Date: Thu, 12 Jul 2012 10:26:37 -0400 From: John Baldwin <jhb@freebsd.org> To: Ian Lepore <freebsd@damnhippie.dyndns.org> Cc: freebsd-hackers@freebsd.org, Paul Albrecht <albrecht@glccom.com> Subject: Re: kqueue periodic timer confusion Message-ID: <201207121026.37849.jhb@freebsd.org> In-Reply-To: <1342101436.1123.52.camel@revolution.hippie.lan> References: <1342036332.8313.8.camel@albrecht-desktop> <201207120834.40745.jhb@freebsd.org> <1342101436.1123.52.camel@revolution.hippie.lan>
next in thread | previous in thread | raw e-mail | index | archive | help
On Thursday, July 12, 2012 9:57:16 am Ian Lepore wrote:
> On Thu, 2012-07-12 at 08:34 -0400, John Baldwin wrote:
> > On Wednesday, July 11, 2012 5:00:47 pm Ian Lepore wrote:
> > > On Wed, 2012-07-11 at 14:52 -0500, Paul Albrecht wrote:
> > > > Hi,
> > > >
> > > > Sorry about this repost but I'm confused about the responses I received
> > > > in my last post so I'm looking for some clarification.
> > > >
> > > > Specifically, I though I could use the kqueue timer as essentially a
> > > > "drop in" replacement for linuxfd_create/read, but was surprised that
> > > > the accuracy of the kqueue timer is much less than what I need for my
> > > > application.
> > > >
> > > > So my confusion at this point is whether this is consider to be a bug or
> > > > "feature"?
> > > >
> > > > Here's some test code if you want to verify the problem:
> > > >
> > > > #include <stdio.h>
> > > > #include <stdlib.h>
> > > > #include <string.h>
> > > > #include <unistd.h>
> > > > #include <errno.h>
> > > > #include <sys/types.h>
> > > > #include <sys/event.h>
> > > > #include <sys/time.h>
> > > >
> > > > int
> > > > main(void)
> > > > {
> > > > int i,msec;
> > > > int kq,nev;
> > > > struct kevent inqueue;
> > > > struct kevent outqueue;
> > > > struct timeval start,end;
> > > >
> > > > if ((kq = kqueue()) == -1) {
> > > > fprintf(stderr, "kqueue error!? errno = %s",
> > strerror(errno));
> > > > exit(EXIT_FAILURE);
> > > > }
> > > > EV_SET(&inqueue, 1, EVFILT_TIMER, EV_ADD | EV_ENABLE, 0, 20, 0);
> > > >
> > > > gettimeofday(&start, 0);
> > > > for (i = 0; i < 50; i++) {
> > > > if ((nev = kevent(kq, &inqueue, 1, &outqueue, 1, NULL)) ==
> > -1) {
> > > > fprintf(stderr, "kevent error!? errno = %s",
> > strerror(errno));
> > > > exit(EXIT_FAILURE);
> > > > } else if (outqueue.flags & EV_ERROR) {
> > > > fprintf(stderr, "EV_ERROR: %s\n",
> > strerror(outqueue.data));
> > > > exit(EXIT_FAILURE);
> > > > }
> > > > }
> > > > gettimeofday(&end, 0);
> > > >
> > > > msec = ((end.tv_sec - start.tv_sec) * 1000) + (((1000000 +
> > end.tv_usec - start.tv_usec) / 1000) - 1000);
> > > >
> > > > printf("msec = %d\n", msec);
> > > >
> > > > close(kq);
> > > > return EXIT_SUCCESS;
> > > > }
> > > >
> > > >
> > >
> > > What you are seeing is "just the way FreeBSD currently works."
> > >
> > > Sleeping (in most all of its various forms, and I've just looked at the
> > > kevent code to verify this is true there) is handled by converting the
> > > amount of time to sleep (usually specified in a timeval or timespec
> > > struct) to a count of timer ticks, using an internal routine called
> > > tvtohz() in kern/kern_time.c. That routine rounds up by one tick to
> > > account for the current tick. Whether that's a good idea or not (it
> > > probably was once, and probably not anymore) it's how things currently
> > > work, and could explain the fairly consistant +1ms you're seeing.
> >
> > This is all true, but mostly irrelevant for his case. EVFILT_TIMER
> > installs a periodic callout that executes KNOTE() and then resets itself (via
> > callout_reset()) each time it runs. This should generally be closer to
> > regulary spaced intervals than something that does:
> >
>
> In what way is it irrelevant? That is, what did I miss? It appears to
> me that the next callout is scheduled by calling timertoticks() passing
> a count of milliseconds, that count is converted to a struct timeval and
> passed to tvtohz() which is where the +1 adjustment happens. If you ask
> for 20ms and each tick is 1ms, then you'd get regular spacing of 21ms.
> There is some time, likely a small number of microseconds, that you've
> consumed of the current tick, and that's what the +1 in tvtohz() is
> supposed to account for according to the comments.
>
> The tvtohz() routine both rounds up in the usual way (value+tick-1)/tick
> and then adds one tick on top of that. That seems not quite right to
> me, except that it is a way to g'tee that you don't return early, and
> that is the one promise made by sleep routines on any OS; those magical
> "at least" words always appear in the docs.
>
> Actually what I'm missing (that I know of) is how the scheduler works.
> Maybe the +1 adjustment to account for the fraction of the current tick
> you've already consumed is the right thing to do, even when that
> fraction is 1uS or less of a 1mS tick. That would depend on scheduler
> behavior that I know nothing about.
Ohhhhh. My bad, sorry. You are correct. It is a bug to use +1 in this
case. That is, the +1 makes sense when you are computing a one-time delta
for things like nanosleep(). It is incorrect when computing a periodic
delta such as for computing the interval for an itimer (setitimer) or
EVFILT_TIMER().
Hah, setitimer()'s callout (realitexpire) uses tvtohz - 1:
sys/kern/kern_time.c:
/*
* Real interval timer expired:
* send process whose timer expired an alarm signal.
* If time is not set up to reload, then just return.
* Else compute next time timer should go off which is > current time.
* This is where delay in processing this timeout causes multiple
* SIGALRM calls to be compressed into one.
* tvtohz() always adds 1 to allow for the time until the next clock
* interrupt being strictly less than 1 clock tick, but we don't want
* that here since we want to appear to be in sync with the clock
* interrupt even when we're delayed.
*/
void
realitexpire(void *arg)
{
struct proc *p;
struct timeval ctv, ntv;
p = (struct proc *)arg;
PROC_LOCK(p);
kern_psignal(p, SIGALRM);
if (!timevalisset(&p->p_realtimer.it_interval)) {
timevalclear(&p->p_realtimer.it_value);
if (p->p_flag & P_WEXIT)
wakeup(&p->p_itcallout);
PROC_UNLOCK(p);
return;
}
for (;;) {
timevaladd(&p->p_realtimer.it_value,
&p->p_realtimer.it_interval);
getmicrouptime(&ctv);
if (timevalcmp(&p->p_realtimer.it_value, &ctv, >)) {
ntv = p->p_realtimer.it_value;
timevalsub(&ntv, &ctv);
callout_reset(&p->p_itcallout, tvtohz(&ntv) - 1,
realitexpire, p);
PROC_UNLOCK(p);
return;
}
}
/*NOTREACHED*/
}
Paul, try this patch for sys/kern/kern_event.c. It uses the same approach as
seitimer() above:
Index: kern_event.c
===================================================================
--- kern_event.c (revision 238365)
+++ kern_event.c (working copy)
@@ -538,7 +538,7 @@ filt_timerexpire(void *knx)
if ((kn->kn_flags & EV_ONESHOT) != EV_ONESHOT) {
calloutp = (struct callout *)kn->kn_hook;
- callout_reset_curcpu(calloutp, timertoticks(kn->kn_sdata),
+ callout_reset_curcpu(calloutp, timertoticks(kn->kn_sdata) - 1,
filt_timerexpire, kn);
}
}
--
John Baldwin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?201207121026.37849.jhb>