collectgarbage ([String:opt [, arg]])
collect, stop, restart, count, step, setpause, setstepmul
https://www.lua.org/manual/5.1/manual.html#pdf-collectgarbage

"collect": performs a full garbage-collection cycle. This is the default option.
"stop": stops the garbage collector.
"restart": restarts the garbage collector.
"count": returns the total memory in use by Lua (in Kbytes).
"step": performs a garbage-collection step. The step "size" is controlled by arg (larger values mean more steps) in a non-specified way. If you want to control the step size you must experimentally tune the value of arg. Returns true if the step finished a collection cycle.
"setpause": sets arg as the new value for the pause of the collector (see 2.10). Returns the previous value for pause.
"setstepmul"