Lua Garbage Collection

In this article, we will explore the Lua garbage collection process, how it works, and its benefits for developers.

Almost all programming languages include garbage collection as part of their memory management, and Lua is no exception.

The Lua garbage collector is an automatic memory management mechanism that releases memory that is no longer being used by the program. As a result, it prevents memory leaks and ensures that the program makes efficient use of memory.

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Lua Garbage Collection:

What is Garbage Collection?

Garbage collection is a technique used by programming languages to manage the memory used by an application. When an application allocates memory to create an object, it needs to free up that memory when the object is no longer needed.

Without garbage collection, developers would have to manually track and release the memory used by an application, which is both time-consuming and error-prone.

Garbage collection is an automatic process that frees up memory when an object is no longer needed. It identifies and deletes objects that are no longer in use, freeing up memory for the application to use.

How does Lua Garbage Collection work?

Lua’s approach to garbage collection involves monitoring the usage of all allocated memory blocks within the program. Upon allocation, the garbage collector adds memory blocks to a list of “reachable” memory, and periodically traverses the memory space during program runtime to identify which blocks are still in use and which ones are not.

This garbage collection process employs a mark-and-sweep algorithm, wherein the garbage collector begins at a set of “roots” (e.g. global variables, call stack, and other active data structures) during the mark phase, and proceeds to follow references to other memory blocks, marking them as “reachable”. In the sweep phase, the garbage collector scans all allocated memory blocks and releases those that are not marked as “reachable”.

Garbage Collector Functions:

The garbage collection process in Lua can be managed and controlled using various built-in functions:

Functions	Overview
collectgarbage(“collect”)	Executes one full cycle of garbage collection.
collectgarbage(“count”)	Retrieves the current amount of memory being used by the program, measured in Kilobytes.
collectgarbage(“restart”)	Restarts the garbage collector if it has been halted.
collectgarbage(“setpause”)	Sets the pause variable of the garbage collector to a value calculated by dividing the given parameter by 100.
collectgarbage(“setstepmul”)	Sets the step multiplier variable of the garbage collector to a value calculated by dividing the given parameter by 100.
collectgarbage(“step”)	Performs one step of the garbage collection process, with the size of the step determined by the second argument. If the step is the last one of the garbage collection cycle, “collectgarbage” will return true.
collectgarbage(“stop”)	Stops the garbage collector if it is currently running.

Here is an example that demonstrates garbage collection in Lua:

Example:

-- Define a simple function that creates a table with a reference to itself function createTable() local t = {} t.myself = t return t end-- Call the function to create a table with a reference to itself local myTable = createTable()-- Print out the amount of memory currently being used by the program print("Memory used before collection:", collectgarbage("count"))-- Set the pause variable of the garbage collector to 0.5 collectgarbage("setpause", 500)-- Call the garbage collector to perform one full cycle of collection collectgarbage("collect")-- Print out the amount of memory currently being used by the program after the collection print("Memory used after collection:", collectgarbage("count"))-- Set the pause variable of the garbage collector to 2.0 collectgarbage("setpause", 2000)-- Call the garbage collector to perform one step of collection collectgarbage("step", 1)-- Print out the amount of memory currently being used by the program after the step print("Memory used after step:", collectgarbage("count"))-- Set the pause variable of the garbage collector to 1.0 collectgarbage("setpause", 1000)-- Call the garbage collector to perform one step of collection collectgarbage("step", 1)-- Print out the amount of memory currently being used by the program after the step print("Memory used after another step:", collectgarbage("count"))

Example Explaination

Above example used the collectgarbage() function to perform garbage collection, which involves freeing up any memory that is no longer being used by the program. It does this by setting the pause variable of the garbage collector to different values and then calling the collectgarbage() function to perform one full cycle of garbage collection or one step of garbage collection, and prints out the amount of memory used by the program before and after each call to collectgarbage().