(* $Id: netmcore.mli 1579 2011-04-12 20:16:22Z gerd $ *)
(** Multi-processing for compute jobs
This library allows it to develop parallel algorithms that can take
advantage of multiple CPU cores. It does not use Ocaml's multi-threading
feature because this would implicitly serialize all computations.
Instead, independent processes are created, and the communication
between processes is made as cheap and unproblematic as possible.
Before using this library, it is required to call {!Netmcore.startup}.
This turns the current process into the master process. The master
process has only a supervising function, and is responsible for
managing global resources, and for starting further worker processes.
The worker processes can start more workers. This is different from
creating a new thread, though: The new worker does not share memory
with its creator. It is also different from calling [Unix.fork],
because new workers are always subprocesses of the master process.
This means it is not initialized with a copy of the state of the
logical creator, but with its real parent process which is always
the master.
Compatibility with multi-threading: You may run into big trouble
when the master process starts further threads (after forking
thread-related resources are in an uncontrolled state). There is
no such problem in the worker processes. In this library,
however, nothing is done to ease the multi-threaded life, so you
should carefully check the compatibility first.
*)
(** {2 Types and exceptions} *)
type res_id =
[ `Resource of int ]
(** This tagged integer identifies resources. This variant type will never
be extended.
*)
type process_id =
[ `Process of int ]
(** This tagged integer identifies processes. This variant type will never
be extended.
*)
type compute_resource_type =
[ `File | `Posix_shm | `Posix_shm_preallocated
| `Posix_sem | `Fork_point | `Join_point
]
type inherit_request =
[ `Resources of res_id list
| `All
]
type compute_resource_repr =
[ `File of string
| `Posix_shm of string
| `Posix_shm_preallocated of string * Netsys_mem.memory
| `Posix_sem of string
| `Fork_point of (inherit_request * Netplex_encap.encap -> process_id)
| `Join_point of (process_id -> Netplex_encap.encap option)
]
(** Centrally managed resources include:
- [`File name]: Files [name] (absolute name)
- [`Posix_shm name]: Shared memory objects with [name]
- [`Posix_shm_preallocated(name,m)]: Shared memory objects already
allocated by the master process. These objects are passed over
to the worker processes by inheritance, and are always mapped at
the same address. [m] is the bigarray mapping the object.
- [`Posix_sem name]: Semaphores with [name]
- [`Fork_point(inh,f)]: Fork points where [let pid=f arg] fork a new process
with argument [arg]. [pid] is the process identifier. The list [inh]
are resources inherited from the master.
- [`Joint_point f]: Joint points where [let res=f pid] wait until
the process [pid] terminates. If [res] is non-[None] it is the
result value. If it is [None], no result was passed back
(including all pathological cases like crashes)
*)
exception No_resource of res_id
(** No such resource, or no resource of the expected type *)
class type compute_resource =
object
method id : res_id
(** an ID of the resource that is valid in the whole Netplex system *)
method typ : compute_resource_type
(** which type of resource *)
method repr : compute_resource_repr
(** the resource detail *)
method release : unit -> unit
(** Notify the manager that the resource is not used any longer by this
process. If all processes release the resource it will be finally
destroyed.
*)
end
(** {2 Defining processes} *)
(** This is the lower-level version of the process API where arguments
and results of processes are dynamically typed. It may have some
uses when writing generic process managers, but for the
normal application the statically typed API in {!Netmcore_process}
is easier to use (and less verbose).
*)
val def_process :
(Netplex_encap.encap -> Netplex_encap.encap) ->
res_id * res_id
(** [let fork_point, join_point = def_process f]:
Defines process types, i.e. ways of starting and finishing processes.
The definition must be done in the master process, e.g. before the first
worker is started.
Once the process type is defined, new processes can be started,
and these processes will run the function [f]. When [f] is
finished, the process will terminate. Starting processes is possible
from other worker processes.
[f] is supplied with its argument by the process starter [start].
The result value of [f] can be retrieved with [join] (by any
process, but only the first [join] for this process will be successful).
If [f] throws exceptions, these will be caught and logged, but not
be passed back to the caller (which just gets [None] as result
of [join]). The same happens for any unplanned termination of the
process.
It is allowed to immediately release the [join_point] if there is
no interest in catching the termination of started processes.
Here is an example how to define a process that takes a [string]
argument and returns an [int]:
{[
module String_encap = Netplex_encap.Make_encap(struct type t=string end)
module Int_encap = Netplex_encap.Make_encap(struct type t=int end)
let my_process_fork, my_process_join =
Netmcore.def_process
(fun s_encap ->
let (s:string) = String_encap.unwrap s_encap in
let (r:int) = ... in
Int_encap.wrap r
)
]}
The wrapping and unwrapping is required for ensuring type-safety
(see {!Netplex_encap} for the details of the idea).
Calling this process is done with (also see below):
{[
let pid = Netmcore.start my_process_fork (String_encap.wrap s) in
let r_encap_opt = Netmcore.join my_process_join pid in
match r_encap_opt with
| None -> failwith "Something went wrong"
| Some r_encap -> Int_encap.unwrap r_encap
]}
*)
(** The following functions can also be called from worker processes
(i.e. Netplex containers)
*)
val start : ?inherit_resources:inherit_request ->
res_id -> Netplex_encap.encap -> process_id
(** [let pid = start fork_point arg]: Starts the process with the
given [fork_point] and the argument [arg].
Raises [No_resource] if there is no such resource.
The function returns a process identifier. This is not the Unix
PID, but a sequentially generated number that is unique for a
running program.
Option [inherit_resources]: Certain resources are only accessible by
the process when they are inherited to it. This is the case for
[`Posix_shm_preallocated]. This can be set to [`All] to inherit
all inheritable resources, or to [`Resources l] to only inherit
the resources of [l]. By default, no resources are inherited.
*)
val join : res_id -> process_id -> Netplex_encap.encap option
(** [let res_opt = join join_point pid]: Waits until the process [pid]
is done, and returns the result value if any.
Raises [No_resource] if there is no such resource.
*)
(** {2 Managing resources from worker processes} *)
(** The following functions can also be called from worker processes
(i.e. Netplex containers)
*)
val get_resource : res_id -> compute_resource
(** Retrieves the resource by ID. This implicitly also marks this resource
as being used by this process. Don't forget to call [release] when
your are done with the resource.
Raises [No_resource] if there is no such resource.
*)
val release : res_id -> unit
(** Release the resource with this ID. (Same as calling the [release]
method on the object.)
*)
val manage_file : string -> compute_resource
(** hands over a file to the manager *)
val get_file : res_id -> string
(** Gets the file with this ID (or raises [No_resource]). As for
[get_resource] the file is marked as being used by the process.
*)
val manage_shm : string -> compute_resource
(** hands over a named shm object to the manager *)
val get_shm : res_id -> string
(** Gets the shm object with this ID (or raises [No_resource]). As
for [get_resource] the shm object is marked as being used by the process.
*)
(** Shared memory objects can be created with {!Netsys_posix.shm_create},
and opened with {!Netsys_posix.shm_open}.
*)
val create_preallocated_shm :
?value_area:bool -> string -> int -> (res_id * string)
(** [create_preallocated_shm prefix size]: Creates a new preallocated
shm object with a unique name based on [prefix], and a length of
[size] bytes. The object is created and mapped into the master
process, and will be available to any newly started process when
the resource ID is inherited to the process.
Returns [(res_id,shm_name)] where [res_id] identifies the new
resource, and [shm_name] is the name of the POSIX shared memory
object.
Note that the process calling this function cannot look up this
resource (using [get_shm] or [get_resource]) because the shm
block cannot be mapped at the right address. Nevertheless, the calling
process counts as a user of the object, and needs to release
the object.
Option [value_area]: if set, the new memory is marked as value
area, so the ocaml runtime allows value comparisons in this
memory area.
*)
val manage_sem : string -> compute_resource
(** hands over a named semaphore to the manager *)
val get_sem : res_id -> string
(** gets the semaphore with this ID (or raises [No_resource]). As
for [get_resource] the semaphore is marked as being used by the process.
*)
(** Semaphores can be opened with {!Netsys_posix.sem_open}, and
created with {!Netsys_posix.sem_create}.
*)
val self_process_id : unit -> process_id
(** Returns the process ID of a worker *)
(** {2 Initialization and system start} *)
(** This module can either be used as Netplex plugin and integrated into
any existing Netplex program, or it can be started in stand-alone mode
*)
val add_plugins : Netplex_types.controller -> unit
(** To enable compute processes for any Netplex program, call this
function with the controller object as argument. This can e.g.
be done in the [post_add_hook] of the processor.
*)
val startup : socket_directory:string ->
?pidfile:string ->
?init_ctrl:(Netplex_types.controller -> unit) ->
first_process:(unit -> process_id) ->
unit ->
unit
(** This function makes the current process the master process.
It starts immediately a new worker process, called the
first process. The [startup] function returns first when this
process is finished, in which case the whole Netplex system is
shut down (which may lead to killing the remaining processes,
following the usual shutdown procedure).
The first process is created by calling [first_process()] at the
right moment. This function normally just invokes [start].
Passing a [socket_directory] is mandatory. This directory will
contain helper files. The must be a separate [socket_directory]
for each running Computeplex instance.
[pidfile]: If passed, the PID of the master process is written
to this file.
*)
val destroy_resources : unit -> unit
(** Destroys all resources that may be left *)
module Debug : sig
val enable : bool ref
end
