[Serusers] SER performance bottleneck ?

Hi dear friends, We tried to benchmarking IPtel SER 0.9.6 on Intel xeon CPU based computer recently,but we met a problem about poort performance.For example on a computer configured with Intel 2*sossaman CPU (four cores)+2G ddr2-400 memory+100M Ethernet card, we didn't configure any database,and registered with only 1-2 users,and we use SIPP 3.0 as the call generator ,but we can only see about 3,000 calls per second for a very simply built-in scenario in SIPP(UAC+UAS),and the CPU idle percentage is about 70-80%.if we tried to increase the call rate ,we got failure calls. We tried to enlarge the memory to 768M for SER,and limited the ser processes number on each port from 8 to 4,and we also made some improvement on config.h & Makefile.defs,but didn't get big improvement on performance . If anybody could help me on this,I will appreciate a lot. Thanks Fang tian.