Huge Graph Structure
I doubt you'll be able to use a memory structure unless you have a LOT of memory at your disposal:
Assume you are talking about 600 directed edges from each node, with a node being 4-bytes (integer key) and a directed edge being JUST the destination node keys (4 bytes each).
Then the raw data about each node is 4 + 600 * 4 = 2404 bytes x 6,000,000 = over 14.4GB
That's without any other overheads or any additional data in the nodes (or edges).
I'm developing an application in which I need a structure to represent a huge graph (between 1000000 and 6000000 nodes and 100 or 600 edges per node) in memory. The edges representation will contain some attributes of the relation. 
I have tried a memory map representation, arrays, dictionaries and strings to represent that structure in memory, but these always crash because of the memory limit.
I would like to get an advice of how I can represent this, or something similar.
By the way, I'm using python.