performance degradation in 6.2 when adding a second quad core chip

[benjamin thielsen]

hi folks-

we've been experiencing some interesting behavior on single quad core  
computers as compared to dual quad core computers.

it appears that adding a second processor to the system (leaving it  
otherwise untouched) actually decreases performance.  we've got a  
small rudimentary test process, built in house, that does postgresql  
queries (selects) via http requests (apache2/php5).

here is a small data set from what we've seen so far:

a user is one iteration of a curl statement and values listed in the  
cpu columns are average time before the page returns, in seconds.

users        1 cpu       2 cpu
-------------------------------
60            2.48        2.18
80            3.34        2.72
100           4.18        5.34
120           9.48       15.61
140           13.2       46.16
160           26.28      66.99

we're confident that nothing else has changed other than the addition  
of the chip, so i'm hoping for some insight on where we might look for  
clues.  the above results used 6.2-RELEASE.  we've started the same  
test using 7.0-RC2 and are seeing similar response times with both  
processors in place.  next on our list is doing a single processor  
iteration of the 7.0-RC2 test to corroborate that data, followed by a  
local test to query pg more directly, removing apache and friends from  
the equation, in hopes of getting some clarity.

below are the basics of this configuration - this is a new list for  
me, so i've been conservative, but i'm happy to provide as much detail  
as is helpful.

computer:
dell poweredge 2900
xeon quad core 2 ghz / 1333 mhz bus speed / 2 x 6mb l2 cache
4 gb 667 mhz memory

all of the software has been installed via ports, the 6.2 kernel is a  
custom kernel with (i believe) mostly rudimentary changes and support  
for the perc6 disk controller (that became available in 7) added, and  
the 7.0 kernel contains the same changes as the 6.2 kernel, excluding  
the custom perc6 support.

thanks!
-ben