Date: Wed, 29 Jun 2005 04:52:41 -0400 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-hackers@FreeBSD.org Cc: Norbert Koch <NKoch@demig.de> Subject: Re: Posix threads: CLOCK_REALTIME/CLOCK_MONOTONIC Message-ID: <200506290452.42480.jhb@FreeBSD.org> In-Reply-To: <000001c57c80$27dbc6e0$4801a8c0@ws-ew-3.W2KDEMIG> References: <000001c57c80$27dbc6e0$4801a8c0@ws-ew-3.W2KDEMIG>
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On Wednesday 29 June 2005 03:57 am, Norbert Koch wrote: > Hello. > > I am working on a multi-threaded application which > may call settimeofday() and therefore may have > serious problems with timing calculations. > > In my applications I calclulate time differences > using clock_gettime(CLOCK_MONOTONIC) under FreeBSD-5. > Under FreeBSD-4 it is a trivial kernel patch in kern_time.c > to have CLOCK_MONOTONIC, as there already is a kernel function > nanouptime(). > > int > clock_gettime(p, uap) > struct proc *p; > struct clock_gettime_args *uap; > { > struct timespec ats; > > switch (SCARG(uap, clock_id)) { > case CLOCK_REALTIME: > nanotime(&ats); > break; > > case CLOCK_MONOTONIC: > nanouptime(&ats); > break; > > default: > return (EINVAL); > }; > return (copyout(&ats, SCARG(uap, tp), sizeof(ats))); > } > > > > > Looking through the sources of the various threading libraries > I found that either gettimeofday() or clock_gettime(CLOCK_REALTIME) > is used for all calculations. > > I am not sure, what posix currently says about this > but found a chapter 'Condition variable wait clock' in [Butenhof] (p.359). > As I understand it, Posix.1j expects an implementation to > - at least for pthread_cond_timedwait() - use CLOCK_MONOTONIC by default. > They introduce a new function pair pthread_condattr_(get|set)clock() > to change pthread_cond_timedwait() to use either CLOCK_MONOTONIC or > CLOCK_REALTIME. > > >From my understanding of the threading libraries' internals, it > > should be trivial to modify them to using CLOCK_MONOTONIC only, but not > quite as trivial to implement pthread_condattr_(get|set)clock(). > > For FreeBSD-4 I already have a modified libc_r, where I call > clock_gettime(CLOCK_MONOTONIC) once in _thread_init() and set > a global variable _sched_clkid to either CLOCK_MONOTONIC or CLOCK_REALTIME > for further calls to clock_gettime(). > > Any comments/ideas/opinions? You probably want to bring this up on threads@FreeBSD.org as that is where = all=20 the guys who implement the thread libraries hang out. :) =2D-=20 John Baldwin <jhb@FreeBSD.org> =A0<>< =A0http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" =A0=3D =A0http://www.FreeBSD.org
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