high sys time, very very slow builds on new 24-core system

[Thor Lancelot Simon <tls%panix.com@localhost>]

I have a new machine with 24 2Ghz Opteron cores.  It has 32GB of RAM.

Building with sources on a fast SSD ("preloaded" into the page
cache before the build using tar > /dev/null) and obj, dest, and rel
dirs on tmpfs, system builds are extraordinarily slow.  The system
takes about 20 minutes to build a netbsd-5 based source tree
with -j24 -- about the same amount of time as an older 8-core Intel
based system running netbsd-5 requires with -j8.

All cores spend well over 50% time in 'sys', even when all or almost
all are running cc1 processes.  The kernel is amd64 -current GENERIC
from about 1 week ago -- no DIAGNOSTIC, DEBUG, KMEMSTATS, LOCKDEBUG,
etc.

Does anyone have any idea what might be wrong here?

-- 
Thor Lancelot Simon                                              
tls%panix.com@localhost
    And now he couldn't remember when this passion had flown, leaving him so
  foolish and bewildered and astray: can any man?
                                                   William Styron