On Thu, 9 Oct 2008, Jeremy Chadwick wrote:
On Fri, Oct 10, 2008 at 03:51:02AM +1100, Ian Smith wrote:
> 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.
Not to pull this too much further OT, but in the original message there
was a comment about HZ and context switching. I care for a number of FBSD
boxes that are stuffed full of qmail processes. Context switches are
always through the roof when the boxes are busy. My layman's
understanding of "context switching" is very vague - in short I assume
it's some type of overhead from the kernel having to move between
servicing different processes. Altering HZ to "tune" this is very
intriguing to me, so if anyone would like to explain, I'm all ears.
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. :-)
Join the f***ing club. I'm still waiting for my honeymoon after two years
of being married. :)
Charles
--
| 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 |
_______________________________________________
[email protected] mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-stable
To unsubscribe, send any mail to "[EMAIL PROTECTED]"
_______________________________________________
[email protected] mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-stable
To unsubscribe, send any mail to "[EMAIL PROTECTED]"
