Home > Articles > Programming > Graphic Programming

Introducing OpenCL

Over the past decade, graphics cards have gone from being simple accelerators to being fast general-purpose computing engines. David Chisnall looks at OpenCL, a new API for running non-graphics applications on modern GPUs.
Like this article? We recommend

Like this article? We recommend

The first desktop graphics cards were frame buffers. They provided a region of memory where you could write color values, and a digital-to-analog converter that generated a signal for a CRT. A modern GPU is a very different beast. Something like the Radeon HD 5800 has 2.15 billion transistors—more than a six-core Xeon.

Whereas the older graphics hardware was only usable for graphics, a modern GPU is a powerful computing engine in its own right. It makes sense to consider using it for tasks other than handling graphics. This is where OpenCL fits in. Like OpenGL, it's an abstract API that hides the details of the GPU implementation. Unlike OpenGL, however, OpenCL is intended for arbitrary computations, rather than just graphics.

In this article, we'll take a look at the concepts behind OpenCL and create a simple program (implementing Conway's Game of Life) that uses OpenCL. Open the syntax-highlighted opencl.c.html file if you want to follow along as we go through the code.

Understanding the GPU

GPUs are often said to be special-purpose processors, with the implication that CPUs are general-purpose processors. This designation is misleading. A modern CPU and a modern GPU are both general-purpose processors; both can implement any algorithm. But they're not built with the same design goals.

In theory, you could run any program on a GPU, just as you can run a complete OpenGL stack on the CPU. It might not run fast, however, because both the CPU and GPU are heavily optimized toward a certain kind of instruction stream. CPUs expect a lot of integer computations and a lot of branches. The code that runs on a CPU expects a branch around every seven instructions, on average. In contrast, a GPU expects very few branches and a lot of floating-point arithmetic.

The CPU and the GPU also have very different memory models: A GPU typically doesn't have memory protection and is designed for streaming. A CPU expects a lot of locality of reference, so it has a big cache for storing the program's working set. The GPU expects you to read from one blob of memory and write to another; for example, reading from vertex lists and textures and writing a picture. Therefore, you can't just recompile your application for a GPU and expect it to run faster. You must select parts that will run efficiently on a GPU and design them carefully.

The OpenCL Model

OpenCL really describes two things:

  • A C99-derived language, called OpenCL C, which is designed to be easy to compile for the GPU
  • A library designed for compiling this code and running it

The most important unit of code in OpenCL is the kernel. A kernel is a self-contained program that runs on some data. Typically, an implementation will run several copies of the kernel in parallel. Kernels are equivalent to public library functions, with some extra constraints on their concurrency support.

Our example for this article will implement Conway's Game of Life, with a kernel that calculates what the next value for a cell should be. Conceptually, this kernel runs in parallel on every single cell in the input. In practice, the OpenCL stack may run it entirely on one CPU core, run 128 copies of it concurrently on a GPU, and so on.

When you write your kernel, you need to remember this fact. A number of instances of the kernel might run on the same input concurrently, so you have to make sure that they don't interact with each other. To make it easier to prevent interaction, you typically will separate input and output completely, which is what we'll do in this example. Our kernel will take pointers to input and output boards as arguments, read nine cells from the input, and write one cell in the output. The OpenCL stack can run one instance of this kernel at a time, one instance per cell in the board concurrently, or anything in between. This statement is a slight oversimplification, of course. The OpenCL memory model allows kernels to lock regions of memory and enforce synchronization, but with some quite complex limitations.

Commands are sent to OpenCL via queues, a form of implicit serialization. If you tell OpenCL to read some data, run a kernel on it, and then read back the results, all of these actions will happen in the background. Typically, you start them going and then wait for the last one—generating the output—to complete.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020