mod operation every 10'000 iterations. Of course this assumes you are running it on a modern and typical CPU, and not some embedded system with unknown specs. This should even be faster than having a counter variable. Suggestion: Test it with and without the timing code, and figure out a complex solution if there is really a problem.

Premature optimisation is the root of all evil. -Knuth

Answer 2

I'd go with the mod check, but maybe with subtractions instead :-)

icount = 0;
progress = 10000;
/* ... */
    if (--progress == 0) {
        progress = 10000;
        printf("Currently at iteration %d0000\n", ++icount);
    }
/* ... */

Answer 3

you could have a global variable for the iterations, which you could monitor from an external thread.

While () {
  Print(iteration);
  Sleep(1000);
}

You may need to watch out for data races though.

Answer 4

mod is about the same speed as division, on most CPU's these days that means about 5-10 cycles... in other words hardly anything, slower than multiply/add/subtract, but not enough to really worry about.

However you are right to want to avoid sting in a loop spinning if you're doing work in another thread or something like that, if you're on a unixish system there's timer_create() or on linux the much easier to use timerfd_create()

But for single threaded, just putting that if in is enough.

Answer 5

Use alarm setitimer to raise SIGALRM signals at regular intervals.

struct itimerval interval;

void handler( int x ) {
    write( STDOUT_FILENO, ".", 1 ); /* Defined in POSIX, not in C */
}

int main() {
    signal( SIGALRM, &handler );
    interval.it_value.tv_sec = 5; /* display after 5 seconds */
    interval.it_interval.tv_sec = 5; /* then display every 5 seconds */
    setitimer( ITIMER_REAL, &interval, NULL );

    /* do computations */

    interval.it_interval.tv_sec = 0; /* don't display progress any more */
    setitimer( ITIMER_REAL, &interval, NULL );
    printf( "\n" ); /* done with the dots! */
}

Note, only a smattering of functions are OK to call inside handler. They are listed partway down this page. If you want to communicate anything for a fancier printout, do it through a sig_atomic_t variable.