Hi phuc_trantan,
Thanks for the file. I will have a look in the morning at it. (it is Sunday 11:40 pm here)
For now, I would like you to point to something Octane based. (I do not make comparisons here, you decided to have Octane for specific reasons, so use them!) I do not have Octane. This means I have to ask you to check into a forum for this specific render.
If you like to use Octane for this, perhaps you use a different set up altogether then:
http://www.aoktar.com/octane/OCTANE HELP MANUAL.html?DistributionSettings.html
Copy the link, they have empty spaces in their URL, which doesn’t work here.
I have set up a similar polygon amount with trees in RedShift:
https://www.cineversity.com/forums/viewthread/4402/
I assume you will be happier with the native use of the renderer you have.
In the morning, I will check your file.
All the best