Concurrent::Stack

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

Synopsis

use Concurrent::Stack;

my $stack = Concurrent::Stack.new;

for 'a'..'z' {
    $stack.push($_);
}

say $stack.elems;       # 26
say $stack.peek;        # z
say $stack.pop;         # z
say $stack.pop;         # y
say $stack.peek;        # x
$stack.push('k');
say $stack.peek;        # k
say $stack.elems;       # 25
say $stack.Seq;         # A Seq iterating a snapshot of the stack
say $stack.list;        # A lazy List with a snapshot of the stack

$stack.pop for ^25;
say $stack.elems;       # 0
my $x = $stack.peek;    # Failure with X::Concurrent::Stack::Empty 
my $y = $stack.pop;     # Failure with X::Concurrent::Stack::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 stack data structure uses a linked list of immutable nodes, the only mutable state being a head pointer to the node representing the stack top and an element counter mintained through atomic increment/decrement operations. The element count updates are not performed as part of the stack update, and so may lag the actual state of the stack. However, since checking the number of elements to decide whether to peek or pop is doomed in a concurrent setting anyway (since another thread may pop the last value in the meantime), this is not problematic. The elems method primarily exists so that the number of elements can be queried once the stack reaches some known stable point (for example, when a bunch of working threads that push to it are all known to have completed their work).

Methods

push(Any $value)

Pushes a value onto the stack. Returns the value that was pushed.

pop()

If the stack is not empty, removes the top value and returns it. Otherwise, returns a Failure containing an exception of type X::Concurrent::Stack::Empty.

peek()

If the stack is not empty, returns the top value. Otherwise, returns a Failure containing an exception of type X::Concurrent::Stack::Empty.

elems()

Returns the number of elements on the stack. This value can only be relied upon when it is known that no threads are pushing/popping from the stack at the point this method is called. Never use the result of elems to decide whether to peek or pop, since another thread may pop in the meantime. Instead, check if peek or pop return a Failure.

Bool()

Returns False if the stack is empty and True if the stack is non-empty. The result can only be relied upon when it is known that no threads are pushing/popping from the stack at the point this method is called. Never use the result of Bool to decide whether to peek or pop, since another thread may pop in the meantime. Instead, check if peek or pop return a Failure.

Seq()

Returns a Seq that will iterate to a snapshot of the stack content, starting from the stack top. The snapshot is made at the time this method is called.

list()

Returns a List that will lazily evaluate to a snapshot of the stack content, starting from the stack top. The snapshot is made at the time this method is called.