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

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,

Does anyone have any idea what might be wrong here?

Thor Lancelot Simon                                     
    And now he couldn't remember when this passion had flown, leaving him so
  foolish and bewildered and astray: can any man?
                                                   William Styron

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