From owner-freebsd-hackers@FreeBSD.ORG Thu Jul 12 15:36:17 2012 Return-Path: Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id D77B91065670; Thu, 12 Jul 2012 15:36:17 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from bigwig.baldwin.cx (bigknife-pt.tunnel.tserv9.chi1.ipv6.he.net [IPv6:2001:470:1f10:75::2]) by mx1.freebsd.org (Postfix) with ESMTP id 98E838FC1C; Thu, 12 Jul 2012 15:36:17 +0000 (UTC) Received: from jhbbsd.localnet (unknown [209.249.190.124]) by bigwig.baldwin.cx (Postfix) with ESMTPSA id 04404B94E; Thu, 12 Jul 2012 11:36:17 -0400 (EDT) From: John Baldwin To: Davide Italiano Date: Thu, 12 Jul 2012 11:25:01 -0400 User-Agent: KMail/1.13.5 (FreeBSD/8.2-CBSD-20110714-p17; KDE/4.5.5; amd64; ; ) References: <1342036332.8313.8.camel@albrecht-desktop> <201207121026.37849.jhb@freebsd.org> In-Reply-To: MIME-Version: 1.0 Content-Type: Text/Plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Message-Id: <201207121125.01228.jhb@freebsd.org> X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.2.7 (bigwig.baldwin.cx); Thu, 12 Jul 2012 11:36:17 -0400 (EDT) Cc: Ian Lepore , Paul Albrecht , freebsd-hackers@freebsd.org Subject: Re: kqueue periodic timer confusion X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 12 Jul 2012 15:36:17 -0000 On Thursday, July 12, 2012 11:08:47 am Davide Italiano wrote: > On Thu, Jul 12, 2012 at 4:26 PM, John Baldwin 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 > >> > > > #include > >> > > > #include > >> > > > #include > >> > > > #include > >> > > > #include > >> > > > #include > >> > > > #include > >> > > > > >> > > > 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 > > _______________________________________________ > > freebsd-hackers@freebsd.org mailing list > > http://lists.freebsd.org/mailman/listinfo/freebsd-hackers > > To unsubscribe, send any mail to "freebsd-hackers-unsubscribe@freebsd.org" > > 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. Given that you are reworking callout already, it would seem a bit odd to rework callout a separate time just to fix this bug. A simple fix like this (which follows the same pattern we already use for setitimer()) is something we can easily merge back to 8 and 9. Reworking callout in a different way than you are already doing, OTOH, is not something we can merge trivially. -- John Baldwin