Googleしてみると、こんな記述が。
http://unix.derkeiler.com/Mailing-Lists/FreeBSD/performance/2005-08/0004.html
Hrm... I'll suggest one tunable:
options KSTACK_MAX_PAGES=32 # Maximum pages to give the kernel stack
Don't crank it up too far, but 64 seems to be a decent number in my
experience. I'd pimp more settings, but given that tuning is still
closer to an art than a science (it wouldn't be if someone catalogued
settings for various applications and hardware combinations, but I
digress), I'll suggest having you take a peak at the various NOTES
files and some of the headers, such as: src/sys/amd64/include/param.h.
I've divulged more in the archives, too. -sc
