Class: Concurrent::Edge::LockFreeLinkedSet

Inherits:
Object
  • Object
show all
Includes:
Enumerable
Defined in:
lib/concurrent/edge/lock_free_linked_set.rb,
lib/concurrent/edge/lock_free_linked_set/node.rb,
lib/concurrent/edge/lock_free_linked_set/window.rb

Overview

This class implements a lock-free linked set. The general idea of this implementation is this: each node has a successor which is an Atomic Markable Reference. This is used to ensure that all modifications to the list are atomic, preserving the structure of the linked list under any circumstance in a multithreaded application.

One interesting aspect of this algorithm occurs with removing a node. Instead of physically removing a node when remove is called, a node is logically removed, by 'marking it.' By doing this, we prevent calls to remove from traversing the list twice to perform a physical removal. Instead, we have have calls to add and remove clean up all marked nodes they encounter while traversing the list.

This algorithm is a variation of the Nonblocking Linked Set found in 'The Art of Multiprocessor Programming' by Herlihy and Shavit.

Defined Under Namespace

Classes: Head, Node, Tail, Window

Instance Method Summary (collapse)

Constructor Details

- (LockFreeLinkedSet) initialize(initial_size = 0, val = nil)

Returns a new instance of LockFreeLinkedSet

Parameters:

  • initial_size (Fixnum) (defaults to: 0)

    the size of the linked_list to initialize



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# File 'lib/concurrent/edge/lock_free_linked_set.rb', line 27

def initialize(initial_size = 0, val = nil)
  @head = Head.new

  initial_size.times do
    val = block_given? ? yield : val
    add val
  end
end

Instance Method Details

- (Object) <<(item)

Atomically adds the item to the set if it does not yet exist.

Parameters:

  • item (Object)

    the item you wish to insert

Returns:

  • (Object)

    the set on which the :<< method was invoked



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# File 'lib/concurrent/edge/lock_free_linked_set.rb', line 71

def <<(item)
  add item
  self
end

- (Boolean) add(item)

Atomically adds the item to the set if it does not yet exist. Note: internally the set uses Object#hash to compare equality of items, meaning that Strings and other objects will be considered equal despite being different objects.

that the item was already in the set.

Parameters:

  • item (Object)

    the item you wish to insert

Returns:

  • (Boolean)

    true if successful. A false return indicates



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# File 'lib/concurrent/edge/lock_free_linked_set.rb', line 47

def add(item)
  loop do
    window = Window.find @head, item

    pred, curr = window.pred, window.curr

    # Item already in set
    return false if curr == item

    node = Node.new item, curr

    if pred.successor_reference.compare_and_set curr, node, false, false
      return true
    end
  end
end

- (Boolean) contains?(item)

Atomically checks to see if the set contains an item. This method compares equality based on the Object#hash method, meaning that the hashed contents of an object is what determines equality instead of Object#object_id

Parameters:

  • item (Object)

    the item you to check for presence in the set

Returns:

  • (Boolean)

    whether or not the item is in the set



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# File 'lib/concurrent/edge/lock_free_linked_set.rb', line 86

def contains?(item)
  curr = @head

  while curr < item
    curr = curr.next_node
    marked = curr.successor_reference.marked?
  end

  curr == item && !marked
end

- (Object) each {|item| ... }

An iterator to loop through the set.

Yields:

  • (item)

    each item in the set

Yield Parameters:

  • item (Object)

    the item you to remove from the set

Returns:

  • (Object)

    self: the linked set on which each was called



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# File 'lib/concurrent/edge/lock_free_linked_set.rb', line 131

def each
  return to_enum unless block_given?

  curr = @head

  until curr.last?
    curr = curr.next_node
    marked = curr.successor_reference.marked?

    yield curr.data unless marked
  end

  self
end

- (Boolean) remove(item)

Atomically attempts to remove an item, comparing using Object#hash.

Parameters:

  • item (Object)

    the item you to remove from the set

Returns:

  • (Boolean)

    whether or not the item was removed from the set



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# File 'lib/concurrent/edge/lock_free_linked_set.rb', line 104

def remove(item)
  loop do
    window = Window.find @head, item
    pred, curr = window.pred, window.curr

    return false if curr != item

    succ = curr.next_node
    removed = curr.successor_reference.compare_and_set succ, succ, false, true

    #next_node unless removed
    next unless removed

    pred.successor_reference.compare_and_set curr, succ, false, false

    return true
  end
end