clocks realtime - gpos running at different speed

Theo Veenker <[email protected]> Mon, 20 Sep 2004 13:11:10 +0200

Newsgroups	gmane.linux.real-time.rtlinux.general
Message-ID	<[email protected]>

Hi all,

This issue has come up before, but I still don't know how to fix it.

On our systems (Asus P4B533/P4PE, RH9, 2.4.20, 3.2-pre2) CLOCK_GPOS
runs about 150us/s faster than CLOCK_REALTIME (I have used attached
module to test this).

Every night the machines call ntpdate <server> to adjust the system
time and call hwclock --systohc to set the hardware clock.

150us/s is about 13 seconds in 24 hours. And in /var/log/messages
I always see ntpdate applying a time step by -13 seconds. So from
this I conclude that ntpdate also thinks the system time goes too
fast. Haven't tested yet if I get the same time step when running
a regular kernel.

If it is a bug in the BIOS or motherboard, is there anyone who had
the same problem and solved it?

Theo

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clktst.c (text/x-csrc, 1.4 KB)

/*
 * clktst.c
 *
 * Gets CLOCK_REALTIME and CLOCK_GPOS once every second and prints
 * the difference between the offsets between the two clock values.
 * If the clocks run at the same pace this difference should be zero
 * on average. On my system CLOCK_GPOS runs 150us/s faster then 
 * CLOCK_REALTIME.
 *
 * Redhat Linux 9, kernel 2.4.20, RTL 3.2-pre2
 * Same results for Asus P4B533 and AsusP4PE motherboard.
 *
 */

#include <rtl.h>
#include <posix/time.h>
#include <pthread.h>

static pthread_t mythread;

#define CLOCK1  CLOCK_REALTIME
#define CLOCK2  CLOCK_GPOS
//#define CLOCK2  CLOCK_MONOTONIC

static void* testTask(void* arg)
{
    hrtime_t realtime, gpostime, offset, prevoffset, diff;
    int n;

    pthread_make_periodic_np(pthread_self(), 
	clock_gethrtime(CLOCK_RTL_SCHED), 1000000000);

    realtime = clock_gethrtime(CLOCK1);
    gpostime = clock_gethrtime(CLOCK2);
    prevoffset = offset = gpostime - realtime;
    diff = 0;

    for (n = 0; n < 60; n++) {
    	pthread_wait_np();
    	realtime = clock_gethrtime(CLOCK1);
    	gpostime = clock_gethrtime(CLOCK2);
    	offset = gpostime - realtime;
	diff = offset - prevoffset;
	prevoffset = offset;
	rtl_printf("%d: %lld\n", n, diff);
    }

    return 0;
}

int init_module(void)
{
    pthread_create(&mythread, NULL, testTask, NULL);

    return 0;
}

void cleanup_module(void)
{
    pthread_cancel(mythread);
    pthread_join(mythread, NULL);
}

Makefile.clktst (text/plain, 262 B)

include /usr/rtlinux/rtl.mk

                   CC = gcc

all: clktst.o

clean:
	$(RM) clktst.o

load:
	rtlinux start clktst

unload:
	rtlinux stop clktst

reload:
	-/sbin/rmmod clktst
	/sbin/insmod clktst.o

clktst.o: clktst.c
	$(CC) -c $(CFLAGS) $(INCLUDE) $<