Home > Articles > Operating Systems, Server > Linux/UNIX/Open Source

POSIX Parallel Programming, Part 1

  • Print
  • + Share This
In the world of multi-core processors, it's important to write parallel code to take advantage of the available power. In the first of a series of three articles, David Chisnall takes a look at some of the standard ways of doing this on UNIX-like platforms.
Read Part 2 in the POSIX programming series here.
Like this article? We recommend

Like this article? We recommend

As I write this article, almost every new CPU has two cores. AMD and Intel are rushing to put four cores on a die; Sun is already shipping chips that can execute 32 threads at once. These days, fast code means parallel code. (See my article "The Future of CPUs: What’s After Multi-Core?" for more on upcoming CPU designs.)

Writing parallel code isn’t much more difficult than writing serial code. A lot of the time, parallelism isn’t even required, and when it is, often algorithms are available that map nicely to a parallel approach. This article describes a few of the ways of approaching the problem on a *NIX platform.

If you’re starting from scratch, choosing the correct language for the project can give you an immediate benefit. A language like Erlang, which is designed for concurrency, can enable you to write code that’s scalable to about 1,000 nodes. Unfortunately, this luxury is uncommon, so for the remainder of this article I’ll assume that the project must be completed in C.

There Is No Spoon

Traditionally, concurrency is implemented in UNIX by multiple processes. These are created with the fork(2) system call. The origins of the fork(2) mechanism date back to a time before computers had the necessary hardware for proper multitasking. A single process at a time would be memory-resident. When context-switching, the entire contents of memory would be written out, and another process image read in.

In this kind of system, creating a new process by forking is a logical mechanism. You already have a copy of the process image in memory; all you need to do is save another copy out-of-core. For modern systems, the process is slightly less sensible. In a modern operating system, the kernel must iterate across the process’ page table and mark each page as copy-on-write (CoW). Then, every time either process writes to a page, it will cause an interrupt and vector into the kernel, which will copy the page.

Consider the following piece of code, which populates an array with random numbers and then calculates the average:

#include <stdlib.h>
#include <stdio.h>

const unsigned int ARRAY_SIZE = 0x500000;

void populate_array(int* array, unsigned int elements)
{
    for(unsigned int i=0 ; i<elements ; i++)
    {
        array[i] = random();
    }
}

int average_array(int* array, unsigned int elements)
{
    long long total = 0;
    for(unsigned int i=0 ; i<elements ; i++)
    {
        total += array[i];
    }
    return total / elements;
}

int main(void)
{
    int * array = calloc(ARRAY_SIZE, sizeof(int));
    populate_array(array, ARRAY_SIZE);
    printf("Average: %d\n", average_array(array, ARRAY_SIZE));
    return 0;
}

Note that this code is for example purposes only and contains a few issues, such as a potential overflow, that would make it unsuitable for production use (even if you were ever going to need to find the average of a set of random numbers in the real world). Now, consider what we could do to parallelize this code. To make it run on two processors, we could split the array in half and generate/average each in, and then average the averages to get the result.

Since our processes don’t share an address space, however, we have a slight problem in getting the average value from our child processes to the parent. Fortunately, POSIX has a simple mechanism to allow us to do exactly that. When a process returns, either by calling exit(3) or by returning from the main() function, the return value is kept by the operating system and can be retrieved by using the wait(2) system call. wait(2) takes a pointer to an integer and stores the result here. The return value is the process ID (pid) of the child to terminate. These days, it’s quite uncommon to use wait(3); it has been superseded largely by waitpid(3), wait3(3), and wait4(3). We’ll use it in this example, though, because it has a simple interface:

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

const unsigned int ARRAY_SIZE = 0x500000;
const unsigned int PROCESSES = 2;

void populate_array(int* array, unsigned int elements)
{
    for(unsigned int i=0 ; i<elements ; i++)
    {
        array[i] = random();
    }
}

int average_array(int* array, unsigned int elements)
{
    long long total = 0;
    for(unsigned int i=0 ; i<elements ; i++)
    {
        total += array[i];
    }
    return total / elements;
}

int main(void)
{
    unsigned int array_size = ARRAY_SIZE / PROCESSES;
    //Fork off PROCESSES children:
    for(unsigned int child=0 ; child<PROCESSES ; child++)
    {
        pid_t pid = fork();
        if(pid == 0)
        {
            //We are in a child process
            int * array = calloc(array_size, sizeof(int));
            populate_array(array, array_size);
            exit(average_array(array, array_size));
        }
    }
    //Array to store child results in:
    int child_averages[PROCESSES];
    for(unsigned int i=0 ; i<PROCESSES ; i++)
    {
        wait(&child_averages[i]);
    }
    printf("Average: %d\n", average_array(child_averages, PROCESSES));
    return 0;
}

The majority of this program is the same as the single-process variant. The differences lie primarily in the main() function. Here, we begin by forking off all of the child processes. Note the test statement after the fork(2). The fork(2) function is called once, but returns twice. In the parent process, it returns the pid of the child. In the child process, it returns 0. This program checks whether we’re in the child; if we are, it allocates a smaller array and returns the average by using exit(3).

  • + Share This
  • 🔖 Save To Your Account

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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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