Crate coco [] [src]

Concurrent collections.

This crate offers several collections that are designed for performance in multithreaded contexts. They can be freely shared among multiple threads running in parallel, and concurrently modified without the overhead of locking.


The following collections are available:

  • Stack: A lock-free stack.
  • deque: A lock-free work-stealing deque.

Which collection should you use?

Use a Stack when:

  • You want a simple shared collection where objects can be insert and removed.
  • You want to avoid performance degradation due to locking.
  • You want the first-in first-out order of elements.

Use a deque when:

  • You want one thread inserting and removing objects, and multiple threads just removing them.
  • You don't care about the order of elements.

Garbage collection

An interesting problem concurrent collections deal with comes from the remove operation. Suppose that a thread removes an element from a lock-free map, while another thread is reading that same element at the same time. The first thread must wait until the second thread stops reading the element. Only then it is safe to destruct it.

Programming languages that come with garbage collectors solve this problem trivially. The garbage collector will destruct the removed element when no thread can hold a reference to it anymore.

This crate implements a basic garbage collection mechanism, which is based on epochs (see the epoch module). When an element gets removed from a concurrent collection, it is inserted into a pile of garbage and marked with the current epoch. Every time a thread accesses a collection, it checks the current epoch, attempts to increment it, and destructs some garbage that became so old that no thread can be referencing it anymore.

That is the general mechanism behind garbage collection, but the details are a bit more complicated. Anyhow, garbage collection is designed to be fully automatic and something users of concurrent collections don't have to worry about.


pub use stack::Stack;



A lock-free work-stealing deque.


Epoch-based garbage collection.


A lock-free stack.