Detecting variable change as soon as possible - c

Question

First of all, this has to do with homework. Just a little hint would be enough.
What I have to do is to detect when a variable(signal) has changed and announce it in 1 microsecond or less. My progress so far:
int main(int argc, char **argv)
{
int i;
N = atoi(argv[1]);
if (argc != 2) {
printf("Usage: %s N\n"
" where\n"
" N : number of signals to monitor\n"
, argv[0]);
return (1);
}
// set a timed signal to terminate the program
signal(SIGALRM, exitfunc);
alarm(20); // after 20 sec
// Allocate signal, time-stamp arrays and thread handles
signalArray = (int *) malloc(N*sizeof(int));
timeStamp = (struct timeval *) malloc(N*sizeof(struct timeval));
pthread_t sigGen;
pthread_t *sigDet = (pthread_t*) malloc(N * sizeof(pthread_t));
long *signalid = (long*) malloc(N * sizeof(long));
for (i=0; i<N; i++) {
signalArray[i] = 0;
}
for (i = 0; i < N; i++)
{
signalid[i] = (long) i;
pthread_create (&sigDet[i], NULL, ChangeDetector, (void*) signalid[i]);
}
pthread_create (&sigGen, NULL, SensorSignalReader, NULL);
// wait here until the signal
for (i = 0; i < N; i++)
{
pthread_join (sigDet[i], NULL);
}
return 0;
}
void *SensorSignalReader (void *arg)
{
char buffer[30];
struct timeval tv;
time_t curtime;
srand(time(NULL));
while (1) {
int t = rand() % 10 + 1; // wait up to 1 sec in 10ths
usleep(t*100000);
int r = rand() % N;
signalArray[r] ^= 1;
if (signalArray[r]) {
gettimeofday(&tv, NULL);
timeStamp[r] = tv;
curtime = tv.tv_sec;
strftime(buffer,30,"%d-%m-%Y %T.",localtime(&curtime));
printf("Changed %5d at Time %s%ld\n",r,buffer,tv.tv_usec);
}
}
}
void *ChangeDetector (void *arg)
{
char buffer[30];
struct timeval tv;
time_t curtime;
long n = (long) arg;
while (1) {
while (signalArray[n] == 0) {}
pthread_mutex_lock(&mutex);
gettimeofday(&tv, NULL);
curtime = tv.tv_sec;
strftime(buffer,30,"%d-%m-%Y %T.",localtime(&curtime));
printf("Detcted %5ld at Time %s%ld after %ld.%06ld sec\n", n, buffer,tv.tv_usec,
tv.tv_sec - timeStamp[n].tv_sec,
tv.tv_usec - timeStamp[n].tv_usec);
pthread_mutex_unlock(&mutex);
while (signalArray[n] == 1) {}
}
}
With this implementation my program achieves to detect up to 3 signals in time <= 1 us. When more than 3 the response delay up to few ms. How could I detect even more signals in that time? I was wondering how thread tasks are allocated in CPU cores? I've read that it's too painful to manage what is executed on each core by code? Would I earn something with that effort?

Answer 1

Your ChangeDetector() relies on busy-waiting on the flag. The problem is that you can't have more processes simultaneously busy-waiting than you have CPUs - only some subset of your ChangeDetector() processes are actually going to be running at any point in time.
This means that quite often, you have to wait for the right ChangeDetector thread to be scheduled back onto the CPU before it can run and notice that the flag has changed.
If you want to have one ChangeDetector thread for every flag, then you will need to use a non-busy-waiting method, like pthread condition variables (you can have one mutex / condition variable pair per flag). I am not sure if you will be able to get sub-microsecond latency this way, though.
If you want really want to stick with the busy-waiting method, you'll need to limit your number of ChangeDetector threads to less than the number of CPUs, by having each thread responsible for checking multiple array locations in every loop.