Monday, 28 July 2014

Poll me, maybe?

{This post is part of a long running series on lock free queues, checkout the full index to get more context here}
The java.util.Queue interface has the ever important poll/offer methods, documented thus:
This allows the caller to assume that if the poll method returns null the queue is empty, and in a single threaded world this is quite straight forward. A problem for multi producer lock free concurrent queues however is that while an element may not be visible the queue may not be empty. "WOT?" I hear you cry, lets look at an example:
The above snippet shows the offer/poll methods of an MPSC linked queue as originated by Mr. Vyukov(MPSC - Multi-Prodcuer, Single-Consumer) and ported into Java by your humble servant (though others have ported this algo before me: Akka/RxJava/Netty, and others...).
Where be dragons?

Multiple Producers Break The Chain

How the above algorithm works for multiple producers:

  • On line 17 we use the same mechanism offered by AtomicReference.getAndSet to atomically swap the producerNode with the new node.
  • This means no producerNode is ever returned more than once, preventing producer threads from overwriting the producerNode reference field and that node's reference to the next node.
  • We use the same mechanism offered by AtomicReference.lazySet to set this new node as the next node from the previous.
  • On the consumer thread side we process nodes by grabbing the next node and replacing the consumerNode with it after we pluck out it's value.

The problem is in the offer method lines 17-19 where we first xchgProducerNode and then set the new node (now the producerNode) to be the next node after the previous producerNode. For a short moment the old producer node has null as the next element, the chain is broken. This short moment is of undefined length, depending on scheduler pressure and the whimsical sense of humour of the gods the producer thread which got interrupted at line 18  (puff) may be held up for a while.
And yet, while this producer thread is sleeping (what dreams may come?), other producers can make progress. They may add any number of nodes to the queue, each linking the producerNode to the next. The producerNode can be at any 'distance' from that suspended node on that suspended thread waiting for it's next field to be patched through and may have any number of further broken links in the way.
Looking at the poll method in this light the problem becomes more obvious. If a node may have it's next set to null due to the timing described above, then poll may return null when the queue is in fact not empty.

Unbreaking The Chain

To fix this issue we could do the following:

And indeed this is how other implementors have chosen to tackle the issue (in spirit, if not in code).
In doing so we have given up quite allot however:
  1. poll() is no longer wait free. Which is a shame, I quite like wait freedom.
  2. The consumer thread is volunteered into spin waiting on the next field to become visible.
  3. In the event of hitting null we now read the producerNode. This introduces the potential for a cache miss. This is a big problem to my mind as this cache miss has an unknown and potentially very large cost.
  4. The producerNode read from the consumer thread will have a negative impact on the producer threads contending to write to it. This has been previously explored here. This will be particularly bad when the consumer is spinning on the poll() method while waiting for the next value.
Is it worth it? I'm not convinced... Given that a Queue already mandates an isEmpty method could we not settle for a relaxed definition of poll? given the above observation of the queue emptiness is also (by it's lock free and concurrent nature) imprecise should we really sacrifice performance/scalability for it?
At the moment I am leaning towards offering weaker guarantees for the lock-free queues offered as part of JCTools, but I'm hoping to get some feedback from prospective users on how important that part of the queue interface is to their purposes.

NOTE: The same issue exists for the offer method when we look at an SPMC queue, as discussed in this issue.

Tuesday, 8 July 2014

Concurrent Bugs: Size Matters

{This post is part of a long running series on lock free queues, checkout the full index to get more context here}As part of my ongoing work on JCTools (no relation to that awfully popular Mexican dude) I've implemented SPSC/MPSC/MPSC/MPMC lock free queues, all of which conform to the java.util.Queue interface. The circular array/ring buffered variants all shared a similar data structure where a consumer thread (calling the poll() method) writes to the consumerIndex and a producer thread (calling the offer() method) writes to the producerIndex. They merrily chase each other around the array as discussed in this previous post.
Recently I've had the pleasure of having Martin Thompson pore over the code and contribute some bits and pieces. Righteous brother that Mr. T is, he caught this innocent looking nugget (we're playing spot the bug, can you see it?):
What could possibly go wrong?

Like bugs? you'll looove Concurrency!

The code above can return a negative number as the size. How can that be you wonder? Let us see the same method with some slow motion comments:

See it?
  • We need not get suspended for this to happen. The independent loads could have the second one hit a cache miss and get delayed, or maybe the consumer and the producer are just moving furiously fast. The important thing to realise is that the loads are independent and so while there may not be much time between the 2, there is the potential for some time by definition.
  • Because the 2 loads are volatile they cannot get re-ordered.
  • Doing it all on one line, or on 3 different lines (in the same order) makes no difference at all.

So how do we solve this?

Is it really solved?
  • This will at least return a value that is in the correct range.
  • This will NOT ALWAYS return the right or correct number. The only way to get the absolutely correct number would be to somehow read both indexes atomically.
  • Atomically reading 2 disjoint longs is not possible... You can (on recent x86) atomically read 2 adjacent longs, but that is:
    • Not possible in Java
    • A recipe for false sharing
  • We could block the producers/consumers from making progress while we read the 2 values, but that would kill lock freedom.
  • This method is lock free, not wait free. In theory the while loop can spin forever in the same way that a CAS loop can spin forever. That is very unlikely however.
This is as good as it gets baby.


We all know that a thread can get suspended and threads execution can interleave, but it is easy to forget/overlook that when looking at a line of code. This issue is in a way just as naive as incrementing a volatile field and expecting atomicity.
Sometimes there is no atomicity to be found and we just have to come to terms with the best approximation to a good answer we can get...
A further lesson here is that having your code reviewed by others is an amazingly effective way to find bugs, and learn :-)