Question about 2.6.5 kernel scalability

I am currently measuring the performance of a directory server, OpenLDAP
on IBM xSeries 455(8-way).
First see the performance result(operations/sec) with increase in the 
number of CPUs.

1 : 6,100
2: 10,000
4: 17,600
5: 14,200
6: 13,200
7: 13,100
8: 11,500

As you can see, the throughput starts degrading when the number of CPUs is 
4.
when the number of CPUs is 8, the throughput is about 65% of 4 CPUs.
During experiment, CPU utilization of all cases are about 93%.
The directory server are multithreaded and in the experiment 16~32 work 
threads were used.
I am currently investigating to find the reason of degradation.
I guess that there might be scheduling anomaly on SMP.
If any one knows any strange behavior of Linux 2.6.5 scheduler, please let 
share the knowledge.
It'll will help me a lot.

Sang Seok