I am developing for a low-power embedded system. It has two ARM9 processors at 48 MHz each. One of the processors has access to 6 MiB of "fast" memory as well as access to 26 MiB of "slow" memory. The other processor does not have direct access to memory, so it's going to be pretty much useless in this scenario (I keep it halted). The fast memory timings are complicated, and memory timings change based on whether it's a read or a write, and whether memory access is sequential or not, as well as whether accesses are 16 or 32 bits. The slow memory is much slower, but is consistently slow.
I need to implement a KDF on this system. I considered Argon2, but I do not know how to decide if it's better to run many passes on the "fast" memory, or a few passes on the "slow" memory, or some combination of the two. How do I decide what the optimal parameters are for a memory-hard KDF if the memory is going to be significantly slower than typical systems? Or is memory-hardness a losing game in this scenario and I should just use PBKDF2 (in which case I could make use of both CPUs)?
Update: I write this a little late (long after I finished this project, but reading the datasheet for another ARM-based device reminded me), but I ended up going with bcrypt in a region of very fast memory called the TCM (Tightly Coupled Memory), which can be accessed at the full speed of the processor. This allowed me to use the second processor simultaneously for improved security despite it not having full access to memory, since it has its own TCM region independent of the other processor. The following pseudocode is roughly what was done to turn
(CPU 0) hash_0 := tcm_bcrypt(password || 0x0)
(CPU 1) hash_1 := tcm_bcrypt(password || 0x1)
(CPU 0) result := hash_0 ^ hash_1