# Module Gc.Memprof

    module Memprof: sig .. end

`Memprof` is a sampling engine for allocated memory words. Every allocated word has a probability of being sampled equal to a configurable sampling rate. Once a block is sampled, it becomes tracked. A tracked block triggers a user-defined callback as soon as it is allocated, promoted or deallocated.

Since blocks are composed of several words, a block can potentially be sampled several times. If a block is sampled several times, then each of the callback is called once for each event of this block: the multiplicity is given in the `n_samples` field of the `allocation` structure.

This engine makes it possible to implement a low-overhead memory profiler as an OCaml library.

Note: this API is EXPERIMENTAL. It may change without prior notice.

    type allocation = private {

`` | `n_samples : `int`;` | `(*` | The number of samples in this block (>= 1). | `*)`
`` | `size : `int`;` | `(*` | The size of the block, in words, excluding the header. | `*)`
`` | `unmarshalled : `bool`;` | `(*` | Whether the block comes from unmarshalling. | `*)`
`` | `callstack : `Printexc.raw_backtrace`;` | `(*` | The callstack for the allocation. | `*)`

`}`

The type of metadata associated with allocations. This is the type of records passed to the callback triggered by the sampling of an allocation.

    type ('minor, 'major) tracker = {

`` | `alloc_minor : `allocation -> 'minor option`;`
`` | `alloc_major : `allocation -> 'major option`;`
`` | `promote : `'minor -> 'major option`;`
`` | `dealloc_minor : `'minor -> unit`;`
`` | `dealloc_major : `'major -> unit`;`

`}`

A `('minor, 'major) tracker` describes how memprof should track sampled blocks over their lifetime, keeping a user-defined piece of metadata for each of them: `'minor` is the type of metadata to keep for minor blocks, and `'major` the type of metadata for major blocks.

If an allocation-tracking or promotion-tracking function returns `None`, memprof stops tracking the corresponding value.

    val null_tracker : ('minor, 'major) tracker

Default callbacks simply return `None` or `()`

    val start : sampling_rate:float ->
           ?callstack_size:int -> ('minor, 'major) tracker -> unit

Start the sampling with the given parameters. Fails if sampling is already active.

The parameter `sampling_rate` is the sampling rate in samples per word (including headers). Usually, with cheap callbacks, a rate of 1e-4 has no visible effect on performance, and 1e-3 causes the program to run a few percent slower

The parameter `callstack_size` is the length of the callstack recorded at every sample. Its default is `max_int`.

The parameter `tracker` determines how to track sampled blocks over their lifetime in the minor and major heap.

Sampling is temporarily disabled when calling a callback for the current thread. So they do not need to be reentrant if the program is single-threaded. However, if threads are used, it is possible that a context switch occurs during a callback, in this case the callback functions must be reentrant.

Note that the callback can be postponed slightly after the actual event. The callstack passed to the callback is always accurate, but the program state may have evolved.

Calling `Thread.exit` in a callback is currently unsafe and can result in undefined behavior.

    val stop : unit -> unit

Stop the sampling. Fails if sampling is not active.

This function does not allocate memory, but tries to run the postponed callbacks for already allocated memory blocks (of course, these callbacks may allocate).

All the already tracked blocks are discarded.

Calling `stop` when a callback is running can lead to callbacks not being called even though some events happened.