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    SubjectRe: boot time, process start time, and NOW time
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    On Mon, 2004-08-16 at 23:56, Tim Schmielau wrote:
    > On Tue, 16 Aug 2004, Albert Cahalan wrote:
    >
    > > If you're interested in reducing (not solving)
    > > the problem for the 2.6.x series, you might change
    > > HZ to something that works better with the PIT.
    >
    > No, that's not needed anymore. We've already started to account for the
    > difference, e.g. by using TICK_NSEC in jiffies64_to_clock_t().

    Well, unfortunately TICK_NSEC just gives the *current* tick length as
    requested by ntpd. So it won't work over an long interval of jiffies
    where TICK_NSEC might have changed.

    > Problem is, we are only halfway through the attempt to remove the use
    > of "jiffies" as a clock, so currently to incompatible time sources get
    > mixed
    > up.
    >
    > The other problem seems to be that this move away from "jiffies" seems to
    > happen on an ad-hoc basis whenever we encounter a problem, rather than
    > with a big picture in mind.

    Indeed you are correct. Since timer interrupts are not precisely or
    accurately delivered, a timer interrupt counter (jiffies), cannot be
    used as a reliable time source (except where there is not other time
    source). The problem is that it is difficult to discern where jiffies is
    just being used as a timer subsystem counter, or where its being used as
    a time of day time source.

    Even worse, this is a userspace visible usage of jiffies as a time
    stamp, so in this case we have to preserve the interface and find a way
    to emulate it. So proc_pid_stats() may need to do the reverse of what
    procps is doing.

    > John Stultz once laid out a concept for a (coordinated) rewrite in 2.7,
    > and I think this still is a good idea.

    Yep, I've been working like crazy on just this (well, when my work isn't
    swamping me). Unfortunately it is a major overhaul and causes cascading
    changes (removal of xtime), so its not going as quickly as I'd like.
    However I feel the design is quite good and I will attach a copy of it
    and the first pass of the code in a reply to this email.

    thanks
    -john

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