Date: Thu, 9 Jul 2009 20:02:09 +0200 From: =?ISO-8859-1?Q?Marius_N=FCnnerich?= <marius@nuenneri.ch> To: Andriy Gapon <avg@freebsd.org> Cc: freebsd-current@freebsd.org Subject: Re: dtrace users opinion solicited (timestamps) Message-ID: <b649e5e0907091102h2bbf3799r4f8b840696a9162b@mail.gmail.com> In-Reply-To: <4A562960.3010801@freebsd.org> References: <4A562960.3010801@freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Thu, Jul 9, 2009 at 19:31, Andriy Gapon<avg@freebsd.org> wrote: > > As you might be aware DTrace timestamps right now are derived from TSC value. > http://en.wikipedia.org/wiki/Time_Stamp_Counter > > DTrace timestamps are measured in nano-seconds and the formula similar to the > following is used for calculations: > rdtsc() * 1000000000 / tsc_freq > where rdtsc is a function that returns current TSC value and tsc_freq is a > frequency of TSC. > > This formula is supposed to produce proper results if tsc_freq stays constant. > But there are environment where this might not be the case. > If a CPU has a non-invariant TSC and processor's clock frequency changes (e.g. > because of powerd), then tsc_freq changes too. > As a result, the formula would produce wildly different values and, most > importantly, was values would non be monotonic. Timestamp values that jump back > and forth would not only be useless for a user, they would also confuse DTrace > internal logic. > > There are at least the following two alternatives: > > 1. Keep things as they are and warn users not to change CPU clock frequency when > they use DTrace and the CPU doesn't have invariant TSC. I think that this should > cause only minor inconveniences to a portion of DTrace users. > > 2. Use raw TSC value as a DTrace timestamp and document this difference from the > original DTrace. Advantage: timestamp value is always monotonic. Disadvantage: > manual conversion is needed to get "real" time (using the same formula). > Please note that in this case timestamps would be in non-linear time dimension if > TSC frequency changes, so to get meaningful timestamps (when needed/important) one > would still have to make sure that TSC frequency stay constant. > > Please share your opinion on these approaches. > Or suggest yest another alternative. What about atomically changing tsc_freq every time the frequency is changed? > > Just in case, related sysctls: > machdep.tsc_freq > kern.timecounter.invariant_tsc
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?b649e5e0907091102h2bbf3799r4f8b840696a9162b>