Openmp has no improvement?

[oym1994]

Hi,
I compared the g2o graph pose optimization time cost with and without openmp. My hardware is :
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 12
On-line CPU(s) list: 0-11
Thread(s) per core: 2
Core(s) per socket: 6
Socket(s): 1
NUMA node(s): 1
Vendor ID: GenuineIntel
CPU family: 6
Model: 166
Model name: Intel(R) Core(TM) i7-10710U CPU @ 1.10GHz
Stepping: 0
CPU MHz: 3127.860
CPU max MHz: 4700.0000
CPU min MHz: 400.0000
BogoMIPS: 3199.92
Virtualization: VT-x
L1d cache: 32K
L1i cache: 32K
L2 cache: 256K
L3 cache: 12288K
NUMA node0 CPU(s): 0-11
memory: 32G

the testing result is:
About 70000 edges costs nearly 140s with 20 iterations
About 74000edges costs nearly 160s with 20 iterations

It seems that openmp has no improvement!

Another problem is running without openmp performs much better than with openmp.

Have anyone also come across this problem?

Thanks!