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)
    sched_yield();
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.

Comments

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

I real glad to uncover this

yepi250's picture

I real glad to uncover this web internet site on bing, just what I was searching for : D likewise saved to bookmarks.

These articles are greatly

kizi's picture

These articles are greatly appreciated; very useful and informative article and every body must visit this article.

Oakleys Sale,Cheap Sunglasses Sale,Oakley Batwolf

podcfzu8's picture

Today, in addition to the bright colors and unusual shapes, persistent resistance to organic pollutants, capture the image of fashion eyewear comprehensive definition than ever before.Oakley sunglasses for the Asian sports and special living area can be seen as timeless and stunning sunglasses.Sports sunglasses are generally polarized lenses can eliminate interference ability to increase depending on the material, and more lightweight frame materials and styles designed for sports wear.The colors of Oakley sunglasses to protect your eyes, the sun goes down, Oakley sunglasses Lenses fading light will automatically adjust.As time goes by, sunglasses, and gradually become a common part of everyday life, fashion jewelry.Darker skin should choose brighter colors; fair-skinned, generally with what color glasses are very nice.Yellow lenses filter blue light, nature scenery can be seen more clearly. While driving, wearing a yellow lens sunglasses, you can more clearly see from the vehicle.In visible light, violet energy, the red energy minimization. UV contains a lot of natural light, due to the high energy of the ultraviolet rays, so interested in corneal and retinal damage, and therefore the sunglasses would expect prevent ultraviolet.On sunglasses to be safe, or else you exercise, it will follow the action, which you found in the running belt loose the same reaction.Do not be flashy there may be some fashion rimmed glasses and confusion you feel, you must wear a framework to see if this is the most important.Oakley sunglasses, inventions and innovative ideas, world-class scientific research and production equipment, provide a strong support.Polarized lenses is recognized worldwide as the most suitable for driving the lens. The light reflected by the surface polarization glare. The negative effects of glare - Enhanced brightness, color saturation weakened; object contours become blurred, glasses fatigue and discomfort.Fashion sport sunglasses PLUTONITE lens manufacturing materials, polarized Oakley sunglasses block 100% of harmful UV blue light, vision than the United States defined in ANSI industrial standards.Oakley in addition to the characteristics of the population unobtanium their ears grips to prevent slipping and High Definition Optics (HDO), it provides a clear, but also to protect the eyes.Oakley sunglasses at least change the color of the object itself, more real, the advantage of the natural landscape, but also to prevent glare.In accordance with the functional use can be divided into many types of sunglasses point. Ordinary sunglasses, decorated sunglasses, drivers sunglasses.Oakley sunglasses are very useful activities, such as driving, fishing and other sports. The polarized lenses bring better visual clarity.Date night, a copy of jewelry features a lot of fashion critics alike, affordable, because these are a good substitute to platinum, gold, and other jewelry sunglasses.Most girls will be ignored is not a compliment, this is mainly because they like to be called a fashionable and stylish than most people know.Oakley sunglasses is the most appropriate choice! Oakley glasses store, especially the discount may be your choice. To select your favorite, you will not regret it!

Reply to comment | Linux Journal

Psychic's picture

Hi, just read your post. Thought you might want to
know that I found you through Google.

what's the solution for

Anonymous's picture

what's the solution for compile error?

Reply to comment | Linux Journal

plastische chirurgie's picture

I do not even know how I ended up here, but I thought this post was good.

I don't know who you are but definitely you are going to a famous blogger if you aren't already ;) Cheers!

Reply to comment | Linux Journal

sacoche homme's picture

This piece of writing provides clear idea in support of the new viewers
of blogging, that genuinely how to do running a blog.

Reply to comment | Linux Journal

http://best-breastpump.net/'s picture

There's certainly a great deal to know about this topic. I love all the points you made.

compiling

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_;
^

Suggestions?

Fixed

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.

Reply to comment | Linux Journal

free psychological tricks to get your ex back's picture

I am sure this article has touched all the internet viewers, its really really pleasant piece of writing on
building up new blog.

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
link.springer.com/chapter/10.1007%2F3-540-57818-8_53
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".

Misprint

A.Krizhanovsky's picture

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

Reply to comment | Linux Journal

http://www.telephone-fixe.info/'s picture

I do consider all of the concepts you have presented for your post.
They are really convincing and can definitely work. Nonetheless, the posts are very short for beginners.
Could you please lengthen them a bit from subsequent time?

Thanks for the post.

Inaccuracy

alastair's picture

The article claims that the "mfence" in the sequence

A
mfence
B

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).

The mfence will cause memory accesses before the fence to complete before memory accesses after the fence (more accurately, it causes memory accesses before the fence to become globally visible — i.e. their effects are apparent to other cores in the system — before those after the fence).

I would like to thank you for

kizi's picture

I would like to thank you for the efforts you have made in writing this article.Thanks.Would be waiting for more stuff from yourside.

Simplification

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.

Reply to comment | Linux Journal

Minisivarao Short Article Author's picture

bookmarked!!, I like your site!

Feel free to surf to my website; Minisivarao Short Article Author

Reply to comment | Linux Journal

diete rapide's picture

excellent put up, very informative. I'm wondering why the other experts of this sector don't realize this.
You must continue your writing. I am confident, you've a huge readers' base already!

my blog post :: diete rapide

Reply to comment | Linux Journal

thecandidacenter.com's picture

I am sure this post has touched all the internet viewers,
its really really nice piece of writing on building up new weblog.

Reply to comment | Linux Journal

yepi's picture

Currently it sounds like Drupal is the preferred blogging platform available right now.

(from what I've read) Is that what you are using on your blog?

Reply to comment | Linux Journal

werbeagentur blog's picture

I'm really inspired with your writing talents and also with the format to your blog. Is this a paid theme or did you customize it yourself? Anyway keep up the excellent high quality writing, it's rare to see a great weblog like
this one nowadays..

Reply to comment | Linux Journal

older ladies's picture

I for all time emailed this website post page to all my associates, because if like to read it
after that my contacts will too.

Re: Inaccuracy

digitas's picture


A
mfence
B

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.

White Paper
Linux Management with Red Hat Satellite: Measuring Business Impact and ROI

Linux has become a key foundation for supporting today's rapidly growing IT environments. Linux is being used to deploy business applications and databases, trading on its reputation as a low-cost operating environment. For many IT organizations, Linux is a mainstay for deploying Web servers and has evolved from handling basic file, print, and utility workloads to running mission-critical applications and databases, physically, virtually, and in the cloud. As Linux grows in importance in terms of value to the business, managing Linux environments to high standards of service quality — availability, security, and performance — becomes an essential requirement for business success.

Learn More

Sponsored by Red Hat

White Paper
Private PaaS for the Agile Enterprise

If you already use virtualized infrastructure, you are well on your way to leveraging the power of the cloud. Virtualization offers the promise of limitless resources, but how do you manage that scalability when your DevOps team doesn’t scale? In today’s hypercompetitive markets, fast results can make a difference between leading the pack vs. obsolescence. Organizations need more benefits from cloud computing than just raw resources. They need agility, flexibility, convenience, ROI, and control.

Stackato private Platform-as-a-Service technology from ActiveState extends your private cloud infrastructure by creating a private PaaS to provide on-demand availability, flexibility, control, and ultimately, faster time-to-market for your enterprise.

Learn More

Sponsored by ActiveState