  Red-blue pebble games model the computation cost of a two-level memory
hierarchy. We present various hardness results in different red-blue pebbling
variants, with a focus on the oneshot model. We first study the relationship
between previously introduced red-blue pebble models (base, oneshot, nodel). We
also analyze a new variant (compcost) to obtain a more realistic model of
computation. We then prove that red-blue pebbling is NP-hard in all of these
model variants. Furthermore, we show that in the oneshot model, a
$\delta$-approximation algorithm for $\delta<2$ is only possible if the unique
games conjecture is false. Finally, we show that greedy algorithms are not good
candidates for approximation, since they can return significantly worse
solutions than the optimum.
