I have a program that creates a TCP server socket and listens on it.
When the program accepts a connection from a client, it calls poll(2) repeatedly to see if there is any input available, like this:
for (int i = 0; i < 1000000; i++) {
struct pollfd fds[] = {{ .fd = clientfd, .events = POLLIN, .revents = 0 }};
int r = poll(fds, 1, 0);
if (r < 0) {
perror("poll");
exit(1);
}
}
On my Intel iMac, this takes about 15 seconds. If I use select(2) instead, as follows, it takes about 550ms.
for (int i = 0; i < 1000000; i++) {
struct timeval timeout = { .tv_sec = 0, .tv_usec = 0 };
fd_set infds;
FD_ZERO(&infds);
FD_SET(clientfd, &infds);
int r = select(1, &infds, 0, 0, &timeout);
if (r < 0) {
perror("select");
exit(1);
}
}
https://gist.github.com/xrme/cd42d3d753ac00861e439e012366f2cf is a C source file that contains a complete example.
If you're inclined to take a look, please download the file from the gist and do cc poll-test.c and then ./a.out. Connect to it with nc 127.0.0.1 6444 and observe that it will take quite a while. (Activity Monitor will show a large number of "idle wake ups", but I'm not clear what that signifies.)
Recompile with cc -DUSE_SELECT, run ./a.out and connect again, and it will complete in under a second.
Advisability of this usage aside (which is adapted from a much larger system), is there anything here to be done to make poll faster? Other systems I have looked at (FreeBSD, OmniOS, Linux) do not exhibit the same slowdown with poll.