Re: System time slowdown after upgrading from 2.4 to 2.6

[john stultz <johnstul@us.ibm.com>]

On Thu, 2004-08-19 at 12:42, Ivan Kalatchev wrote:
> We have a project that uses PC-104 (ZFx86 100 MHz CPU) Linux box to acquire
> some seismological signals. System resides on a M-Systems DiskOnChip 2000.
> We have an ISA acquisition card that acquires data into FIFO buffer and
> triggers interrupt as soon as 341 points were acquired (then this data is
> read out in interrupt handler routine). System worked perfectly well when we
> used 2.4.18 kernel. When I printk-ed system time (do_gettimeofday)  in
> interrupt routine, at 1kHz sampling frequency interrupts were reported every
> 341 msec as they should do.
> But after I've switched to 2.6 kernel (2.6.7 - preempted, then I tried
> 2.6.8 - non-preempted) time between successive interrupts at 1kHz became 335
> msec, losing 6 msec at each interrupt. What is interesting, when sampling
> frequency is 10 kHz, with 2.4.18 kernel interrupts are reported every 34
> msec, which is right, but with 2.6 kernels interrupts are coming at 28 msec
> interval, again losing same 6 msec!
> 
> Any ideas why this might happen?

Hmm. Strange. Just to remove some variables, are you running NTP on the
system? Could you also send your 2.6 dmesg?

thanks
-john