Yes, I disabled device polling on my NIC. It's in my kernel so that I can
experiment with it. I use ipfw, but not dummynet.

I find your numbers interesting, Ian. But how did you determine when a
kern.hz setting was too high or too low -- and was this very sensitive to
different workloads?

I'd also like to learn more about this subject, and I'm hoping that some
of the developers will take the time to include some discussion of this
parameter in an updated and expanded tuning(7) manpage.

Best wishes for some enjoyable holidays soon,

b.

--- On Thu, 10/9/08, Charles Sprickman wrote:

> From: Charles Sprickman
> Subject: Re: Recent Problems with RELENG_7 i386
> To: "Jeremy Chadwick"
> Cc: "Ian Smith" , "bf" , freebsd-stable@freebsd.org
> Date: Thursday, October 9, 2008, 2:13 PM
> 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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> >

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> >




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