Module Netmcore

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.

Please read Netmcore_basics for an introduction into using this module. There is also Netmcore_tut with more advanced techniques.

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.

Netmcore controls the lifetime of certain system resources in addition to worker processes:

• Temporary files
• Shared memory objects
• Named semaphores

• disable_pmanage parameter for the Netmcore.startup call, or by setting the
• unlink parameter in the same call. This second option only disables the deletion, but the management file is updated.

Note that the int argument is not the Unix PID, but just a self-generated identifier that is unique for the lifetime of the program.

• `File name: (Temporary) file name (absolute name)
• `Posix_shm name: Shared memory objects with name
• `Posix_shm_sc(name,p): Shared memory objects with name, and attached container for semaphores with prefix p
• `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_shm_preallocated_sc(name,m.p): Same, plus attached container for semaphores with prefix p
• `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)

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
      

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, all resources are inherited. (This changed in Ocamlnet-3.5 - before, no resources were inherited, which turned out to be quite dangerous as default.)

Raises No_resource if there is no such resource.

It is not possible to call join from the master process (as this process must not block). You can, however, get the result of an already terminated process with join_nowait, e.g. after Netmcore.run returned.

Raises No_resource if there is no such resource.

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.

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 Unix PID of the master process is written to this file.

disable_pmanage: Disable that persistent kernel objects are managed via the "netplex.pmanage" file in the socket directory.

no_unlink: Disable that old persistent kernel objects are deleted at startup. This may be useful when startup is called a second time from the same program.

You need to call Netmcore.join_nowait to get the results from processes.

The result of extract_result is the result of run.