From owner-freebsd-alpha  Tue May  1  2:56:42 2001
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To: freebsd-alpha@FreeBSD.ORG
Subject: Re: determine cycle counter frequency in user space
References: <200104302147.OAA12404@usr01.primenet.com>
From: Rolf Neugebauer <neugebar@dcs.gla.ac.uk>
Date: 01 May 2001 10:56:27 +0100
In-Reply-To: Terry Lambert's message of "Mon, 30 Apr 2001 21:47:48 +0000 (GMT)"
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Terry Lambert <tlambert@primenet.com> writes:

> > I try to determine the cycle counter frequency from a user space
> > program to do some accurate time counting. Ideally, I would like to
> > access the rpb_cc_freq field Hardware Restart Parameter Block (rpb) as
> > possible under OSF/1 using getsysinfo(2) or the calibrated
> > cycles_per_sec variable from sys/alpha/alpha/machdep.c.
> > 
> > While it would be easy to introduce a new sysctl or similar, I was
> > wondering if there is already a 'standard' way under FreeBSD to access
> > this information (ideally, also under ix86).
> > 
> > I searched around a bit, but couldn't find any obvious way.
> 
> The closest you will find is hw.cpu_mhz, via sysctl.

Not on my 4.2-STABLE, late February vintage. I could parse
hw.model, but that reports 533MHz, while dmesg reports 531MHz.

In a followup, Mike mentioned fluctuations of clock frequency. Any
idea, by how much this fluctuation can be? percentage of MHz, kHz?

In general, I am not too fussed about these fluctuations, as I can
take some fine grained samples using cycle counters together with more
coarse grained once obtained from gettimeofday().

However, to get an estimate for cycles per second, I was looking for a
programmatic way to access above mentioned variables instead of
providing them as command line arguments. I'll just add an appropriate
sysctl to get cycles_per_sec. If people are interested, I'll submit a
patch.

Thanks
Rolf


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