[lttng-dev] Coding blog: lockless queues

[lttng-dev] Coding blog: lockless queues

[Mathieu Desnoyers]

Hi Rusty!

I've stumbled on your blog post: http://rusty.ozlabs.org/?p=317

and I find that you looked at the RCU lock-free queue implementation in
the Userspace RCU project, but not at the wait-free concurrent queue
implementation we have (wait-free enqueue, blocking dequeue), which does
not require RCU at all and might be a much better fit for what you are
trying to achieve.

A quick pointer to the implementation:

git://git.lttng.org/userspace-rcu.git  master branch
files:

urcu/wfcqueue.h (public API)
urcu/static/wfcqueue.h (API implementation)
(https://git.lttng.org/?p=userspace-rcu.git;a=blob;f=urcu/static/wfcqueue.h;h=4b3535aaa83207a4f4eb801dc63150bef8f28bbc;hb=HEAD)

Some nice features of this wfcqueue is to allow this:

- enqueue is wait-free (uses xchg() and store),
- splice (a kind of dequeue_all) from queue is only needs mutual
  exclusion with other dequeue, but not with other splice. We call it
  "blocking" because it may have to busy-wait if enqueue is preempted
  between xchg and store,
- synchronization needed when iterating on the queue (so we wait for
  store to complete if we reach a node being enqueued) is performed
  at the very last moment where it is needed: this is why we provide
  "first" and "next" iterators.
- we also support "dequeue", to dequeue nodes one by one. The dequeue
  operation needs mutual exclusion against other dequeue acting on that
  queue (and against first/next iterators).

I'd be very interested to hear feedback from you on this implementation.

Thanks!

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

[lttng-dev] Coding blog: lockless queues

[Rusty Russell]

Mathieu Desnoyers <mathieu.desnoyers at efficios.com> writes:
> Hi Rusty!
>
> I've stumbled on your blog post: http://rusty.ozlabs.org/?p=317
>
> and I find that you looked at the RCU lock-free queue implementation in
> the Userspace RCU project, but not at the wait-free concurrent queue
> implementation we have (wait-free enqueue, blocking dequeue), which does
> not require RCU at all and might be a much better fit for what you are
> trying to achieve.

Clearly, I stopped reading too early!  It's a very nice
implementation...

If I revisit this, I'll do a code review and post it on my blog.

Thanks!
Rusty.