SnapQueue

Lock-Free Queue with Constant Time Snapshots

Aleksandar Prokopec / @alexprokopec

Single-Shot Queues

• head - position of the first element in the queue
• last - position less than or equal to the first empty entry

Single-Shot Queues - Enqueue

Single-Shot Queues - Dequeue

Correctness

• Every array entry is initially null, then some element, and finally the special value REMOVED
• The numbers in head increase monotonically from 0 until the array length

Eventual Correctness

• Assuming no contention, last eventually points to the first empty entry (eventually fast)
• Eventually, the reference to the removed element is released (eventual reclamation)

Single-Shot Queue Freeze Operation

SnapQueue

SnapQueue Enqueue

SnapQueue Enqueue

Support data structure is any immutable data structure into which we can push single-shot queues.

The support data structure must be a sequence, i.e. it must retain order.

SnapQueue Dequeue

Dequeue must occasionally steal the support data structure from the enqueue (right) side.

SnapQueue Dequeue

SnapQueue Dequeue

Left support structure non-empty, no need to steal.

SnapQueue Dequeue

What about snapshots?

It's hard to get a consistent view of a fast-moving object...

...unless you can freeze it.

SnapQueue Freeze

How to atomically prevent subsequent enqueue and dequeue operations?

SnapQueue Freeze

SnapQueue Freeze

SnapQueue Freeze

SnapQueue Freeze

SnapQueue Freeze

The freeze operation is as good as a snapshot.

After a freeze, the previously mutable data structure becomes an immutable data structure.

SnapQueue Snapshot

SnapQueue Concatenation

SnapQueue Concatenation

If we can find an immutable data structure that:

• has efficient push-right and pop-left operations
• has efficient concatenation

Then we have an efficient lock-free concatenation operation.

SnapQueue Concatenation

Conc-Trees as the support data structure:

• O(1) pop-left and push-right
• O(log n) concatenation

More information in the paper.

Performance - 1 Thread Enqueue

Performance - N Thread Enqueue

Performance - 1 Thread Dequeue

Performance - N Thread Dequeue

Performance - Segment Length

Thank you!