Date: Thu, 9 Jul 2009 15:39:32 -0400 From: David Schultz <das@FreeBSD.ORG> To: Andriy Gapon <avg@FreeBSD.ORG> Cc: freebsd-current@FreeBSD.ORG Subject: Re: dtrace users opinion solicited (timestamps) Message-ID: <20090709193932.GA54408@zim.MIT.EDU> In-Reply-To: <4A562960.3010801@freebsd.org> References: <4A562960.3010801@freebsd.org>
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On Thu, Jul 09, 2009, Andriy Gapon 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. Doesn't Solaris DTRT and compensate for TSC frequency changes? Why can't we do the same thing?
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