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Real World OCaml by Jason Hickey, Anil Madhavapeddy, Yaron Minsky

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Chapter 21. Understanding the Garbage Collector

We’ve described the runtime format of individual OCaml variables earlier, in Chapter 20. When you execute your program, OCaml manages the lifecycle of these variables by regularly scanning allocated values and freeing them when they’re no longer needed. This in turn means that your applications don’t need to manually implement memory management, and it greatly reduces the likelihood of memory leaks creeping into your code.

The OCaml runtime is a C library that provides routines that can be called from running OCaml programs. The runtime manages a heap, which is a collection of memory regions that it obtains from the operating system. The runtime uses this memory to hold heap blocks that it fills up with OCaml values in response to allocation requests by the OCaml program.

Mark and Sweep Garbage Collection

When there isn’t enough memory available to satisfy an allocation request from the pool of allocated heap blocks, the runtime system invokes the garbage collector (GC). An OCaml program can’t explicitly free a value when it is done with it. Instead, the GC regularly determines which values are live and which values are dead, i.e., no longer in use. Dead values are collected and their memory made available for reuse by the application.

The GC doesn’t keep constant track of values as they are allocated and used. Instead, it regularly scans them by starting from a set of root values that the application always has access to (such as the ...

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