This chapter describes the runtime events tracing system which enables continuous extraction of performance information from the OCaml runtime with very low overhead. The system and interfaces are low-level and tightly coupled to the runtime implementation, it is intended for end-users to rely on tooling to consume and visualise data of interest.
Data emitted includes:
There are three main classes of events emitted by the runtime events system:
The runtime events tracing system is designed to be used in different contexts:
The runtime events tracing system logs events to a ring buffer. Consequently, old events are being overwritten by new events. Consumers can either continuously consume events or choose to only do so in response to some circumstance, e.g if a particular query or operation takes longer than expected to complete.
The runtime tracing system conceptually consists of two parts: 1) the probes which emit events and 2) the events transport that ingests and transports these events.
Probes collect events from the runtime system. These are further split in to two sets: 1) probes that are always available and 2) probes that are only available in the instrumented runtime. Probes in the instrumented runtime are primarily of interest to developers of the OCaml runtime and garbage collector and, at present, only consist of major heap allocation size counter events.
The full set of events emitted by probes and their documentation can be found in Module Runtime_events.
The events transport part of the system ingests events emitted by the probes and makes them available to consumers.
Events are transported using a data structure known as a ring buffer. This data structure consists of two pointers into a linear backing array, the tail pointer points to a location where new events can be written and the head pointer points to the oldest event in the buffer that can be read. When insufficient space is available in the backing array to write new events, the head pointer is advanced and the oldest events are overwritten by new ones.
The ring buffer implementation used in runtime events can be written by at most one producer at a time but can be read simultaneously by multiple consumers without coordination from the producer. There is a unique ring buffer for every running domain and, on domain termination, ring buffers may be re-used for newly spawned domains. The ring buffers themselves are stored in a memory-mapped file with the processes identifier as the name and the extension .events, this enables them to be read from outside the main OCaml process. See Runtime_events for more information.
The runtime event tracing system provides both OCaml and C APIs which are cursor-based and polling-driven. The high-level process for consuming events is as follows:
We start with a simple example that prints the name, begin and end times of events emitted by the runtime event tracing system:
let runtime_begin _ ts phase = Printf.printf "Begin\t%s\t%Ld\n" (Runtime_events.runtime_phase_name phase) (Runtime_events.Timestamp.to_int64 ts) let runtime_end _ ts phase = Printf.printf "End\t%s\t%Ld\n" (Runtime_events.runtime_phase_name phase) (Runtime_events.Timestamp.to_int64 ts) let () = Runtime_events.start (); let cursor = Runtime_events.create_cursor None in let callbacks = Runtime_events.Callbacks.create ~runtime_begin ~runtime_end () in while true do let list_ref = ref  in (* for later fake GC work *) for _ = 1 to 100 do (* here we do some fake GC work *) list_ref := ; for _ = 1 to 10 do list_ref := (Sys.opaque_identity(ref 42)) :: !list_ref done; Gc.full_major (); done; ignore(Runtime_events.read_poll cursor callbacks None); Unix.sleep 1 done
The next step is to compile and link the program with the runtime_events library. This can be done as follows:
ocamlopt -I +runtime_events -I +unix unix.cmxa runtime_events.cmxa example.ml -o example
When using the dune build system, this example can be built as follows:
(executable (name example) (modules example) (libraries unix runtime_events))
Running the compiled binary of the example gives an output similar to:
Begin explicit_gc_full_major 24086187297852 Begin stw_leader 24086187298594 Begin minor 24086187299404 Begin minor_global_roots 24086187299807 End minor_global_roots 24086187331461 Begin minor_remembered_set 24086187331631 Begin minor_finalizers_oldify 24086187544312 End minor_finalizers_oldify 24086187544704 Begin minor_remembered_set_promote 24086187544879 End minor_remembered_set_promote 24086187606414 End minor_remembered_set 24086187606584 Begin minor_finalizers_admin 24086187606854 End minor_finalizers_admin 24086187607152 Begin minor_local_roots 24086187607329 Begin minor_local_roots_promote 24086187609699 End minor_local_roots_promote 24086187610539 End minor_local_roots 24086187610709 End minor 24086187611746 Begin minor_clear 24086187612238 End minor_clear 24086187612580 End stw_leader 24086187613209 ...
This is an example of self-monitoring, where a program explicitly starts listening to runtime events and monitors itself.
For external monitoring, a program does not need to be aware of the existence of runtime events. Runtime events can be controlled via the environment variable OCAML_RUNTIME_EVENTS_START which, when set, will cause the runtime tracing system to be started at program initialization.
We could remove Runtime_events.start (); from the previous example and, instead, call the program as below to produce the same result:
Environment variables can be used to control different aspects of the runtime event tracing system. The following environment variables are available:
The size of the runtime events ring buffers can be configured via OCAMLRUNPARAM, see section 15.2 for more information.
To receive events that are only available in the instrumented runtime, the OCaml program needs to be compiled and linked against the instrumented runtime. For our example program from earlier, this is achieved as follows:
ocamlopt -runtime-variant i -I +runtime_events -I +unix unix.cmxa runtime_events.cmxa example.ml -o example
And for dune:
(executable (name example) (modules example) (flags "-runtime-variant=i") (libraries unix runtime_events))
Programmatic access to events is intended primarily for writers of observability libraries and tooling that end-users use. The flexible API enables use of the performance data from runtime events for logging and monitoring purposes.
In this section we cover several utilities in the runtime_events_tools package which provide simple ways of extracting and summarising data from runtime events. The trace utility in particular produces similar data to the previous ’eventlog’ instrumentation system available in OCaml 4.12 to 4.14.
First, install runtime_events_tools in an OCaml 5.0+ opam switch:
opam install runtime_events_tools
This should install the olly tool in your path. You can now generate runtime traces for programs compiled with OCaml 5.0+ using the trace subcommand:
olly trace trace.json 'your_program.exe .. args ..'
Runtime tracing data will be generated in the json Trace Event Format to trace.json. This can then be loaded into the Chrome tracing viewer or into Perfetto to visualize the collected trace.
The olly utility also includes a latency subcommand which consumes runtime events data and on program completion emits a parseable histogram summary of pause durations. It can be run as follows:
olly latency 'your_program.exe .. args ..'
This should produce an output similar to the following:
GC latency profile: #[Mean (ms): 2.46, Stddev (ms): 3.87] #[Min (ms): 0.01, max (ms): 9.17] Percentile Latency (ms) 25.0000 0.01 50.0000 0.23 60.0000 0.23 70.0000 0.45 75.0000 0.45 80.0000 0.45 85.0000 0.45 90.0000 9.17 95.0000 9.17 96.0000 9.17 97.0000 9.17 98.0000 9.17 99.0000 9.17 99.9000 9.17 99.9900 9.17 99.9990 9.17 99.9999 9.17 100.0000 9.17