# Module Runtime_events

    module Runtime_events: sig .. end

Runtime events - ring buffer-based runtime tracing

This module enables users to enable and subscribe to tracing events from the Garbage Collector and other parts of the OCaml runtime. This can be useful for diagnostic or performance monitoring purposes. This module can be used to subscribe to events for the current process or external processes asynchronously.

When enabled (either via setting the OCAML_RUNTIME_EVENTS_START environment variable or calling Runtime_events.start) a file with the pid of the process and extension .events will be created. By default this is in the current directory but can be over-ridden by the OCAML_RUNTIME_EVENTS_DIR environment variable. Each domain maintains its own ring buffer in a section of the larger file into which it emits events.

There is additionally a set of C APIs in runtime_events.h that can enable zero-impact monitoring of the current process or bindings for other languages.

The runtime events system's behaviour can be controlled by the following environment variables:

- OCAML_RUNTIME_EVENTS_START if set will cause the runtime events system to be started as part of the OCaml runtime initialization.

- OCAML_RUNTIME_EVENTS_DIR sets the directory where the runtime events ring buffers will be located. If not present the program's working directory will be used.

- OCAML_RUNTIME_EVENTS_PRESERVE if set will prevent the OCaml runtime from removing its ring buffers when it terminates. This can help if monitoring very short running programs.

    type runtime_counter =

`|` | `EV_C_FORCE_MINOR_ALLOC_SMALL`
`|` | `EV_C_FORCE_MINOR_MAKE_VECT`
`|` | `EV_C_FORCE_MINOR_SET_MINOR_HEAP_SIZE`
`|` | `EV_C_FORCE_MINOR_MEMPROF`
`|` | `EV_C_MINOR_PROMOTED`
`|` | `EV_C_MINOR_ALLOCATED`
`|` | `EV_C_REQUEST_MAJOR_ALLOC_SHR`
`|` | `EV_C_REQUEST_MAJOR_ADJUST_GC_SPEED`
`|` | `EV_C_REQUEST_MINOR_REALLOC_REF_TABLE`
`|` | `EV_C_REQUEST_MINOR_REALLOC_EPHE_REF_TABLE`
`|` | `EV_C_REQUEST_MINOR_REALLOC_CUSTOM_TABLE`

The type for counter events emitted by the runtime

    type runtime_phase =

`|` | `EV_EXPLICIT_GC_SET`
`|` | `EV_EXPLICIT_GC_STAT`
`|` | `EV_EXPLICIT_GC_MINOR`
`|` | `EV_EXPLICIT_GC_MAJOR`
`|` | `EV_EXPLICIT_GC_FULL_MAJOR`
`|` | `EV_EXPLICIT_GC_COMPACT`
`|` | `EV_MAJOR`
`|` | `EV_MAJOR_SWEEP`
`|` | `EV_MAJOR_MARK_ROOTS`
`|` | `EV_MAJOR_MARK`
`|` | `EV_MINOR`
`|` | `EV_MINOR_LOCAL_ROOTS`
`|` | `EV_MINOR_FINALIZED`
`|` | `EV_EXPLICIT_GC_MAJOR_SLICE`
`|` | `EV_FINALISE_UPDATE_FIRST`
`|` | `EV_FINALISE_UPDATE_LAST`
`|` | `EV_INTERRUPT_REMOTE`
`|` | `EV_MAJOR_EPHE_MARK`
`|` | `EV_MAJOR_EPHE_SWEEP`
`|` | `EV_MAJOR_FINISH_MARKING`
`|` | `EV_MAJOR_GC_CYCLE_DOMAINS`
`|` | `EV_MAJOR_GC_PHASE_CHANGE`
`|` | `EV_MAJOR_GC_STW`
`|` | `EV_MAJOR_MARK_OPPORTUNISTIC`
`|` | `EV_MAJOR_SLICE`
`|` | `EV_MAJOR_FINISH_CYCLE`
`|` | `EV_MINOR_CLEAR`
`|` | `EV_MINOR_FINALIZERS_OLDIFY`
`|` | `EV_MINOR_GLOBAL_ROOTS`
`|` | `EV_MINOR_LEAVE_BARRIER`
`|` | `EV_STW_API_BARRIER`
`|` | `EV_STW_HANDLER`
`|` | `EV_STW_LEADER`
`|` | `EV_MAJOR_FINISH_SWEEPING`
`|` | `EV_MINOR_FINALIZERS_ADMIN`
`|` | `EV_MINOR_REMEMBERED_SET`
`|` | `EV_MINOR_REMEMBERED_SET_PROMOTE`
`|` | `EV_MINOR_LOCAL_ROOTS_PROMOTE`
`|` | `EV_DOMAIN_CONDITION_WAIT`
`|` | `EV_DOMAIN_RESIZE_HEAP_RESERVATION`

The type for span events emitted by the runtime

    type lifecycle =

`|` | `EV_RING_START`
`|` | `EV_RING_STOP`
`|` | `EV_RING_PAUSE`
`|` | `EV_RING_RESUME`
`|` | `EV_FORK_PARENT`
`|` | `EV_FORK_CHILD`
`|` | `EV_DOMAIN_SPAWN`
`|` | `EV_DOMAIN_TERMINATE`

Lifecycle events for the ring itself

    val lifecycle_name : lifecycle -> string

Return a string representation of a given lifecycle event type

    val runtime_phase_name : runtime_phase -> string

Return a string representation of a given runtime phase event type

    val runtime_counter_name : runtime_counter -> string

Return a string representation of a given runtime counter type

    type cursor

Type of the cursor used when consuming

    module Timestamp: sig .. end

    module Callbacks: sig .. end

    val start : unit -> unit

`start ()` will start the collection of events in the runtime if not already started.

Events can be consumed by creating a cursor with `create_cursor` and providing a set of callbacks to be called for each type of event.

    val pause : unit -> unit

`pause ()` will pause the collection of events in the runtime. Traces are collected if the program has called `Runtime_events.start ()` or the OCAML_RUNTIME_EVENTS_START environment variable has been set.

    val resume : unit -> unit

`resume ()` will resume the collection of events in the runtime. Traces are collected if the program has called `Runtime_events.start ()` or the OCAML_RUNTIME_EVENTS_START environment variable has been set.

    val create_cursor : (string * int) option -> cursor

`create_cursor path_pid` creates a cursor to read from an runtime_events. Cursors can be created for runtime_events in and out of process. A runtime_events ring-buffer may have multiple cursors reading from it at any point in time and a program may have multiple cursors open concurrently (for example if multiple consumers want different sets of events). If `path_pid` is None then a cursor is created for the current process. Otherwise the pair contains a string `path` to the directory that contains the `pid`.events file and int `pid` for the runtime_events of an external process to monitor.

    val free_cursor : cursor -> unit

Free a previously created runtime_events cursor

    val read_poll : cursor -> Callbacks.t -> int option -> int

`read_poll cursor callbacks max_option` calls the corresponding functions on `callbacks` for up to `max_option` events read off `cursor`'s runtime_events and returns the number of events read.