= Garbage Collection Roots =

The "roots" are the set of pointers that the GC starts traversing from, i.e. the roots of the live object graph.

Most roots belong to a particular Capability.  Traversing the roots of a capbility is done by `markSomeCapabilities()` in [[GhcFile(rts/Capability.c)]].  The roots of a Capability are:

 * The run queue (head and tail)
 * The wakeup queue (head and tail)
 * For each Task on the `suspended_ccalling_tasks` list, the TSO for that Task
 * The Spark Pool
 * Only for the non-threaded RTS: The blocked queue (head and tail), and the sleeping queue

In addition, each Capability has a [wiki:Commentary/Rts/Storage/GC/RememberedSets remembered set] for each generation.  A remembered set is a source of roots if that generation is ''not'' being collected during this cycle; otherwise the remembered set is discarded.  During GC, all remembered sets are discarded and new ones will be constructed for each generation and Capability; see `scavenge_capability_mut_lists()` in [[GhcFile(rts/sm/Scav.c)]].

There are also roots from other parts of the system:

 * Signal handlers (only in the non-threaded RTS; in the threaded RTS signal handlers are maintained by the IO manager in `GHC.Conc` rather than the RTS).
 * [wiki:Commentary/Rts/Storage/GC/Weak Weak pointers]
 * [wiki:Commentary/Rts/Stable Stable pointers]