Threading model:

Only one thread is running Async code at a time. This is enforced by a single lock in Async's scheduler data structure. There are any number of threads running code without holding the lock that get data from the outside world and want to affect the Async world. They do this by calling Thread_safe.run_in_async*, which acquires the lock, does a computation (e.g. fills an ivar), and then runs a "cycle" of Async computations.

type t = Raw_scheduler.t
val t : unit -> t
t () returns the Async scheduler. If the scheduler hasn't been created yet, this will create it and acquire the Async lock.
val go : ?raise_unhandled_exn:bool -> unit -> Core.Std.never_returns
go ?raise_unhandled_exn () passes control to Async, at which point Async starts running handlers, one by one without interruption, until there are no more handlers to run. When Async is out of handlers it blocks until the outside world schedules more of them. Because of this, Async programs do not exit until shutdown is called.

go () calls handle_signal Sys.sigpipe, which causes the SIGPIPE signal to be ignored. Low-level syscalls (e.g. write) still raise EPIPE.

If any Async job raises an unhandled exception that is not handled by any monitor, Async execution ceases. Then, by default, Async pretty prints the exception, and exits with status 1. If you don't want this, pass ~raise_unhandled_exn:true, which will cause the unhandled exception to be raised to the caller of go ().

val go_main : ?raise_unhandled_exn:bool ->
?file_descr_watcher:Import.Config.File_descr_watcher.t ->
?max_num_open_file_descrs:int ->
?max_num_threads:int -> main:(unit -> unit) -> unit -> Core.Std.never_returns
go_main is like go, except that one supplies a main function that will be run to initialize the Async computation, and that go_main will fail if any Async has been used prior to go_main being called. Moreover it allows to configure more static options of the scheduler.
type 'a with_options = ?monitor:Import.Monitor.t -> ?priority:Import.Priority.t -> 'a
val within_context : Import.Execution_context.t -> (unit -> 'a) -> ('a, unit) Core.Std.Result.t
within_context context f runs f () right now with the specified execution context. If f raises, then the exception is sent to the monitor of context, and Error () is returned.
val within' : ((unit -> 'a Import.Deferred.t) -> 'a Import.Deferred.t) with_options
within' f ~monitor ~priority runs f () right now, with the specified block group, monitor, and priority set as specified. They will be reset to their original values when f returns. If f raises, then the result of within' will never become determined, but the exception will end up in the specified monitor.
val within : ((unit -> unit) -> unit) with_options
within is like within', but doesn't require thunk to return a deferred.
val within_v : ((unit -> 'a) -> 'a option) with_options
within_v is like within, but allows a value to be returned by f.
val with_local : 'a Core.Std.Univ_map.Key.t -> 'a option -> f:(unit -> 'b) -> 'b
with_local key value ~f, when run in the current execution context, e, runs f right now in a new execution context, e', that is identical to e, except that find_local key = value. As usual, e' will be in effect in asynchronous computations started by f. When with_local returns, the execution context is restored to e.
val find_local : 'a Core.Std.Univ_map.Key.t -> 'a option
find_local key returns the value associated to key in the current execution context.
val schedule' : ((unit -> 'a Import.Deferred.t) -> 'a Import.Deferred.t) with_options
Just like within', but instead of running thunk right now, adds it to the Async queue to be run with other Async jobs.
val schedule : ((unit -> unit) -> unit) with_options
Just like schedule', but doesn't require thunk to return a deferred.
val preserve_execution_context : ('a -> unit) -> ('a -> unit) Core.Std.Staged.t
preserve_execution_context t f saves the current execution context and returns a function g such that g a runs f a in the saved execution context. g a becomes determined when f a becomes determined.
val preserve_execution_context' : ('a -> 'b Import.Deferred.t) ->
('a -> 'b Import.Deferred.t) Core.Std.Staged.t
val cycle_start : unit -> Core.Std.Time.t
cycle_start () returns the result of Time.now () called at the beginning of cycle.
val cycle_times : unit -> Core.Std.Time.Span.t Import.Stream.t
cycle_times () returns a stream that will have one element for each cycle that Async runs, with the amount of time that the cycle took (as determined by calls to Time.now at the beginning and end of the cycle).
val report_long_cycle_times : ?cutoff:Core.Std.Time.Span.t -> unit -> unit
report_long_cycle_times ?cutoff () sets up something that will print a warning to stderr whenever there is an Async cycle that is too long, as specified by cutoff, whose default is 1s.
val cycle_count : unit -> int
cycle_count () returns the total number of Async cycles that have happened.
val force_current_cycle_to_end : unit -> unit
force_current_cycle_to_end () causes no more normal priority jobs to run in the current cycle, and for the end-of-cycle jobs (i.e. writes) to run, and then for the cycle to end.
val is_running : unit -> bool
is_running () returns true if the scheduler has been started.
val set_max_num_jobs_per_priority_per_cycle : int -> unit
set_max_num_jobs_per_priority_per_cycle int sets the maximum number of jobs that will be done at each priority within each Async cycle. The default is 500.
val set_max_inter_cycle_timeout : Core.Std.Time.Span.t -> unit
set_max_inter_cycle_timeout span sets the maximum amount of time the scheduler will remain blocked (on epoll or select) between cycles.
val set_check_invariants : bool -> unit
set_check_invariants do_check sets whether Async should check invariants of its internal data structures. set_check_invariants true can substantially slow down your program.
val set_detect_invalid_access_from_thread : bool -> unit
set_detect_invalid_access_from_thread do_check sets whether Async routines should check if they are being accessed from some thread other than the thread currently holding the Async lock, which is not allowed and can lead to very confusing behavior.
val set_record_backtraces : bool -> unit
set_record_backtraces do_record sets whether Async should keep in the execution context the history of stack backtraces (obtained via Backtrace.get) that led to the current job. If an Async job has an unhandled exception, this backtrace history will be recorded in the exception. In particular the history will appean in an unhandled exception that reaches the main monitor. This can have a substantial performance impact, both in running time and space usage.
type 'b folder = {
folder : 'a. 'b -> t -> (t, 'a) Core.Std.Field.t -> 'b;
}
fold_fields ~init folder folds folder over each field in the scheduler. The fields themselves are not exposed -- folder must be a polymorphic function that can work on any field. So, it's only useful for generic operations, e.g. getting the size of each field.
val fold_fields : init:'b -> 'b folder -> 'b
val is_ready_to_initialize : unit -> bool
val reset_in_forked_process : unit -> unit
If a process that has already created, but not started, the Async scheduler would like to fork, and would like the child to have a clean Async, i.e. not inherit any of the Async work that was done in the parent, it can call reset_in_forked_process at the start of execution in the child process. After that, the child can do Async stuff and then start the Async scheduler.
val add_busy_poller : (unit -> [ `Continue_polling | `Stop_polling of 'a ]) -> 'a Import.Deferred.t
Async supports "busy polling", which runs a thread that busy loops running user-supplied polling functions. The busy-loop thread is distinct from Async's scheduler thread.

Busy polling is useful for a situation like a shared-memory ringbuffer being used for IPC. One can poll the ringbuffer with a busy poller, and then when data is detected, fill some ivar that causes Async code to handle the data.

add_busy_poller poll adds poll to the busy loop. poll will be called continuously, once per iteration of the busy loop, until it returns `Stop_polling a at which point the result of add_busy_poller will become determined. poll will hold the Async lock while running, so it is fine to do ordinary Async operations, e.g. fill an ivar. The busy loop will run an ordinary Async cycle if any of the pollers add jobs.

poll will run in monitor in effect when add_busy_poller was called; exceptions raised by poll will be sent asynchronously to that monitor. If poll raises, it will still be run on subsequent iterations of the busy loop.

val handle_thread_pool_stuck : (stuck_for:Core.Std.Time.Span.t -> unit) -> unit
handle_thread_pool_stuck f causes f to run whenever Async detects its thread pool is stuck (i.e. hasn't completed a job for over a second and has no available threads). Async checks every second. By default, if thread pool has been stuck for less than 60s, Async will eprintf a message. If more than 60s, Async will send an exception to the main monitor, which will abort the program unless there is a custom handler for the main monitor.

Calling handle_thread_pool_stuck replaces whatever behavior was previously there.

val sexp_of_t : t -> Sexplib.Sexp.t