Just a few comments to add to what’s already been said:
Yes, it does need a lot of RAM, but I can assure you that the memory load is entirely reasonable considering what’s going on. A lot of effort went into keeping these requirements to a minimum, and while there may be ways to reduce this further, they’re unlikely to make a dramatic difference. The fundamental problem is we need the fixel-fixel correlation matrix to be computed, and as @rsmith mentioned, it’s large. On top of that, it needs to be resident in RAM since the streamlines are randomly located - there’s no way to predict which bit of the matrix will need to be updated at any point during processing, so we need to have it available in RAM. Any alternative would involve a lot of random disk reads/writes and processing would slow down to a crawl.
What is lacking though are more overt warnings about these RAM requirements in the documentation. There is one mention of this as a note at the end of the fixel mask computation step, but nothing in the most relevant section, or upfront in the introduction. I think we could definitely do something about this…