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    SubjectRe: 2.6.1: process start times by procps
    Thanks for answering,

    On Wed, Jan 28, 2004 at 06:21:52PM -0800, john stultz wrote:
    > On Tue, 2004-01-27 at 07:52, Petri Kaukasoina wrote:
    > > I made an experiment shown below. I know nothing about kernel programming,
    > > so this is probably not correct, but at least btime seemed to stay constant.
    > > (I don't believe this fixes procps, though. If HZ if off by 180 ppm then I
    > > guess ps can't possibly get its calculations involving HZ right. But at
    > > least the bootup time reported by procinfo stays constant.)
    >
    >
    > Uh, what does your /etc/ntp/drift file show?

    ntp.drift is -22.251 on linux-2.6.1. But on linux-2.4.24 it stabilizes at
    about -5.4.

    > The basic equation is:
    > btime ~= gettimeodfay() - uptime
    >
    > Thus if your time of day is adjusted by NTP, btime will change as well.
    > Uptime is calculated calculated by jiffies/HZ, and HZ is not NTP
    > adjusted, so if your system is running 180ppm too fast or slow, btime
    > would be expected to change.
    >

    Yes, on linux-2.2.24 I can see that /proc/uptime is just the jiffies and
    btime is current time - jiffies. But in linux-2.6.1 /proc/uptime is now
    do_posix_clock_monotonic_gettime(), whatever that means, and /proc/uptime
    gives a correct value. But btime is still gettimeofday-jiffies and it does
    not stay constant. My patch changed btime to be
    gettimeofday-do_posix_clock_monotonic_gettime() and after that it stays
    constant.

    In other words, on linux-2.2.24, if I print the current time with 'date +%s'
    and subtract uptime (/proc/uptime) from that, I do get the same number that
    is in the btime field in /proc/stat. But not on linux-2.6.1 without my
    patch. (ntp running in both cases.)

    > I'm not yet sure how that is related to the ps start time being wrong.

    I guess it's not. The relative error was just the same in both. On
    linux-2.2.24, ps start time is correct but on linux-2.6.1 it shows times in
    future. How much in future, about minute per four days of uptime. But if I
    multiply Hertz by e.g. 1.000172 in the ps source, then I get the right
    results on linux-2.6.1.

    I'll do an experiment and boot to linux-2.6.1 without ntpd next...

    Thanks,

    -Petri
    -
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