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https://github.com/jnthn/p6-concurrent-queue

A lock-free concurrent queue data structure.
https://github.com/jnthn/p6-concurrent-queue

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A lock-free concurrent queue data structure.

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# Concurrent::Queue



A lock-free queue data structure, safe for concurrent use.



## Synopsis



    use Concurrent::Queue;



    my $queue = Concurrent::Queue.new;

    $queue.enqueue('who');

    $queue.enqueue('what');

    $queue.enqueue('why');

    say $queue.dequeue;         # who

    say ?$queue;                # True

    say $queue.elems;           # 2

    say $queue.Seq;             # (what why)

    say $queue.list;            # (what why)

    say $queue.dequeue;         # what

    $queue.enqueue('when');

    say $queue.dequeue;         # why

    say $queue.dequeue;         # when

    say ?$queue;                # False

    say $queue.elems;           # 0



    my $val = $queue.dequeue;

    say $val.WHO;               # Failure

    say $val.exception.WHO;     # X::Concurrent::Queue::Empty



## Overview



Lock-free data structures may be safely used from multiple threads, yet do not

use locks in their implementation. They achieve this through the use of atomic

operations provided by the hardware. Nothing can make contention between

threads cheap - synchronization at the CPU level is still synchronization -

but lock-free data structures tend to scale better.



This lock-free queue data structure implements an [algorithm described by

Maged M. Michael and Michael L. Scott](https://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf).

The only (intended) differences are:



* A `Failure` is returned to indicate emptiness, rather than a combination of

  boolean return value and out parameter, in order that this type feels more

  natural to Perl 6 language users

* There is an out-of-band element count (which doesn't change the algorithm at

  all, just increments and decrements the count after an enqueue/dequeue)

* Perl 6 doesn't need ABA-problem mitigation thanks to having GC



The `elems` and `Bool` method should not be used to decide whether to dequeue,

unless it is known that no other thread could be performing an enqueue or

dequeue. Their only use in the presence of concurrent use of the queue is for

getting an approximate idea of queue size. In the presence of a single thread,

the element count will be accurate (so if many workers were to enqueue data,

and are known to have completed, then at that point the `elems` will be an

accurate reflection of how many values were placed in the queue).



Note that there is no blocking dequeue operation. If looking for a blocking

queue, consider using the Perl 6 built-in `Channel` class. (If tempted to

write code that sits in a loop testing if `dequeue` gives back a `Failure` -

don't. Use `Channel` instead.)



### Methods



#### enqueue(Any $value)



Puts the value into the queue at its tail. Returns `Nil`.



#### dequeue()



If the queue is not empty, removes the head value and returns it. Otherwise,

returns a `Failure` containing an exception of type `X::Concurrent::Queue::Empty`.



#### elems()



Returns the number of elements in the queue. This value can only be relied upon

when it is known that no threads are interacting with the queue at the point this

method is called. Never use the result of `elems` to decide whether to `dequeue`,

since another thread may `enqueue` or `dequeue` in the meantime. Instead, check

if `dequeue` returns a `Failure`.



#### Bool()



Returns `False` if the queue is empty and `True` if the queue is non-empty.

The result can only be relied upon when it is known that no threads are

interacting with the queue at the point this method is called. Never use

the result of `Bool` to decide whether to `dequeue`, since another thread

may `enqueue` or `dequeue` in the meantime. Instead, check if `dequeue`

returns a `Failure`.



#### Seq



Returns a `Seq` that will iterate the queue contents. The iteration will

include all values that had not been dequeued at the point the `Seq`

method was called. Additionally, it will incorporate any values that

are enqueued during the iteration, meaning that if values are being

enqueued at a rate at least as fast as the iteration is visiting them

then the iteration may not terminate.



If wanting to prevent this, consider limiting the result length to

that of the result of `elems` (e.g. `$cq.head($cq.elems)`). If

using the queue to collect values in many threads and then iterate

them in one thread afterwards, this is not a concern, since nothing

will be enqueueing further values at that point. However, consider

using `Concurrent::Stack` instead, since a concurrent stack's

operations are cheaper than those of a concurrent queue (the same

algorithmic order, but a lower constant factor).



#### list



Equivalent to `.Seq.list`; see the description of `Seq` for caveats,

and remember that a `List` preserves its elements, so the potentially

endless iteration could also eat endless memory.