Date: Thu, 12 Jul 2012 17:08:47 +0200 From: Davide Italiano <davide@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: Ian Lepore <freebsd@damnhippie.dyndns.org>, Paul Albrecht <albrecht@glccom.com>, freebsd-hackers@freebsd.org Subject: Re: kqueue periodic timer confusion Message-ID: <CACYV=-G8Hip5-8G6Cyjw%2BGmrW3TvZMxZrKvEv7cUXuRXWyHwJw@mail.gmail.com> In-Reply-To: <201207121026.37849.jhb@freebsd.org> References: <1342036332.8313.8.camel@albrecht-desktop> <201207120834.40745.jhb@freebsd.org> <1342101436.1123.52.camel@revolution.hippie.lan> <201207121026.37849.jhb@freebsd.org>
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On Thu, Jul 12, 2012 at 4:26 PM, John Baldwin <jhb@freebsd.org> wrote:
> 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
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John,
I don't think it's good to decrease by a unit the 'ticks' you pass to
callout_reset_* KPI.
If this have to be fixed it should be fixed at the callout level and
not at the consumer level. In other words, subsystems that makes use
of callout_reset_* should not deal with the inherent limitations of
callout precision, as it is right now.
Davide
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