Many thanks for prompt help. Yep, They are ICX-6610 and also ICX-7250.
In summary, I would test the performance of egress port against (different buffer sizes). It seems that ICX enables buffer sizing, but it is not clear at all in the configuration guide.
For example, one command like: qd-buffer 0 1 y, another command like qd-descriptor 0 1 y. Please note above 0 means device number, 1 means any 1Gb port, y means buffer size (in terms of number of buffers).
Here, I got confused! The commands are not clear, why we must allocate all 1Gb ports the same buffer size, what are the descriptors, why do we need to allocate y for descriptors.