# class Fiber::ExecutionContext::Parallel

- Fiber::ExecutionContext::Parallel
- Reference
- Object

##
Overview

Parallel execution context.

Fibers running in the same context run both concurrently and in parallel to each others, in addition to the other fibers running in other execution contexts.

The context internally keeps a number of fiber schedulers, each scheduler runs on a system thread, so multiple schedulers can run in parallel. The fibers are resumable by any scheduler in the context, and can thus move from one system thread to another at any time.

The actual parallelism is dynamic. As the need for parallelism increases, for example more fibers running longer, the more schedulers will start (and thus system threads), as the need decreases, for example not enough fibers, the schedulers will pause themselves and parallelism will decrease.

The parallelism can be as low as 1, in which case the context becomes a concurrent context (no parallelism) until resized.

For example: we can start a parallel context to run consumer fibers, while the default context produces values. Because the consumer fibers can run in parallel, we must protect accesses to the shared value variable. Running the example without `Atomic#add` would produce a different result every time!

    require "wait_group"

    consumers = Fiber::ExecutionContext::Parallel.new("consumers", 8)
    channel = Channel(Int32).new(64)
    wg = WaitGroup.new(32)

    result = Atomic.new(0)

    32.times do
      consumers.spawn do
        while value = channel.receive?
          result.add(value)
        end
      ensure
        wg.done
      end
    end

    1024.times { |i| channel.send(i) }
    channel.close

    # wait for all workers to be done
    wg.wait

    p result.get # => 523776

NOTE The `Parallel` execution context isn't tied to a fixed set of system threads, and execution can switch to other system threads, for example when a fiber is blocked on a syscall.

##
Included Modules

- Fiber::ExecutionContext

##
Direct Known Subclasses

- Fiber::ExecutionContext::Concurrent

##
Defined in:

fiber/execution_context/parallel.cr
fiber/execution_context/parallel/scheduler.cr

##
Constructors

- .new(name : String, maximum : Int32) : self

  Starts a `Parallel` context with a maximum parallelism.

- .new(name : String, size : Range(Nil, Int32)) : self

  DEPRECATED Use Fiber::ExecutionContext::Parallel.new(String, Int32) instead.

- .new(name : String, size : Range(Int32, Int32)) : self

  DEPRECATED Use Fiber::ExecutionContext::Parallel.new(String, Int32) instead.

##
Instance Method Summary

- #capacity : Int32

  The maximum number of schedulers that can be started, aka how many fibers can run in parallel or maximum parallelism of the context.

- #inspect(io : IO) : Nil

  Appends a String representation of this object which includes its class name, its object address and the values of all instance variables.

- #name : String
- #resize(maximum : Int32) : Nil

  Resizes the context to the new maximum parallelism.

- #to_s(io : IO) : Nil

  Appends a short String representation of this object which includes its class name and its object address.

### Instance methods inherited from module `Fiber::ExecutionContext`

spawn(*, name : String | Nil = nil, &block : -> ) : Fiber spawn

### Constructor methods inherited from module `Fiber::ExecutionContext`

current : ExecutionContext current

### Class methods inherited from module `Fiber::ExecutionContext`

current? : ExecutionContext | Nil current?, default : ExecutionContext::Parallel default, default_workers_count : Int32 default_workers_count, each(&) : Nil each, thread_keepalive : Time::Span thread_keepalive, thread_keepalive=(thread_keepalive : Time::Span) thread_keepalive=

### Instance methods inherited from class `Reference`

==(other : self)
==(other : JSON::Any)
==(other : YAML::Any)
==(other) ==, dup dup, hash(hasher) hash, initialize initialize, inspect(io : IO) : Nil inspect, object_id : UInt64 object_id, pretty_print(pp) : Nil pretty_print, same?(other : Reference) : Bool
same?(other : Nil) same?, to_s(io : IO) : Nil to_s

### Constructor methods inherited from class `Reference`

new new, unsafe_construct(address : Pointer, *args, **opts) : self unsafe_construct

### Class methods inherited from class `Reference`

pre_initialize(address : Pointer) pre_initialize

### Instance methods inherited from class `Object`

! : Bool !, !=(other) !=, !~(other) !~, ==(other) ==, ===(other : JSON::Any)
===(other : YAML::Any)
===(other) ===, =~(other) =~, as(type : Class) as, as?(type : Class) as?, class class, dup dup, hash(hasher)
hash hash, in?(collection : Object) : Bool
in?(*values : Object) : Bool in?, inspect(io : IO) : Nil
inspect : String inspect, is_a?(type : Class) : Bool is_a?, itself itself, nil? : Bool nil?, not_nil!(message)
not_nil! not_nil!, pretty_inspect(width = 79, newline = "\n", indent = 0) : String pretty_inspect, pretty_print(pp : PrettyPrint) : Nil pretty_print, responds_to?(name : Symbol) : Bool responds_to?, tap(&) tap, to_json(io : IO) : Nil
to_json : String to_json, to_pretty_json(indent : String = " ") : String
to_pretty_json(io : IO, indent : String = " ") : Nil to_pretty_json, to_s(io : IO) : Nil
to_s : String to_s, to_yaml(io : IO) : Nil
to_yaml : String to_yaml, try(&) try, unsafe_as(type : T.class) forall T unsafe_as

### Class methods inherited from class `Object`

from_json(string_or_io : String | IO, root : String)
from_json(string_or_io : String | IO) from_json, from_yaml(string_or_io : String | IO) from_yaml

### Macros inherited from class `Object`

class_getter(*names, &block) class_getter, class_getter!(*names) class_getter!, class_getter?(*names, &block) class_getter?, class_property(*names, &block) class_property, class_property!(*names) class_property!, class_property?(*names, &block) class_property?, class_setter(*names) class_setter, def_clone def_clone, def_equals(*fields) def_equals, def_equals_and_hash(*fields) def_equals_and_hash, def_hash(*fields) def_hash, delegate(*methods, to object) delegate, forward_missing_to(delegate) forward_missing_to, getter(*names, &block) getter, getter!(*names) getter!, getter?(*names, &block) getter?, property(*names, &block) property, property!(*names) property!, property?(*names, &block) property?, setter(*names) setter

##
Constructor Detail

def self.new(name : String, maximum : Int32) : self #

Starts a `Parallel` context with a maximum parallelism. The context starts with an initial parallelism of zero. It will grow to one when a fiber is spawned, then the actual parallelism will keep increasing and decreasing as needed, but will never go past the configured maximum.

def self.new(name : String, size : Range(Nil, Int32)) : self #

DEPRECATED Use Fiber::ExecutionContext::Parallel.new(String, Int32) instead.

def self.new(name : String, size : Range(Int32, Int32)) : self #

DEPRECATED Use Fiber::ExecutionContext::Parallel.new(String, Int32) instead.

##
Instance Method Detail

def capacity : Int32 #

The maximum number of schedulers that can be started, aka how many fibers can run in parallel or maximum parallelism of the context.

def inspect(io : IO) : Nil #
Description copied from class Reference

Appends a String representation of this object which includes its class name, its object address and the values of all instance variables.

    class Person
      def initialize(@name : String, @age : Int32)
      end
    end

    Person.new("John", 32).inspect # => #<Person:0x10fd31f20 @name="John", @age=32>

def name : String #
def resize(maximum : Int32) : Nil #

Resizes the context to the new maximum parallelism.

The new maximum can grow, in which case more schedulers are created to eventually increase the parallelism.

The new maximum can also shrink, in which case the overflow schedulers are removed and told to shutdown immediately. The actual shutdown is cooperative, so running schedulers won't stop until their current fiber tries to switch to another fiber.

def to_s(io : IO) : Nil #
Description copied from class Reference

Appends a short String representation of this object which includes its class name and its object address.

    class Person
      def initialize(@name : String, @age : Int32)
      end
    end

    Person.new("John", 32).to_s # => #<Person:0x10a199f20>