Perform multicore performance test took only 1 core to work

Hello,

I have Yade-1.07.0 which I compiled from source. I want to test its performance up to 3 cores by running
yade-1.07.0 -j3 --performance
and in the same time I look at system monitor, I found that from beginning to the end, only one core is used for this performance test (I have 8-core PC). (See attached image: http://i.imgur.com/XtYzL4o.png)

System info:
Ubuntu 12.04 64 bit
Yade 1.07.0 compiled normally (OpenMP successfully included)
bogomips : 4789.08
clflush size : 64
cache_alignment : 64
address sizes : 39 bits physical, 48 bits virtual
processor : 7
vendor_id : GenuineIntel
cpu family : 6
model : 60
model name : Intel(R) Core(TM) i7-4700MQ CPU @ 2.40GHz
stepping : 3
microcode : 0x12
cpu MHz : 800.000
cache size : 6144 KB
physical id : 0
siblings : 8
core id : 3
cpu cores : 4
apicid : 7
initial apicid : 7
fpu : yes
fpu_exception : yes
cpuid level : 13
wp : yes
----------------------------------

Did I do the right thing? I also see there is --threads/--cores syntax, how to use them?
And then how to correctly perform a multicore performance check with Yade?

Thanks in advance!