How to prevent SGE from assigning more nodes than I actually want for my VASP calculations?

[hwang4]

Every node of our computer has 2 dual-core cpu, i.e., 1 node=4 cores. I am using SGE to submit my parallel jobs. If I specify #$ -pe mpi 8 in my script, I am requesting 8 cores. But I don't have controlled on getting 2 nodes only. When some nodes are partially taken by some serial jobs of others, SGE will assign 3 nodes for my job, which greatly reduces the speed of the calculations. So I wonder whether SGE has some parameter to specify exactly how many nodes I want to use instead of cores? Thanks a lot

[job]

Ask your system admin to create a parallel environment with

allocation_rule 4

instead of the usual

allocation_rule $fill_up

[hwang4]

Thanks a lot!