Lock-Free Multi-Producer Multi-Consumer Queue on Ring Buffer

Table 1. Two-Thread Scenario

Thread 1 Thread 2
read tail_ read tail_
read head_ read head_
(scheduled) push an element
push an element

If we have only one free place in the ring buffer, we override the pointer to the oldest queued element. We can solve the problem by incrementing the shared head_ before the loop and use a temporal local variable (that is, we reserve a place to which we're going to insert an element and wait when it is free):

unsigned long tmp_head =
    __sync_fetch_and_add(&head_, 1);
while (tail_ + Q_SIZE < tmp_head)
ptr_array_[tmp_head & Q_MASK] = x;

We can write similar code for pop()—just swap head and tail. However, the problem still exists. Two producers can increment head_, check that they have enough space and re-schedule at the same time just before inserting x. A consumer can wake up instantly (it sees that head_ moved forward to two positions) and read a value from the queue that was not inserted yet.

Before solving the issue, let's consider the following example, where we have two producers (P1 and P2) and two consumers (C1 and C2):

             LT                          LH
| _ | _ | _ | x | x | x | x | x | x | x | _ | _ | _ |
              ^   ^                       ^   ^
              |   |                       |   |
              C1  C2                      P1  P2

In this example, "_" denotes free slots and "x" denotes inserted elements. C1 and C2 are going to read values, and P1 and P2 are going to write an element to currently free slots. Let LT be the latest (lowest) tail value among all the consumers, which is stored in tmp_tail of the latest consumer, C1 above. Consumer C1 currently can work on the queue at the LT position (that is, it is in the middle of fetching the element). And, let LH correspondingly be the lowest value of tmp_head among all the producers. At each given time, we cannot push an element to a position equal to or greater than LT, and we should not try to pop an element at a position equal to or greater than LH. This means all the producers should care about the current LT value, and all consumers should care about the current LH value. So, let's introduce the two helping class members for LH and LT:

volatile unsigned long last_head_;
volatile unsigned long last_tail_;

Thus, we should check for the last_tail_ value instead of tail_ in the loop above. We need to update the values from multiple threads, but we're going to do this via plain write operations, without RMW, so the members do not have to be of the atomic type. I just specified the variables as volatile to prevent their values from caching in local processor registers.

Now the question is who should update the last_head_ and last_tail_ values, and when. We do expect that in most cases, we are able to perform push and/or pop operations on the queue without a wait. Thus, we can update the two helping variables only when we really need them—that is, inside the waiting loop. So when a producer realizes that it cannot insert a new element because of a too-small last_tail_ value, it falls into the wait loop and tries to update the last_tail_ value. To update the value, the thread must inspect the current tmp_tail of each consumer. So we need to make the temporal value visible to other threads. One possible solution is to maintain an array of tmp_tail and tmp_head values with the size equal to the number of running threads. We can do this with the following code:

struct ThrPos {
    volatile unsigned long head, tail;

ThrPos thr_p_[std::max(n_consumers_, n_producers_)];

where n_consumers_ is the number of consumers, and n_producers_ is the number of producers. We can allocate the array dynamically, but leave it statically sized for simplicity for now. Many threads read the elements of the array, but only one thread with a plain move instruction (no RMW operation) can update them, so we also can use regular reads on the variables.

Because thr_p_ values are used only to limit moving of the current queue pointers, we initialize them to the maximum allowed values—that is, we do not limit head_ and tail_ movements until somebody pushes or pops into the queue.


Alexander Krizhanovsky is the software architect and founder of NatSys-Lab. Before NatSys-Lab, he worked as a Senior Software Developer at IBM, Yandex and Parallels. He specializes in high-performance solutions for UNIX environments.


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what's the solution for

Anonymous's picture

what's the solution for compile error?

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John Ellson's picture

Doesn't compile for me using: g++ (GCC) 4.8.1 20130603 (Red Hat 4.8.1-1)

The error messages fill a screen, but the first few lines are:

lockfree_rb_q.cc: In member function ‘void NaiveQueue::push(T*)’:
lockfree_rb_q.cc:72:31: error: capture of non-variable ‘NaiveQueue::head_’
cond_overflow_.wait(lock, [&head_, &tail_]() {
lockfree_rb_q.cc:98:17: note: ‘long unsigned int NaiveQueue::head_’ declared here
unsigned long head_, tail_;
lockfree_rb_q.cc:72:39: error: capture of non-variable ‘NaiveQueue::tail_’
cond_overflow_.wait(lock, [&head_, &tail_]() {
lockfree_rb_q.cc:98:24: note: ‘long unsigned int NaiveQueue::tail_’ declared here
unsigned long head_, tail_;



A.Krizhanovsky's picture

John, thank you for the bug report!

I've fixed compilation errors for GCC 4.8. Please, fetch the new version of the code from GitHub.

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Temporal variable

Anonymous's picture

Aside: I believe I am the registered user rhkramer (at least, I used to be) but this posting thingie wouldn't let me use that name--I think if someone at LJ looks it up, they'll see that my email and the email of registered user rhkramer are the same.

Do you really mean temporal variable or do you just mean a temporary variable.

I had never heard of a temporal variable before I read this article, then I did some googling to find it.

In looking at a page of 10 google hits, I then investigated 3 or 4 of those. At least one of them definitely simply meant temporary variable, and, at the time I looked at the article, it used the phrase temporary variable. I'm guessing that at the time google indexed the article it might have said temporal variable--but there were no remaining instances of temporal in the article. OTOH, maybe google decided that I meant temporary and used temporary instead of temporal in the query.

One hit on the page of hits did give me some hints as to what might be meant by a temporal variable:

On the semantics of (Bi)temporal variable databases - Springer
Numerous proposals for extending the relational data model to incorporate the
temporal dimension of data have appeared during the past several years.

I guess my point is (especially as an old guy trying to keep up with some of this stuff), is that it sure would help if terminology didn't change unnecessarily. If the variable in this article truly is something more or different than a temporary variable, fine (but then please provide a definition or a pointer to a definition), but, if it is no different, then just please use "temporary variable".


A.Krizhanovsky's picture

Yes, sure, I did mean temporary variable, not temporal variable. Thank you for the indication of the error.

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The article claims that the "mfence" in the sequence


will cause the instruction "A" to execute before the instruction "B".

This is untrue (and is explicitly contradicted by the Intel manuals, which state categorically that “mfence does not serialize the instruction stream”; i.e. the instructions can still execute out of order).

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A.Krizhanovsky's picture

This was just a simplification for gentle introduction to memory ordering and when and why barriers are used. Unfortunately, the article has limited size, so there is no opportunity to carefully and fully describe this and some other interesting points.

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Re: Inaccuracy

digitas's picture


If effects of the instruction A and B are to be visible outside the processor core they must somehow access the memory (or to be precise maybe the cache). The article explains inter core or inter processors relations, so IMHO the explanation in the article is a little simplification but it is not inaccurate.