When setting -t 215 or greater, an integer overflow happens when this is multiplied by 10M in fastmap.c (opt->chunk_size * opt->n_threads). This results in behavior where batches seem to become very small for -t greater than 214. I sometimes want to set -t to greater than 214 on Knight's Landing systems, which is how I noticed. These commits seem to fix it by promoting the relevant ints to int64_ts.