# 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` | `(*` | Triggering of a minor collection due to a full minor heap. Since 5.0 | `*)`
`|` | `EV_C_FORCE_MINOR_MAKE_VECT` | `(*` | Triggering of a minor collection due to Array.make. Since 5.0 | `*)`
`|` | `EV_C_FORCE_MINOR_SET_MINOR_HEAP_SIZE` | `(*` | Triggering of a minor collection due to Gc.minor_heap_size. Since 5.0 | `*)`
`|` | `EV_C_FORCE_MINOR_MEMPROF` | `(*` | Triggering of a minor collection during memprof young sampling. Since 5.3 | `*)`
`|` | `EV_C_MINOR_PROMOTED` | `(*` | Total words promoted from the minor heap to the major in the last minor collection. Since 5.0 | `*)`
`|` | `EV_C_MINOR_ALLOCATED` | `(*` | Total bytes allocated in the minor heap in the last minor collection. Since 5.0 | `*)`
`|` | `EV_C_REQUEST_MAJOR_ALLOC_SHR` | `(*` | Major slice requested due to allocation in major heap. Since 5.0 | `*)`
`|` | `EV_C_REQUEST_MAJOR_ADJUST_GC_SPEED` | `(*` | Major slice requested by `caml_adjust_gc_speed`. Since 5.0 | `*)`
`|` | `EV_C_REQUEST_MINOR_REALLOC_REF_TABLE` | `(*` | Triggering of a minor collection due to ref table reallocation. Since 5.0 | `*)`
`|` | `EV_C_REQUEST_MINOR_REALLOC_EPHE_REF_TABLE` | `(*` | Triggering of a minor collection due to ephe_ref table reallocation. Since 5.0 | `*)`
`|` | `EV_C_REQUEST_MINOR_REALLOC_CUSTOM_TABLE` | `(*` | Triggering of a minor collection due to custom table reallocation. Since 5.0 | `*)`
`|` | `EV_C_MAJOR_HEAP_POOL_WORDS` | `(*` | Total words in a Domain's major heap pools. This is the sum of unallocated and live words in each pool. Since 5.1 | `*)`
`|` | `EV_C_MAJOR_HEAP_POOL_LIVE_WORDS` | `(*` | Current live words in a Domain's major heap pools. Since 5.1 | `*)`
`|` | `EV_C_MAJOR_HEAP_LARGE_WORDS` | `(*` | Total words of a Domain's major heap large allocations. A large allocation is an allocation larger than the largest sized pool. Since 5.1 | `*)`
`|` | `EV_C_MAJOR_HEAP_POOL_FRAG_WORDS` | `(*` | Words in a Domain's major heap pools lost to fragmentation. This is due to there not being a pool with the exact size of an allocation and a larger sized pool needing to be used. Since 5.1 | `*)`
`|` | `EV_C_MAJOR_HEAP_POOL_LIVE_BLOCKS` | `(*` | Live blocks of a Domain's major heap pools. Since 5.1 | `*)`
`|` | `EV_C_MAJOR_HEAP_LARGE_BLOCKS` | `(*` | Live blocks of a Domain's major heap large allocations. Since 5.1 | `*)`
`|` | `EV_C_MAJOR_HEAP_WORDS` | `(*` | Major heap size in words of a Domain. Since 5.3 | `*)`
`|` | `EV_C_MAJOR_ALLOCATED_WORDS` | `(*` | Allocations to the major heap of this Domain in words, since the last major slice. Since 5.3 | `*)`
`|` | `EV_C_MAJOR_ALLOCATED_WORK` | `(*` | The amount of major GC 'work' needing to be done as a result of allocations to the major heap of this Domain in words, since the last major slice. Since 5.3 | `*)`
`|` | `EV_C_MAJOR_DEPENDENT_WORK` | `(*` | The amount of major GC 'work' needing to be done as a result of dependent allocations to the major heap of this Domain in words, since the last major slice. Dependent memory is non-heap memory that depends on heap memory being collected in order to be freed. Since 5.3 | `*)`
`|` | `EV_C_MAJOR_EXTRA_WORK` | `(*` | The amount of major GC 'work' needing to be done as a result of extra non-memory resources that are dependent on heap memory being collected in order to be freed. Since 5.3 | `*)`
`|` | `EV_C_MAJOR_WORK_COUNTER` | `(*` | The global amount of major GC 'work' done by all domains since the program began. Since 5.3 | `*)`
`|` | `EV_C_MAJOR_ALLOC_COUNTER` | `(*` | The global words of major GC allocations done by all domains since the program began. Since 5.3 | `*)`
`|` | `EV_C_MAJOR_SLICE_TARGET` | `(*` | The target amount of global 'work' that should be done by all domains at the end of the major slice (see EV_C_MAJOR_SLICE_COUNTER). Since 5.3 | `*)`
`|` | `EV_C_MAJOR_SLICE_BUDGET` | `(*` | The budget in 'work' that a domain has to do during the major slice. Since 5.3 | `*)`

The type for counter events emitted by the runtime. Counter events are used to measure a quantity at a point in time or record the occurence of an event. In the latter case their value will be one.

    type runtime_phase =

`|` | `EV_EXPLICIT_GC_SET` | `(*` | Event spanning a call to Gc.set. Since 5.0 | `*)`
`|` | `EV_EXPLICIT_GC_STAT` | `(*` | Event spanning a call to Gc.stat. Since 5.0 | `*)`
`|` | `EV_EXPLICIT_GC_MINOR` | `(*` | Event spanning a call to Gc.minor, which forces a minor collection. Since 5.0 | `*)`
`|` | `EV_EXPLICIT_GC_MAJOR` | `(*` | Event spanning a call to Gc.major, which forces a major collection. Since 5.0 | `*)`
`|` | `EV_EXPLICIT_GC_FULL_MAJOR` | `(*` | Event spanning a call to Gc.full_major, which forces a full major collection. Since 5.0 | `*)`
`|` | `EV_EXPLICIT_GC_COMPACT` | `(*` | Event spanning a call to Gc.compact, which triggers a compaction. Since 5.0 | `*)`
`|` | `EV_MAJOR` | `(*` | Event spanning any major GC work. Since 5.0 | `*)`
`|` | `EV_MAJOR_SWEEP` | `(*` | Event spanning the sweeping work of a major GC. Since 5.0 | `*)`
`|` | `EV_MAJOR_MARK_ROOTS` | `(*` | Event spanning the marking of roots in a major GC. Since 5.0 | `*)`
`|` | `EV_MAJOR_MEMPROF_ROOTS` | `(*` | Event spanning the marking of memprof roots in a major GC. Since 5.3 | `*)`
`|` | `EV_MAJOR_MARK` | `(*` | Event spanning the marking of the heap in a major GC. Since 5.0 | `*)`
`|` | `EV_MINOR` | `(*` | Event spanning any minor GC work. Since 5.0 | `*)`
`|` | `EV_MINOR_LOCAL_ROOTS` | `(*` | Event spanning the scanning and major allocation of local roots during a minor GC. Since 5.0 | `*)`
`|` | `EV_MINOR_MEMPROF_ROOTS` | `(*` | Event spanning the scanning and promotion of memprof roots in a minor GC. Since 5.3 | `*)`
`|` | `EV_MINOR_MEMPROF_CLEAN` | `(*` | Event spanning cleaning and updating of memprof structures at the end of a minor GC. Since 5.3 | `*)`
`|` | `EV_MINOR_FINALIZED` | `(*` | Event spanning the running of finalisers for dead custom blocks at the end of a minor GC. Since 5.0 | `*)`
`|` | `EV_EXPLICIT_GC_MAJOR_SLICE` | `(*` | Event spanning a call to Gc.major_slice. Since 5.0 | `*)`
`|` | `EV_FINALISE_UPDATE_FIRST` | `(*` | Event spanning time spent in the first phase of finalisation at the end of a major GC cycle. Since 5.0 | `*)`
`|` | `EV_FINALISE_UPDATE_LAST` | `(*` | Event spanning time spent in the last phase of finalisation at the end of a major GC cycle. Since 5.0 | `*)`
`|` | `EV_INTERRUPT_REMOTE` | `(*` | Event spanning work triggered by an interrupt from another domain. This is usually a stop-the-world request. Since 5.0 | `*)`
`|` | `EV_MAJOR_EPHE_MARK` | `(*` | Event spanning the marking of ephemeron tables in a major GC. Since 5.0 | `*)`
`|` | `EV_MAJOR_EPHE_SWEEP` | `(*` | Event spanning the sweeping of ephemeron tables in a major GC. Since 5.0 | `*)`
`|` | `EV_MAJOR_FINISH_MARKING` | `(*` | Event spanning work done at the end of marking in a major GC. Since 5.0 | `*)`
`|` | `EV_MAJOR_GC_CYCLE_DOMAINS` | `(*` | Event spanning work done at the end of a major GC cycle. This includes a minor collection. Since 5.0 | `*)`
`|` | `EV_MAJOR_GC_PHASE_CHANGE` | `(*` | Event spanning the change of phase in the major GC which involves a global barrier. Since 5.0 | `*)`
`|` | `EV_MAJOR_GC_STW` | `(*` | Event spanning the stop-the-world phase done at the end of a major GC cycle. Since 5.0 | `*)`
`|` | `EV_MAJOR_MARK_OPPORTUNISTIC` | `(*` | Event spanning the work done during opportunistic marking in a major GC. Since 5.0 | `*)`
`|` | `EV_MAJOR_SLICE` | `(*` | Event spanning the work done during a major slice in a major GC. Since 5.0 | `*)`
`|` | `EV_MAJOR_FINISH_CYCLE` | `(*` | Event spanning attempts to drive all domains to the end of a major GC cycle. Since 5.0 | `*)`
`|` | `EV_MINOR_CLEAR` | `(*` | Event spanning the cleaning of the minor heap and supporting structures at the end of a minor GC. Since 5.0 | `*)`
`|` | `EV_MINOR_FINALIZERS_OLDIFY` | `(*` | Event spanning the promotion of finalisers during a minor GC. Since 5.0 | `*)`
`|` | `EV_MINOR_GLOBAL_ROOTS` | `(*` | Event spanning the scanning and major allocation of global roots during a minor GC. Since 5.0 | `*)`
`|` | `EV_MINOR_LEAVE_BARRIER` | `(*` | Event spanning the time spent in the barrier at the end of a minor GC, waiting for all domains to finish their work. Since 5.0 | `*)`
`|` | `EV_STW_API_BARRIER` | `(*` | Event spanning the time spent waiting for all other domains to reach the stop-the-world entry barrier. Since 5.0 | `*)`
`|` | `EV_STW_HANDLER` | `(*` | Event spanning the time spent in the stop-the-world handler, including time spent in the stop-the-world callback itself. Since 5.0 | `*)`
`|` | `EV_STW_LEADER` | `(*` | Event spanning the time spent as the leader of a stop-the-world. Since 5.0 | `*)`
`|` | `EV_MAJOR_FINISH_SWEEPING` | `(*` | Event spanning the time spent finishing sweeping when forced to as part of domain termination. Since 5.0 | `*)`
`|` | `EV_MAJOR_MEMPROF_CLEAN` | `(*` | Event spanning the time spent cleaning memprof structures at the end of a major GC. Since 5.3 | `*)`
`|` | `EV_MINOR_FINALIZERS_ADMIN` | `(*` | Event spanning finalisers book-keeping at the end of a minor GC. Since 5.0 | `*)`
`|` | `EV_MINOR_REMEMBERED_SET` | `(*` | Event spanning the scanning and major allocation of remembered sets during a minor GC. Since 5.0 | `*)`
`|` | `EV_MINOR_REMEMBERED_SET_PROMOTE` | `(*` | Event spanning the promotion of blocks in the remembered set and global roots during a minor GC. Since 5.0 | `*)`
`|` | `EV_MINOR_LOCAL_ROOTS_PROMOTE` | `(*` | Event spanning the promotion of local roots during a minor GC. Since 5.0 | `*)`
`|` | `EV_DOMAIN_CONDITION_WAIT` | `(*` | Event spanning waiting in Condition.wait. Since 5.0 | `*)`
`|` | `EV_DOMAIN_RESIZE_HEAP_RESERVATION` | `(*` | Event spanning resizing the domain heap reservation, as a result of minor heap size changes. Since 5.0 | `*)`
`|` | `EV_COMPACT` | `(*` | Event spanning compaction of the heap during a call to Gc.compact. Since 5.2 | `*)`
`|` | `EV_COMPACT_EVACUATE` | `(*` | Event spanning evacuating major GC pools during a compaction. Since 5.2 | `*)`
`|` | `EV_COMPACT_FORWARD` | `(*` | Event spanning the walking of the heap to update changed pointers after an evacuation during a compaction. Since 5.2 | `*)`
`|` | `EV_COMPACT_RELEASE` | `(*` | Event spanning releasing the evacuated pools at the end of a compaction. Since 5.2 | `*)`
`|` | `EV_EMPTY_MINOR` | `(*` | Event spanning a domain needing to empty its minor heap for a new allocation. This includes time spent trying to become stop-the-world leader. Since 5.4 | `*)`

The type for span events emitted by the runtime.

    type lifecycle =

`|` | `EV_RING_START` | `(*` | Event indicating that the Runtime_events ring buffer has been started. Includes the PID of the process as an argument. Since 5.0 | `*)`
`|` | `EV_RING_STOP` | `(*` | Event indicating that the Runtime_events ring buffer has been stopped. Since 5.0 | `*)`
`|` | `EV_RING_PAUSE` | `(*` | Event indicating that the Runtime_events ring buffer has been paused. Since 5.0 | `*)`
`|` | `EV_RING_RESUME` | `(*` | Event indicating that the Runtime_events ring buffer has been resumed. Since 5.0 | `*)`
`|` | `EV_FORK_PARENT` | `(*` | Event indicating that a fork has occurred and the current domain is the parent. Includes the PID of the child as an argument. Since 5.0 | `*)`
`|` | `EV_FORK_CHILD` | `(*` | Event indicating that a fork has occurred and the current domain is the child. Since 5.0 | `*)`
`|` | `EV_DOMAIN_SPAWN` | `(*` | Event indicating that a new domain has been spawned. Includes the PID of the new domain as an argument. Since 5.0 | `*)`
`|` | `EV_DOMAIN_TERMINATE` | `(*` | Event indicating that a domain has terminated. Includes the PID of the domain as an argument. Since 5.0 | `*)`

Lifecycle events for Runtime_events and domains.

    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 Type: sig .. end

    module User: 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 path : unit -> string option

If runtime events are being collected, `path ()` returns `Some p` where `p` is a path to the runtime events file. Otherwise, it returns None.

    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.