Recent Problems with RELENG_7 i386

[Jeremy Chadwick]

On Fri, Oct 10, 2008 at 03:51:02AM +1100, Ian Smith wrote:
> I see bf has since posted these values, but I'd already clipped stuff 
> from the original post with kernel config and verbose dmesg, already 
> wondering why the two didn't match, like:
> 
>  | options         HZ=1000
>  | options         DEVICE_POLLING
> 
> but
> 
>  | CPU: AMD Athlon(tm) Processor (906.35-MHz 686-class CPU)
>  | Timecounter "ACPI-safe" frequency 3579545 Hz quality 850
>  | inittimecounter(0)... Timecounters tick every 10.000 msec
> 
> ie HZ=100, as mentioned, and using ACPI-safe as later confirmed.  So 
> it's either a different kernel or bf updated kern.hz from loader.conf?

Yep -- his original mail had loader.conf shown, with this in it (near the
bottom):

kern.hz=100

>  > Well, I believe HZ was increased from 100 to 1000 long ago (RELENG_6?)
>  > as a default.  I'm really not sure of the implications of decreasing it,
>  > besides having less granularity for some things (the only things I know
>  > of would be something pertaining to firewalls, I just can't remember
>  > what.  My brain is full.  :-) )
> 
> You need a day off :)  But yes, RELENG_5 still had HZ=100 default, long 
> after the 'average' CPU clock frequency was 10 or more times faster than 
> the 166MHz Pentiums and such (mostly then on only 100Mbps ethernet) that 
> were comfortable at 100Hz slicing.  1000Hz was a big shift to catch up.
> 
> In a day or so playing around with it years ago, I found 200-250Hz good 
> for 300MHz, 500Hz a bit much, 1000Hz way too busy, and find my 1133MHz 
> P3-M happy enough at 1000Hz, though I've done no specific tests on it.
> 
> Some people had perhaps similar clock issues when their fast processors 
> were throttling/stepping down to very low speeds (100, even 75MHz) while 
> still slicing at 1000Hz, which I didn't find too surprising.  Limiting 
> minimum CPU freq to 300Mz or more seemed to solve many such issues, but 
> I haven't your perseverance for digging up the relevant threads ..
> 
> Even in 5.5-S (/sys/conf/NOTES and /sys/i386/conf/NOTES) HZ=1000 or 2000 
> was suggested for DEVICE_POLLING (which bf included in config, though 
> maybe it's not enabled?) and HZ=1000 or more was recommended when using 
> DUMMYNET with ipfw - to provide smoother queue dispatching, I gather.
> 
> Bottom line, IMHO, bf should probably run the default 1000Hz, 500 at 
> least, on an Athlon 900.  With powerd, maybe set min. freq >= 150MHz?

Wow, this is fantastic information.  You've just educated me a great bit
about the history and use of HZ.  I've always had a "general" idea of
its importance and key role, but I was never fully aware of the history.

P.S. -- I need more like 6 months off.  I've never taken an official
(read: real) vacation my entire life.  Maybe some day I'll get to travel
to Seoul and visit Pyun Yong-Hyeon and drink lots of soju.  :-)

-- 
| Jeremy Chadwick                                jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |