Recent Problems with RELENG_7 i386

Charles Sprickman spork at bway.net
Thu Oct 9 11:13:26 PDT 2008


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 |
>
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