Linux – Using hardware clock on linux

I using an old version of RHEL that I have recently upgraded from 5.4 to 5.8, with a customised 2.30.9 kernel.

The system clock drifts wildly (> 5 seconds/day) unless it is controlled using an external time source such as NTP.

The hardware clock on the BIOS appears to be much more accurate (< 1 second/week).

I have several Linux boxes available, and if I inspect /etc/adjtime I find that the drift is measured as 0.000004, -0.000050, 10.000107 and -0.000234 seconds / day on different machines with identical Hardware, OS & Applications.

We need to minimise the clock drift when the externel time source is not available.

I have tried various kernel boot options with varying success to improve things:

Initially kernel set up line in /boot/grub/grub.conf

kernel /vmlinuz-2.6.30.9 ro root=LABEL=/ pmtr=0x608 acpi_pm_good clocksource=acpi_pm noapic ide_core.noprobe=1.1 mem=512M ramdisk_size=262144

The clocksource files contain:

cat /sys/devices/system/clocksource/clocksource0/available_clocksource
tsc acpi_pm jiffies

cat /sys/devices/system/clocksource/clocksource0/current_clocksource
acpi_pm

This drifts at more than 5 seconds/day!

First I tried to enable the HPET:

kernel /vmlinuz-2.6.30.9 ro root=LABEL=/ pmtr=0x608 acpi_pm_good hpet=enable clocksource=hpet noapic ide_core.noprobe=1.1 mem=512M ramdisk_size=262144

The clocksource files contain:

cat /sys/devices/system/clocksource/clocksource0/available_clocksource
tsc acpi_pm jiffies

cat /sys/devices/system/clocksource/clocksource0/current_clocksource
tsc

This still seems to drift but at only about 4 seconds/day.

Next I tried turning off tsc:
kernel /vmlinuz-2.6.30.9 ro root=LABEL=/ pmtr=0x608 acpi_pm_good notsc divider=10 acpi_pm_good clocksource=acpi_pm noapic ide_core.noprobe=1.1 mem=512M ramdisk_size=262144

The clocksource files contain:

cat /sys/devices/system/clocksource/clocksource0/available_clocksource
tsc acpi_pm jiffies

cat /sys/devices/system/clocksource/clocksource0/current_clocksource
acpi_pm

This has been running for about a day and there is no noticable drift.

My questions are these:

1) According to this website http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1006427 the divider=10 clocksource=acpi_pm requirement was removed at RHEL 5.3, so why does it improve things for RHEL 5.8?

2) What are the effects of the kernel parameters that I have been changing?

3) How accurate is the drift in /etc/adjtime?

4) Is it possible to set up to update hwclock when ntp is working?

5) Is there a better way of achieving what I am trying to do?

thanks