Messages in this thread | | | Date | Tue, 11 Jan 2005 15:58:37 +0100 (MET) | From | Jan Engelhardt <> | Subject | Re: New Linux System time proposal |
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> I think that Linux time should be re-thought and done over once and > for all. I think that time should be the sum of three variables:
It has been rethought a number of times, the last one being the switch to a 64-bit jiffy counter.
> unsigned boot_time Read from RTC upon boot and never changed. > signed delta_time Starts at 0, is adjusted as required. > unsigned jiffies Starts at 0 upon boot, bumped by tick only. > > Any time there is a requirement to obtain the system time, these > three values are summed. At the time that occurs, the current value > of jiffies is read and compared against a previously-saved value. > The new value is saved in the old value after. If the new jiffies > is less than the old, delta_time is adjusted accordingly. > > What this means is that jiffies can remain a 32-bit variable and > one only needs to read the system time once in awhile (before > it wraps twice) to keep everything synchronized.
jiffies is both 32 and 64 at the same time (using some linker magic).
> It also means that hardware time-outs don't get disrupted when > the system clock gets set because jiffies is never touched > except by the timer-tick interrupt. > > Also, time doesn't get destroyed in the process of reading it. > A simple user-mode daemon can periodically check certain hardware > timers (like the RTC), and adjust delta_time accordingly (and
One can do rdtsc() in kernelspace.
> slowly, like a PLL). A hook can be provided so that the > delta_time variable can be tweaked by a user-mode code that > synchronizes to time-servers as well. As a side-effect, this > daemon does the "before jiffies wraps" read to keep everything > in sync. > > The actual size of the variables boot_time and delta_time can > be determined by the implementor with due consideration for > the resolution required.
I don't see a change necessary unless I'm a physics professor experimenting with atomic clocks.
Jan Engelhardt -- ENOSPC - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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