Re: Missing in sigaction
Re: Missing in sigaction
- Subject: Re: Missing in sigaction
- From: Greg Parker <email@hidden>
- Date: Wed, 28 Mar 2012 10:25:13 -0700
On Mar 28, 2012, at 7:43 AM, Tilghman Lesher <email@hidden> 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);
}
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