dup() scales badly on multicore platform

Dear all;

       i benchchmark the dup() system call on 32 cores machine in
FreeBSD-current8.0.

       The results are bad. The phenomenon is easy to come out. Each
process(not thread) on the core

       dup() its private file, and close() in a tight loop. The time
completing the parallel workload

       is considered as performance. At first,  i think there are some
locks. However, lock profiling

       in FreeBSD is strange and interesting. I attach the graph and lock
profiling. Any ideas?