kern/118124: HZ value on slow computers

[Charlie &]

>Number:         118124
>Category:       kern
>Synopsis:       HZ value on slow computers
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          change-request
>Submitter-Id:   current-users
>Arrival-Date:   Mon Nov 19 13:50:01 UTC 2007
>Closed-Date:
>Last-Modified:
>Originator:     hsu at bbnetworks.net
>Release:        FreeBSD 6.2-STABLE i386
>Organization:
bbnetworks.net
>Environment:
System: FreeBSD ranta.suonsivu.net 6.2-STABLE FreeBSD 6.2-STABLE #3: Sat Nov 17 15:33:51 EET 2007 root at ranta.suonsivu.net:/usr/obj/usr/src/sys/EBOX2300 i386

This is relevant to slow computers, such as anything based on
SoC things, Sis550, Geode, very old computers.


>Description:

HZ was bumped to 1000 couple of years ago.  This is fine for quick computers,
but becomes a problem on old computers and various embedded devices.

On Vortex86 cpu at 200MHz, and HZ set to 1000, with DUMMYNET and IPFW built
into the kernel, cpu seems to spend around 12-14% of time in interrupt
and overall performance is really low.


>How-To-Repeat:

Install FreeBSD on a slow computer.  Try a 486, old pentiums, anything Sis550
or Geode based.  Use a kernel with all firewall, dummynet and other heavy
networking options on.

>Fix:

HZ should probably be dynamic, based on cpu, rather than a fixed value
in kernel config. 

Workaround is setting HZ in loader.conf

>Release-Note:
>Audit-Trail:
>Unformatted: