Home > Articles > Programming > Java

This chapter is from the book

The Load Generators

Load generators (part of a performance test tool, as discussed in Chapter 8) generate the test environment "traffic." A performance test requires hundreds or thousands of users to simulate production conditions. The load generator uses prewritten test scripts to simulate the users and their activities at the web site. Generating large numbers of "virtual users" requires the right supporting equipment. In the field, we often find test teams running expensive, state-of-the-art performance test tools on old, underpowered PCs retrieved from storage. Without sufficient supporting equipment, the best tools on the market cannot generate sufficient load to properly test the web site. If the load generator cannot provide sufficient load to the web site, the web site never achieves the target throughput. Often the test team misreads the symptoms of this condition and spends weeks tuning the web site when, in fact, they need to add capacity to the load generator machines.

Likewise, the test team needs a network analysis of the traffic generated by each client machine. This includes inbound and outbound requests, just as we discussed earlier in this chapter. The traffic generated by the test tools sometimes overloads the network subnet supporting the client machines. Also, the traffic burden sometimes overloads the network cards in individual client machines.

In short, take the load generator environment seriously. The load generator requires more capacity than most teams originally estimate. If the client machines reach 75% CPU utilization or the network traffic passes the safety threshold, increase capacity on these devices. You cannot drive load against your web site if the load cannot make it to the intended servers.

Master/Slave Configurations

Many industrial performance tools use the master-slave test configuration, shown in Figure 9.6. The "master" machine manages each test and collects data. The "slaves" actually manage the threads and sockets representing the virtual users, and run the corresponding test scripts. For extremely small configurations (10 to 20 virtual users), the master and slave both run on the same machine. However, for larger tests, the master runs on a different machine, with one or more slaves also running on different boxes.

Figure 9.6 Figure 9.6 Master/slave load generator configuration


The best practices to remember with a master/slave configuration include the following:

  • Keep traffic between the master and slaves to a minimum. Many performance test tools allow the test manager to define the frequency with which updates and status travel between the slave and the master. If you're using a new performance test tool for this first time, use a protocol analyzer to determine the actual network burden on the client subnet.

  • The master (also known as the "controller" or "coordinator" machine) often requires less CPU capacity than the slave machines. Conversely, if it stores reports for the test cluster, it may require a larger hard drive than a slave machine.

  • Watch the CPU on all test machines during the performance test. If CPU utilization exceeds 75%, the machine needs more capacity.

  • Watch the logs and hard disk on the client machines. Often the clients accumulate large log files and pass these logs back to the master after the test completes. Frequent logging, of course, increases the disk I/O burden for the client machine and impacts testing. Likewise, if these logs accumulate over time, the client machines may not have enough room for subsequent runs. Before starting a run, make sure the client machines contain sufficient free disk space for any logging they may perform.

  • Recycle the machines frequently. The slaves and the controller sometimes throw odd errors or stop responding after several testing cycles. In general, we find it best to recycle the test machines once or twice a day during periods of extended testing.

  • Try to keep the test slave hardware homogenous. Often, because of load balancing techniques, one or two client machines may drive all the load a server in a clustered environment receives. If the test cluster contains one machine significantly more powerful than the others, some servers in the web site cluster may not achieve full loading.

  • It's often useful to simulate traffic from a number of different client IP addresses, especially when performing a test that utilizes an IP sprayer in front of several HTTP servers. Web sites often configure IP sprayers for affinity routing between the incoming user and a particular sever in the cluster.

You need sufficient test client machines, NICs, and supporting performance test software to make IP affinity work during your testing. See Chapter 8 for more details.

After the Performance Test

Many companies treat the performance test environment as a transient entity. The environment exists for a few weeks to test the performance and scalability of the web site. Afterward, the team pulls the test environment apart and uses the components to build other test environments or sends the equipment to production. This works well for most test scenarios, but major production web sites often require a permanent, separate test environment. A permanent test environment allows you to

  • Safely test new features and bug fixes prior to their introduction into the production web site.

  • Recreate problems seen on the production site without using production resources. Because of the multi-threaded nature of web applications, some problems only appear under load conditions. You need a reliable test environment to find these problems.

At a minimum, keep enough test client capacity to stress at least one production server. These clients may reside either with the test environment, if one exists, or may be a part of the production environment. Ideally, configure these clients to drive load in either the production or test environment, if needed.

Few companies dedicate test machines to a particular web site or web application, but this increases another risk: If you run into a problem in production, you don't have machines set aside where you can immediately try to reproduce the situation. The problem determination cycle often takes much longer when you can only work with the problem in production. Having available machines dedicated to testing and problem determination often makes it simpler and cheaper to recreate and debug production problems.

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