
Depends on the application, there might not be a price. My primepi.lua, which count numbers of primes <= n, uses lots of mod's For prime.pi(10^12), it called mod 16,055,541 times. On my old Pentium 3, LuaJIT 1.1.8 were using fmod to implenent %. Just to be sure, I edit away the %, replacing with fmod: luajit O primepi.lua 1e12 => 37607912018, take 54.8 seconds Recompile LuaJIT using Lua 5.1 definition of mod for math.fmod, i.e. ((a)  floor((a)/(b))*(b)), it run slower (64.0 seconds) !? My guess is if a and b are not extremely far off, fmod is faster, because it avoided the expensive division, using subtractions instead. 