Re: severely bad timekeeping

[Michael <macallan%netbsd.org@localhost>]

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Hello,

On Apr 17, 2009, at 12:53 AM, der Mouse wrote:

> The machine started life as a 7600/132 - for example, that's what's
> printed on the case - but (I'm told) it's got a G3 upgrade in it.
> dmesg.boot describes the CPU thus (I am not comptent to say whether
> this means it really is a G3 or not, which is why I put it that way):
>
> cpu0 at mainbus0: 750 (Revision 2.2), ID 0 (primary)
> cpu0: HID0 8090c0a4<EMCP,DOZE,DPM,ICE,DCE,SGE,BTIC,BHT>, powersave: 1
> cpu0: 305.89 MHz L2 cache present but not enabled

It's a G3 alright but the firmware didn't enable its L2 cache.

> The time it keeps is wrong by almost a percent - it runs approximately
> half a second per minute slow.  ntp fails to sync under these
> circumstances (yes, I did check for an old ntp.drift file; this is
> being tested after destroying ntp.drift).  Killing ntpd and running
> ntpdate -b once a minute produces matching results (a correction of
> about .48 seconds each time around).

The CPU's timer is fed by a separate clock input, the frequency of  
which depends on the hardware and w merely believe what OpenFirmware  
tells us about it. The time keeping code ( powerpc/clock.c IIRC )  
takes precautions against missing clock interrupts, you'd have to lose  
a hell of a lot of them to actually lose time. So I guess the time  
base frequency isn't quite right on your machine. There used to be a  
way to override it by hand but I'm not 100% sure it's still there.

> In passing, what would I need to do to figure out what L2CR_CONFIG
> settings I want for this CPU?  "L2 cache present but not enabled"
> sounds like a performance killer, and the macppc FAQ points to list
> mail indicating that setting L2CR_CONFIG is the right magic, but
> doesn't give much guidance on what to set it to.

It definitely kills performance.
You'll need to find out what size your cache is - 1MB at 1/2 CPU clock  
is very common with 300MHz upgrades.

have fun
Michael

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