Add persistent_term built-in measurement (#75)

Author: whatyouhide

src/telemetry_poller.erl

@@ -48,12 +48,13 @@ The following measurements are supported:
   * `memory` (default)
   * `total_run_queue_lengths` (default)
   * `system_counts` (default)
+  * `persistent_term` (default)
   * `{process_info, Proplist}`
   * `{Module, Function, Args}`
 
 We will discuss each measurement in detail. Also note that the
 `telemetry_poller` application ships with a built-in poller that
-measures `memory`, `total_run_queue_lengths` and `system_counts`. This takes
+measures `memory`, `total_run_queue_lengths`, `system_counts`, and `persistent_term`. This takes
 the VM measurement out of the way so your application can focus
 on what is specific to its behaviour.
 
@@ -83,6 +84,14 @@ The measurement includes:
   * `atom_count` - the number of atoms currently existing at the local node
   * `port_count` - the number of ports currently existing at the local node
 
+### Persistent term (since 1.2.0)
+
+An event emitted as `[vm, persistent_term]`. The measurement includes information
+about persistent terms in the system, as returned by `persistent_term:info/0`:
+
+  * `count` - The number of persistent terms
+  * `memory` - The total amount of memory (measured in bytes) used by all persistent terms
+
 ### Process info
 
 A measurement with information about a given process. It must be specified
@@ -315,6 +324,7 @@ A measurement for the poller.
     memory
     | total_run_queue_lengths
     | system_counts
+    | persistent_term
     | {process_info, [{name, atom()} | {event, [atom()]} | {keys, [atom()]}]}
     | {module(), atom(), list()}.
 
@@ -425,6 +435,8 @@ parse_measurement(total_run_queue_lengths) ->
     {telemetry_poller_builtin, total_run_queue_lengths, []};
 parse_measurement(system_counts) ->
     {telemetry_poller_builtin, system_counts, []};
+parse_measurement(persistent_term) ->
+    {telemetry_poller_builtin, persistent_term, []};
 parse_measurement({process_info, List}) when is_list(List) ->
     Name = case proplists:get_value(name, List) of
         undefined -> erlang:error({badarg, "Expected `name' key to be given under process_info measurement"});

src/telemetry_poller_app.erl

@@ -13,7 +13,12 @@ start(_StartType, _StartArgs) ->
             PollerOpts ->
                 Default = #{
                             name => telemetry_poller_default,
-                            measurements => [memory, total_run_queue_lengths, system_counts]
+                            measurements => [
+                                memory,
+                                total_run_queue_lengths,
+                                system_counts,
+                                persistent_term
+                            ]
                            },
                 FinalOpts = maps:to_list(maps:merge(Default, maps:from_list(PollerOpts))),
                 [telemetry_poller:child_spec(FinalOpts)]

src/telemetry_poller_builtin.erl

@@ -5,6 +5,7 @@
   memory/0,
   total_run_queue_lengths/0,
   system_counts/0,
+  persistent_term/0,
   process_info/3
 ]).
 
@@ -65,3 +66,8 @@ total_run_queue_lengths() ->
             port_count => PortCount
         }).
 -endif.
+
+-spec persistent_term() -> ok.
+persistent_term() ->
+    Info = persistent_term:info(),
+    telemetry:execute([vm, persistent_term], Info, #{}).

test/telemetry_poller_SUITE.erl

@@ -12,6 +12,7 @@ all() -> [
   can_configure_sampling_period,
   dispatches_custom_mfa,
   dispatches_memory,
+  dispatches_persistent_term,
   dispatches_process_info,
   dispatches_system_counts,
   dispatches_total_run_queue_lengths,
@@ -154,6 +155,22 @@ dispatches_process_info(_Config) ->
           ct:fail(timeout_receive_echo)
   end.
 
+dispatches_persistent_term(_Config) ->
+  {ok, _Poller} = telemetry_poller:start_link([{measurements, [persistent_term]}, {period, 100}]),
+  HandlerId = attach_to([vm, persistent_term]),
+  receive
+    {event, [vm, persistent_term], #{count := Count, memory := Memory}, _} ->
+      ?assert(is_integer(Count)),
+      ?assert(Count >= 0),
+      ?assert(is_integer(Memory)),
+      ?assert(Memory >= 0),
+      telemetry:detach(HandlerId),
+      ?assert(true)
+  after
+      1000 ->
+          ct:fail(timeout_receive_echo)
+  end.
+
 attach_to(Event) ->
   HandlerId = make_ref(),
   telemetry:attach(HandlerId, Event, fun test_handler:echo_event/4, #{caller => erlang:self()}),