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For large MSB radix sorts you should do 6 bits at a time (64 bins) to avoid spilling the TLB cache. You can switch to 8 bits when you get below 64 pages (256k).

Also, for sorting 64-bit integers I like to do a bitwise or of all the integers while computing the histogram. Then if the bits you are looking at are zero (plausible for unsigned 64-bit integers) you can skip to the ones that matter for the next histogram.