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parmap.mli
(**************************************************************************)
(* ParMap: a simple library to perform Map computations on a multi-core   *)
(*                                                                        *)
(*  Author(s):  Marco Danelutto, Roberto Di Cosmo                         *)
(*                                                                        *)
(*  This library is free software: you can redistribute it and/or modify  *)
(*  it under the terms of the GNU Lesser General Public License as        *)
(*  published by the Free Software Foundation, either version 2 of the    *)
(*  License, or (at your option) any later version.  A special linking    *)
(*  exception to the GNU Lesser General Public License applies to this    *)
(*  library, see the LICENSE file for more information.                   *)
(**************************************************************************)

(** Module [Parmap]: efficient parallel map, fold and mapfold on lists and 
    arrays on multicores. 

    All the primitives allow to control the granularity of the parallelism 
    via an optional parameter [chunksize]: if [chunksize] is omitted, the
    input sequence is split evenly among the available cores; if [chunksize]
    is specified, the input data is split in chunks of size [chunksize] and
    dispatched to the available cores using an on demand strategy that 
    ensures automatic load balancing.

    A specific primitive [array_float_parmap] is provided for fast operations on float arrays.
    *)

(** {6 Sequence type, subsuming lists and arrays} *)

type 'a sequence = L of 'a list | A of 'a array;;

(** The [parmapfold], [parfold] and [parmap] generic functions, for efficiency reasons,
    convert the input data into an array internally, so we provide the ['a sequence] type
    to allow passing an array directly as input.
    If you want to perform a parallel map operation on an array, use [array_parmap] or [array_float_parmap] instead.
    *)

(** {6 Parallel mapfold} *)

val parmapfold : ?ncores:int -> ?chunksize:int -> ('a -> 'b) -> 'a sequence -> ('b-> 'c -> 'c) -> 'c -> ('c->'c->'c) -> 'c

  (** [parmapfold ~ncores:n f (L l) op b concat ] computes [List.fold_right op (List.map f l) b] 
      by forking [n] processes on a multicore machine. 
      You need to provide the extra [concat] operator to combine the partial results of the
      fold computed on each core. If 'b = 'c, then [concat] may be simply [op]. 
      The order of computation in parallel changes w.r.t. sequential execution, so this 
      function is only correct if [op] and [concat] are associative and commutative.
      If the optional [chunksize] parameter is specified,
      the processes compute the result in an on-demand fashion
      on blocks of size [chunksize].
      [parmapfold ~ncores:n f (A a) op b concat ] computes [Array.fold_right op (Array.map f a) b] 
      *)

(** {6 Parallel fold} *)

val parfold: ?ncores:int -> ?chunksize:int -> ('a -> 'b -> 'b) -> 'a sequence -> 'b -> ('b->'b->'b) -> 'b
  (** [parfold ~ncores:n op (L l) b concat] computes [List.fold_right op l b] 
      by forking [n] processes on a multicore machine.
      You need to provide the extra [concat] operator to combine the partial results of the
      fold computed on each core. If 'b = 'c, then [concat] may be simply [op]. 
      The order of computation in parallel changes w.r.t. sequential execution, so this 
      function is only correct if [op] and [concat] are associative and commutative.
      If the optional [chunksize] parameter is specified,
      the processes compute the result in an on-demand fashion
      on blocks of size [chunksize].
      [parfold ~ncores:n op (A a) b concat] similarly computes [Array.fold_right op a b].
      *)

(** {6 Parallel map} *)

val parmap : ?ncores:int -> ?chunksize:int -> ('a -> 'b) -> 'a sequence -> 'b list
  (** [parmap  ~ncores:n f (L l) ] computes [List.map f l] 
      by forking [n] processes on a multicore machine.
      [parmap  ~ncores:n f (A a) ] computes [Array.map f a] 
      by forking [n] processes on a multicore machine.
      If the optional [chunksize] parameter is specified,
      the processes compute the result in an on-demand fashion
      on blocks of size [chunksize]; this provides automatic
      load balancing for unbalanced computations, but the order
      of the result is no longer guaranteed to be preserved. *)


(** {6 Parallel map on arrays} *)

val array_parmap : ?ncores:int -> ?chunksize:int -> ('a -> 'b) -> 'a array -> 'b array
  (** [array_parmap  ~ncores:n f a ] computes [Array.map f a] 
      by forking [n] processes on a multicore machine.
      If the optional [chunksize] parameter is specified,
      the processes compute the result in an on-demand fashion
      on blochs of size [chunksize]; this provides automatic
      load balancing for unbalanced computations, but the order
      of the result is no longer guaranteed to be preserved. *)

(** {6 Parallel map on float arrays } *)

exception WrongArraySize

type buf

val init_shared_buffer : float array -> buf
  (** [init_shared_buffer a] creates a new memory mapped shared buffer big enough to hold a float array of the size of [a].
      This buffer can be reused in a series of calls to [array_float_parmap], avoiding the cost of reallocating it each time. *)

val array_float_parmap : ?ncores:int -> ?chunksize:int -> ?result: float array -> ?sharedbuffer: buf -> ('a -> float) -> 'a array -> float array
  (** [array_float_parmap  ~ncores:n f a ] computes [Array.map f a] by forking 
      [n] processes on a multicore machine, and preallocating the resulting
      array as shared memory, which allows significantly more efficient
      computation than calling the generic array_parmap function.  If the
      optional [chunksize] parameter is specified, the processes compute the
      result in an on-demand fashion on blochs of size [chunksize]; this
      provides automatic load balancing for unbalanced computations, *and* the
      order of the result is still guaranteed to be preserved.

      In case you already have at hand an array where to store the result, you
      can squeeze out some more cpu cycles by passing it as optional parameter
      [result]: this will avoid the creation of a result array, which can be
      costly for very large data sets. Raises [WrongArraySize] if [result] is too
      small to hold the data.

      It is possible to share the same preallocated shared memory space across
      calls, by initialising the space calling [init_shared_buffer a] and
      passing the result as the optional [sharedbuffer] parameter to each
      subsequent call to [array_float_parmap].  Raises WrongArraySize if
      [sharedbuffer] is too small to hold the input data. *)
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