Ocamlviz gives the ability to instrument an existing code, in real time, with lightweight monitoring annotations. ocamlviz can also be used as a debugging tool.

here are a few possibilities provided by ocamlviz:
* observe details about the garbage collector
* observe how many times the program goes through a point
* make a set of values (any) and count its cardinal number and its
size in the heap
* observe how much time passed between two points of the program
* observe the value of integers, floating-point numbers, booleans
and strings
* observe details about hash tables, like the number of empty
buckets, or the filling rate
* etc
this package provides the ocamlviz library.