The GC parameters are given as a control record. Note that these parameters can also be initialised by setting the OCAMLRUNPARAM environment variable. See the documentation of ocamlrun.
This value controls the GC messages on standard error output. It is a sum of some of the following flags, to print messages on the corresponding events: 0x001 Start of major GC cycle. 0x002 Minor collection and major GC slice. 0x004 Growing and shrinking of the heap. 0x008 Resizing of stacks and memory manager tables. 0x010 Heap compaction. 0x020 Change of GC parameters. 0x040 Computation of major GC slice size. 0x080 Calling of finalisation functions. 0x100 Bytecode executable and shared library search at start-up. 0x200 Computation of compaction-triggering condition. 0x400 Output GC statistics at program exit. Default: 0.
The maximum size of the stack (in words). This is only relevant to the byte-code runtime, as the native code runtime uses the operating system's stack. Default: 1024k.
The major GC speed is computed from this parameter. This is the memory that will be "wasted" because the GC does not immediatly collect unreachable blocks. It is expressed as a percentage of the memory used for live data. The GC will work more (use more CPU time and collect blocks more eagerly) if space_overhead is smaller. Default: 80.
The size (in words) of the minor heap. Changing this parameter will trigger a minor collection. Default: 256k.
Heap compaction is triggered when the estimated amount of "wasted" memory is more than max_overhead percent of the amount of live data. If max_overhead is set to 0, heap compaction is triggered at the end of each major GC cycle (this setting is intended for testing purposes only). If max_overhead >= 1000000, compaction is never triggered. If compaction is permanently disabled, it is strongly suggested to set allocation_policy to 1. Default: 500.
How much to add to the major heap when increasing it. If this number is less than or equal to 1000, it is a percentage of the current heap size (i.e. setting it to 100 will double the heap size at each increase). If it is more than 1000, it is a fixed number of words that will be added to the heap. Default: 15.
The policy used for allocating in the heap. Possible values are 0 and 1. 0 is the next-fit policy, which is quite fast but can result in fragmentation. 1 is the first-fit policy, which can be slower in some cases but can be better for programs with fragmentation problems. Default: 0.