What's factors will cause weird cpu usage?

• We have a performance test tool we owned, which written by java. Recently I run three tests with 200/400/600 threads, and found the system CPU usage as the chart: (red-200,blue-600,green-400). I can't explain what factors will cause this kind of result, any ideas?

The GC activities are normal:

CPU Usage increasing on redhat5.4 platform

                                            It's fine on redhat6.0

                                          netpackage also increasing.