On Mar 28, 2012, at 7:43 AM, Tilghman Lesher <tilghman@meg.abyt.es> wrote:

1/10th of a second is the maximum amount of time each thread is given to execute before the kernel preempts the thread, to allow another thread (or process) to execute. It's heavily ingrained in all Unix-based systems, a somewhat arbitrary decision made in the 1960s by Dennis Ritchie, the father of Unix. There have been attempts by others to modify that value, with results that caused performance to suffer, and the takeaway is that nobody has tried to modify the maximum timeslot since. When you wait in a signal handler longer than the maximum length of a context switch, other processes (and not other threads in the same process) will get multiple timeslots, but the process in which a signal handler sleeps will be stuck, unable to do anything else, because _nothing_ in that process other than other signal handlers (if not blocked) can be invoked while that signal handler is still running.

I don't know much about UNIX history, but Darwin doesn't work this way. When a signal arrives, one thread handles the signal and other threads in that process continue unimpeded. (I'd also expect Darwin to pre-empt threads much faster than 1/10s, and I'd expect "timeslots" to be a wild oversimplification on a multiprocessor system.) % % clang test.c && ./a.out signal second thread still running second thread still running second thread still running second thread still running second thread still running signal done % cat test.c #include <stdio.h> #include <signal.h> #include <unistd.h> #include <pthread.h> void *fn(void *arg) { while (1) { printf("second thread still running\n"); sleep(1); } return arg; } void handler(int arg) { write(STDOUT_FILENO, "signal\n", 7); sleep(5); write(STDOUT_FILENO, "signal done\n", 7+5); } int main() { pthread_t th; pthread_create(&th, NULL, &fn, NULL); signal(SIGUSR1, &handler); kill(getpid(), SIGUSR1); } _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-dev mailing list (Darwin-dev@lists.apple.com) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/darwin-dev/site_archiver%40lists.app... This email sent to site_archiver@lists.apple.com