Re: Initial 6.1 questions

[Scott Long <scottl_(_at_)_samsco_(_dot_)_org>]

Danial Thom wrote:
> --- Robert Watson <rwatson_(_at_)_FreeBSD_(_dot_)_org> wrote:
>
>
> > On Mon, 12 Jun 2006, Danial Thom wrote:
> >
> >
> > > first, why is the default for HZ now 1000? It
> >
> > seems that 900 extra clock 
> >
> > > interrupts aren't a performance enhancement.
> >
> > This is a design change that is in the process
> > of being reconsidered.  I 
> > expect that HZ will not be 1000 in 7.x, but
> > can't tell you whether it will go 
> > back to 100, or some middle ground.  There are
> > a number of benefits to a 
> > higher HZ, not least is more accurate timing of
> > some network timer events. 
> > Since I don't have my hands in the timer code,
> > I can't speak to what the 
> > decision process here is, or when any change
> > might happen, but I do expect to 
> > see some change.
>
>
> Will anything break if I tweek this downward?

I run a number of high-load production systems that
do a lot of network and filesystem activity, all
with HZ set to 100.  It has also been shown in the
past that certain things in the network area where
not fixed to deal with a high HZ value, so it's
possible that it's even more stable/reliable with
an HZ value of 100.

My personal opinion is that HZ should gop back down
to 100 in 7-CURRENT immediately, and only be incremented
back up when/if it's proven to be the right thing to do.
And, I say that as someone who (errantly) pushed for the
increase to 1000 several years ago.

Scott

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