Fan Control Success on IBM T40? (another quick Q)

[Kevin Oberman]

> Date: Wed, 28 Apr 2004 13:15:13 -0700 (PDT)
> From: Nate Lawson <nate at root.org>
> Sender: owner-freebsd-acpi at freebsd.org
> 
> I forgot to add -- the reason the clock rate announced at boot time is
> different is because the test for CPU TSC is done before acpi is enabled.
> At some point when the SMI is disabled and acpi enabled, the clock rate is
> switched by your BIOS to the lower rate.  You can see this because the
> clock rate announced by the TSC timecounter will be ~600 mhz while the
> initial boot clock rate will list ~1600 mhz.

This all makes sense, but it conflicts a bit with my
observations. (Probably implies something bad about my powers of
observation.)

I boot and the system (T30) is running at 1.8 GHz. I throttle the CPU.
Testing clearly shows that the throttling is working. I use the test you
suggested of calculating an MD5 hash of a big string of zeros.

But, when I "count cycles" to test the CPU speed (code appended), I
still see 1.8 GHz. 

Why don't I see the speed reduced when throttling? I suspect my lack of
fundamental understanding of the interactions of throttling and the ACPI
clock.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634

/*
 *  $Id: MHz.c,v 1.4 2001/05/21 18:58:01 davej Exp $
 *  This file is part of x86info.
 *  (C) 2001 Dave Jones.
 *
 *  Licensed under the terms of the GNU GPL License version 2.
 *
 * Estimate CPU MHz routine by Andrea Arcangeli <andrea at suse.de>
 * Small changes by David Sterba <sterd9am at ss1000.ms.mff.cuni.cz>
 *
 */

#include <stdio.h>
#include <sys/time.h>
#include <string.h>
#include <unistd.h>

__inline__ unsigned long long int rdtsc()
{
	unsigned long long int x;
	__asm__ volatile (".byte 0x0f, 0x31" : "=A" (x));
	return x;
}

char *estimate_MHz()
{
	char *buffer;
	struct timezone tz;
        struct timeval tvstart, tvstop;
        unsigned long long int cycles[2]; /* gotta be 64 bit */
	unsigned int microseconds; /* total time taken */
	
	memset(&tz, 0, sizeof(tz));

	/* get this function in cached memory */
	gettimeofday(&tvstart, &tz);
	cycles[0] = rdtsc();
	gettimeofday(&tvstart, &tz);

	/* we don't trust that this is any specific length of time */
	usleep(100);
	
	cycles[1] = rdtsc();
	gettimeofday(&tvstop, &tz);
	microseconds = ((tvstop.tv_sec-tvstart.tv_sec)*1000000) +
		(tvstop.tv_usec-tvstart.tv_usec);

	buffer = malloc(sizeof(char)*512);
	sprintf(buffer, "%lldMHz",
		(cycles[1]-cycles[0])/microseconds);

	return buffer;
}