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    SubjectPIT, TSC and power management [was: Re: 2.6.0-test3 "loosing ticks"]
    john stultz wrote:
    > On Thu, 2003-08-14 at 10:17, Jamie Lokier wrote:
    >>john stultz wrote:
    >>>Sounds like either your PIT is running slowly or something is
    >>>consistently keeping the timer interrupt from being handled. In 2.4 do
    >>>you have any time related issues at all? Does the "Loosing too many
    >>>ticks!" message correlate to any event on the system (boot, heavy load)?
    >>>Also listing system type, motherboard, any sort of funky devices you've
    >>>got might be helpful.
    >>I am seeing something similar on my dual Athlon MP 1800 box.
    >>It is running NTP to synchronise with another machine over the LAN,
    >>but ntpdc reports that it develops a larger and larger offset relative
    >>to the server - ntpd clearly is not managing to regulate the clock.

    I also see the time offset problem (Athlon MP 2000+ x2, Tyan S2460 m/b,
    2.6.0-test{1,2,3}) but it is most noticeable when I have amd76x_pm
    installed (it's not in 2.6.x yet, but a late 2.5.x patch was posted to
    LKML a little while back).

    amd76x_pm is roughly equivalent to ACPI C2 idling, but since my BIOS
    doesn't export any C-state functionality to the kernel ACPI code, I am
    stuck with letting amd76x_pm frob the chipset registers. A quick look at
    AMD's datasheets does not indicate that a return from C2 should cause
    much delay at all-- if I understand the timing requirements correctly,
    it would have to sit for more than 1 ms to miss more than one interrupt.
    That said, I don't see any missing interrupts indicated in
    /proc/interrupts, nor do any such messages appear in the kernel logs.

    Brings up another question: does the "try HZ=100" suggestion still apply
    for these faster machines? I would think that if HZ=1000 is too fast,
    then at least an occasional lost interrupt would be logged.

    When using the TSC for time-of-day, I generally have to set tick to
    10200 or somewhere thereabouts. ntpd usually gives up after a few hours,
    though, so I presume that this value for tick is only good for a certain
    combination of processor load and planetary alignment.

    I booted with clock=pit to test that, and now I need tick=9963
    (according to adjtimex's configuration routine). However, that makes the
    clock jump all over the place, with ntpd making step adjustments +/- 2
    seconds every 5 minutes.

    > Approximately at what rate does it skew?

    Well, it's not constant, and I don't trust the tick values given above,
    since they don't seem to hold true for long.

    > Does ntpdate -b <server> set it properly?

    I'm confused. Are there cases where a step time adjustment would fail?
    Is there a possibility that the kernel is rejecting ntpd's step
    adjustments? (I presume that these use the same as 'ntpdate -b';
    specifically, the time is not slewed.)

    > Are you also seeing the "Loosing too many ticks!" message?

    Never seen it.

    Other miscellaneous info:

    > Enabling APIC mode: Flat. Using 1 I/O APICs
    > CPU: CLK_CTL MSR was 6003d22f. Reprogramming to 2003d22f

    (does this have anything to do with the TSC?)

    > Using local APIC timer interrupts.
    > calibrating APIC timer ...
    > ..... CPU clock speed is 1666.0503 MHz.
    > ..... host bus clock speed is 266.0640 MHz.
    > checking TSC synchronization across 2 CPUs: passed.

    (note this still appears when using clock=pit)


    00:00.0 Host bridge: Advanced Micro Devices [AMD] AMD-760 MP [IGD4-2P]
    System Controller (rev 11)
    00:01.0 PCI bridge: Advanced Micro Devices [AMD] AMD-760 MP [IGD4-2P]
    AGP Bridge
    00:07.0 ISA bridge: Advanced Micro Devices [AMD] AMD-766 [ViperPlus] ISA
    (rev 02)
    00:07.1 IDE interface: Advanced Micro Devices [AMD] AMD-766 [ViperPlus]
    IDE (rev 01)
    00:07.3 Bridge: Advanced Micro Devices [AMD] AMD-766 [ViperPlus] ACPI
    (rev 01)

    CPU-selection portions of .config:


    (rest available on request)

    I am open to suggestions for testing.

    Also, how much has the kernel changed with respect to the PLL used by ntpd?


    Charles Lepple <!clepple>

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