[ntp:questions] linux system clock egregiously slow!

David Wuertele dave-gnus at bfnet.com
Wed Jan 21 00:29:07 UTC 2004

I realize this may be off-topic, but I figured the time-heads on this
list might be better prepared to point me in the right direction than
on the linux kernel lists...

I run linux 2.4.18 on an embedded MIPS little-endian CPU, but I'm
getting distressingly poor system clock performance.  My CPU clock is
supposed to be good to about 30 PPM, but my system time is always slow
to the tune of 800 PPM!

I do a lot of PIO, so I wonder if it is possible that the PIO makes
the kernel lose timer interrupts?  I notice that my timer is implemented
as a countdown: in arch/mips/kernel/time.c:timer_interrupt(), it says:

 * high-level timer interrupt service routines.  This function
 * is set as irqaction->handler and is invoked through do_IRQ.
void timer_interrupt(int irq, void *dev_id, struct pt_regs *regs)
	if (mips_cpu.options & MIPS_CPU_COUNTER) {
		unsigned int count;

		 * The cycle counter is only 32 bit which is good for about
		 * a minute at current count rates of upto 150MHz or so.
		count = read_32bit_cp0_register(CP0_COUNT);
		timerhi += (count < timerlo);   /* Wrap around */
		timerlo = count;

		 * set up for next timer interrupt - no harm if the machine
		 * is using another timer interrupt source.
		 * Note that writing to COMPARE register clears the interrupt
		write_32bit_cp0_register (CP0_COMPARE,
					  count + cycles_per_jiffy);


Does it make sense that the average latency to the
write_32bit_cp0_register() command would be .8ms?

I understand that I can use ntpd to correct this problem, but 800PPM
seems egreggiously bad to me.  Any suggestions are welcome!


More information about the questions mailing list