2.5 – Garbage Collection

Lua performs automatic memory management. This means that you do not have to worry about allocating memory for new objects or freeing it when the objects are no longer needed. Lua manages memory automatically by running a garbage collector to collect all dead objects (that is, objects that are no longer accessible from Lua). All memory used by Lua is subject to automatic management: strings, tables, userdata, functions, threads, internal structures, etc.

Lua implements an incremental mark-and-sweep collector. It uses two numbers to control its garbage-collection cycles: the garbage-collector pause and the garbage-collector step multiplier. Both use percentage points as units (e.g., a value of 100 means an internal value of 1).

The garbage-collector pause controls how long the collector waits before starting a new cycle. Larger values make the collector less aggressive. Values smaller than 100 mean the collector will not wait to start a new cycle. A value of 200 means that the collector waits for the total memory in use to double before starting a new cycle.

The garbage-collector step multiplier controls the relative speed of the collector relative to memory allocation. Larger values make the collector more aggressive but also increase the size of each incremental step. You should not use values smaller than 100, because they make the collector too slow and can result in the collector never finishing a cycle. The default is 200, which means that the collector runs at “twice” the speed of memory allocation.

If you set the step multiplier to a very large number (larger than 10% of the maximum number of bytes that the program may use), the collector behaves like a stop-the-world collector. If you then set the pause to 200, the collector behaves as in old Lua versions, doing a complete collection every time Lua doubles its memory usage.

You can change these numbers by calling lua_gc in C or collectgarbage in Lua. You can also use these functions to control the collector directly (e.g., stop and restart it).

2.5.1 – Garbage-Collection Metamethods

You can set garbage-collector metamethods for tables and, using the C API, for full userdata (see §2.4). These metamethods are also called finalizers. Finalizers allow you to coordinate Lua’s garbage collection with external resource management (such as closing files, network or database connections, or freeing your own memory).

For an object (table or userdata) to be finalized when collected, you must mark it for finalization. You mark an object for finalization when you set its metatable and the metatable has a field indexed by the string “__gc“. Note that if you set a metatable without a __gc field and later create that field in the metatable, the object will not be marked for finalization.

When a marked object becomes garbage, it is not collected immediately by the garbage collector. Instead, Lua puts it in a list. After the collection, Lua goes through that list. For each object in the list, it checks the object’s __gc metamethod: If it is a function, Lua calls it with the object as its single argument; if the metamethod is not a function, Lua simply ignores it.

At the end of each garbage-collection cycle, the finalizers for objects are called in the reverse order that the objects were marked for finalization, among those collected in that cycle; that is, the first finalizer to be called is the one associated with the object marked last in the program. The execution of each finalizer may occur at any point during the execution of the regular code.

Because the object being collected must still be used by the finalizer, that object (and other objects accessible only through it) must be resurrected by Lua. Usually, this resurrection is transient, and the object memory is freed in the next garbage-collection cycle. However, if the finalizer stores the object in some global place (e.g., a global variable), then the resurrection is permanent. Moreover, if the finalizer marks a finalizing object for finalization again, its finalizer will be called again in the next cycle where the object is unreachable. In any case, the object memory is freed only in a GC cycle where the object is unreachable and not marked for finalization.

When you close a state (see lua_close), Lua calls the finalizers of all objects marked for finalization, following the reverse order that they were marked. If any finalizer marks objects for collection during that phase, these marks have no effect.

2.5.2 – Weak Tables

A weak table is a table whose elements are weak references. A weak reference is ignored by the garbage collector. In other words, if the only references to an object are weak references, then the garbage collector will collect that object.

A weak table can have weak keys, weak values, or both. A table with weak values allows the collection of its values, but prevents the collection of its keys. A table with both weak keys and weak values allows the collection of both keys and values. In any case, if either the key or the value is collected, the whole pair is removed from the table. The weakness of a table is controlled by the __mode field of its metatable. If the __mode field is a string containing the character ‘k‘, the keys in the table are weak. If __mode contains ‘v‘, the values in the table are weak.

A table with weak keys and strong values is also called an ephemeron table. In an ephemeron table, a value is considered reachable only if its key is reachable. In particular, if the only reference to a key comes through its value, the pair is removed.

Any change in the weakness of a table may take effect only at the next collect cycle. In particular, if you change the weakness to a stronger mode, Lua may still collect some items from that table before the change takes effect.

Only objects that have an explicit construction are removed from weak tables. Values, such as numbers and light C functions, are not subject to garbage collection, and therefore are not removed from weak tables (unless their associated values are collected). Although strings are subject to garbage collection, they do not have an explicit construction, and therefore are not removed from weak tables.

Resurrected objects (that is, objects being finalized and objects accessible only through objects being finalized) have a special behavior in weak tables. They are removed from weak values before running their finalizers, but are removed from weak keys only in the next collection after running their finalizers, when such objects are actually freed. This behavior allows the finalizer to access properties associated with the object through weak tables.

If a weak table is among the resurrected objects in a collection cycle, it may not be properly cleared until the next cycle.