Description of the information provided by the -gc_info option of the C back-end compiler. The information is printed to std_output after each GC cycle.
For each reference type and each NATIVE_ARRAY type, the main part lists the following information:
The number of objects created since program start. This is not the number of currently existing objects.
The same as Number created but the unit is bytes.
The number of free objects left in the "store chunk" of the specified type.
This number is mainly interesting because it is zero for the type that triggered the collection cycle. Here's why: the GC will initially allocate new objects in the store chunk. Only when there is no store left will the GC start looking for space elsewhere, which will result in one of the following outcomes:
- Previously reclaimed garbage is recycled as a new object.
- A new store chunk is allocated.
- A garbage collection cycle is started. If garbage of the correct type was reclaimed, garbage is recycled. Else, a new store chunk is allocated.
The final line shows a summary across all types.
xx is the number of pointers in the stack, i.e. the number of conservative roots.
xx is the number of memory chunks currently allocated. yyyy is the number of chunks that the main table can hold. When xx grows larger than yyyy the GC needs to reallocate a larger main table (not a very costly operation unless yyyy is huge).
xx is the number of memory chunks currently allocated to fixed-size objects (all reference objects except NATIVE_ARRAYs). yy is the number of free memory chunks, that is chunks that are devoted to fixed-size objects, but that do not hold any objects. zz is the number of chunks that can be allocated to fixed-size objects before a GC cycle is triggered.
When the GC needs to allocate a new object and it has exhausted that type's store chunk and reclaimed garbage, it tries to find a new store chunk for that type. If yy is non-null, a free memory chunk is converted to a store chunk. Else, if xx is smaller than zz then a chunk is allocated from the system. Else, a GC cycle is started.
xx is the number of memory chunks currently allocated to resizable objects (NATIVE_ARRAYs). yy is the number of chunks that can be allocated to resizable objects before a GC cycle is triggered.
Currently, there is no information about memory freed during the last GC cycle, i.e. the table provides no information about the effect of the previous GC cycle.