Dr. Dobb's is part of the Informa Tech Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Channels ▼


Writing a Generalized Concurrent Queue

A Two-Lock Multiproducer/Consumer Queue

The queue data structure itself is a singly linked list. To make the code simpler for the empty-queue boundary case, the list always contains a dummy node at the head, and so the first element logically in the queue is the one contained in the node after first. Figure 1 shows the layout of an empty queue, and Figure 2 shows a queue that holds some objects.

Figure 1: Structure of an empty queue.

Figure 2: A queue containing four T objectsempty queue.

Each node holds the T object by pointer and adds padding:

template <typename T>
struct LowLockQueue {
  struct Node {
    Node( T* val ) : value(val), next(nullptr) { }
    T* value;
    atomic<Node*> next;
    char pad[CACHE_LINE_SIZE - sizeof(T*)- sizeof(atomic<Node*>)];

Like any shared variable, the next pointer needs to be protected by a mutex or, as here, declared as an ordered atomic type (C++0x atomic<> or Java/.NET volatile). The padding here is to ensure that two Node objects won't occupy the same cache line; in particular, in a nonempty queue, having the first and last nodes in the same cache line would penalize performance by causing invisible contention between the producer and consumer. Note that the amount of padding shown here and later on errs on the side of being too conservative: Each Node will be allocated on the heap, and a heap allocator may add its own extra overhead to each allocated block for alignment and housekeeping information, which effectively adds extra padding. If so, and if we know how much that will be, we could reduce our internal padding proportionately to make a heap-allocated Node exactly fill one cache line.

Next, here are our queue control variables:

    char pad0[CACHE_LINE_SIZE];
  // for one consumer at a time
  Node* first;
  char pad1[CACHE_LINE_SIZE
           - sizeof(Node*)];
  // shared among consumers
  atomic<bool> consumerLock;
  char pad2[CACHE_LINE_SIZE 
           - sizeof(atomic<bool>)];
  // for one producer at a time
  Node* last; 
  char pad3[CACHE_LINE_SIZE 
           - sizeof(Node*)];
  // shared among producers
  atomic<bool> producerLock;
  char pad4[CACHE_LINE_SIZE 
           - sizeof(atomic<bool>)];

Again, we add padding to make sure hat data used by different threads stay physically separate in memory and cache. Clearly, we want the consumer-end data and the producer-end data to be on separate cache lines; but even though only one producer and one consumer will be active at a time, we want to keep the locking variable separate so that waiting consumers spinning on consumerLock won't contend on the cache line that contains first which the active consumer is updating, and that waiting producers spinning on producerLock won't slow down the active producer who is updating last.

The constructor just sets up the initial empty state, and the destructor (in .NET or Java, this would be the disposer) just walks the internal list and tears it down:

  LowLockQueue() {
    first = last = new Node( nullptr );
    producerLock = consumerLock = false;
  ~LowLockQueue() {
    while( first != nullptr ) {      // release the list
      Node* tmp = first;
      first = tmp->next;
      delete tmp->value;       // no-op if null
      delete tmp;

Related Reading

More Insights

Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.