Performance Testing, LoadRunner Tips&Tricks

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General: Maximum amount of Vusers generated by a single load generator

How many vusers can a Load Generator generate?
There is a straight forward answer in this KB article, KB 11808, "How many Vusers can run on a given computer" from Mercury/HP. Also, you can refer to the following article, KB 3629, "Recommended hardware and OS tuning for optimal Vuser performance" to get information of recommended hardware for vuser performance. As per metioned in the articles, the question of maximum load relates closely to the CPU and Memory on a given hardware.

Before we dwell deeper into the technical details, it is best to ensure that common blunders are removed, such as, (1) the Controller have sufficient license to run the amount of vusers stated in the Scenario. (2) Furthermore, ensure that there are no limits enforced on the Load Generators with are configured in the Scenario Settings.

To start with, we must understand what happens during a scenarion execution. Refer to General: Scenario Execution for an understanding of the scenarion execution. Furthermore, when vusers are executed in a scenario, the vusers are actually running in memory instead of the disk. That is why memory is an important factor to consider.
Also, the article, KB 10291, "Vuser Memory requirements - memory footprints" provides a list of memory footprint of different types of protocols. This is only a measurement of a simple script of its kind but it can be used to estimate roughly the amount of load generated on a given hardware specification.
For Citrix, SAP GUI and RTE, it is closely related to GDI resources. GDI resources is a hard-coded value in the OS and cannot be changed. For more information, you can refer to the Microsoft article on GDI Objects. To monitor GDI resource, you can refer to KB 30684, "How to monitor the GDI resources used".
If you like to maximize the amount of load that can be generated, you can refer to the following article, KB 3645, "How to increase the number of Vusers you can run on a Windows Machine" by Mercury/HP. However, this is not applicable to GDI resources-related protocols.
WIth all suggestions above, the best bet to determine the amount of load generated, is to monitor the Load Generators itself (instead of the machines under load in a load test).

Take note that the above Mercury/HP links required a valid login to their support site. Please apply an account with them first before you click on them.
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2 Responses to “General: Maximum amount of Vusers generated by a single load generator”

  1. # Blogger Ben Simo

    There is no straight forward answer to this question. Mercury does try to give some answers, but the truth is that is depends on the script, the application under test, and the duration of the test. There are no rules of thumb. If there were, then there would be little need for products like LoadRunner.

    As you state, the best way to determine how many vusers can be run on a piece of hardware is to monitor it just like we monitor resources on the systems under test.

    We need to monitor memory and CPU usage on the injector. We also need not attempt to put more data out of or pull more data into a single box or network than the bandwidth supports.

    I have some scripts that I can run over 1000 web vusers on a single 3ghz system with 2gb of RAM. I have other scripts that only support 50 vusers on the same hardware. It depends on the script.

    Also, a major memory leak was introduced in one of the v8.1 feature packs. This memory leak shows up in some scripts and not in others. I've spent dozens of hours investigating this with Mercury and the source has not yet been identified. This memory leak creates a new requirement -- in some cases -- for more injectors for tests of longer duration than are required for shorter tests.

    My bottom line feeling on this is that anyone that seeks a single one-size-fits-all answer to the question of how many load generators are required should not be leading any load testing. They should not be designing tests. They should not be analyzing results. They may be able to safely script what other people have designed.

    The question is really asking how much load a load generator can support without impacting performance. This is one of the major questions that load testing attempts to answer.

    Even new load testers quickly learn (if they are paying attention) that even minor changes in activity within an application can impact performance and the load generated on the system under test.

    There are no one-size-fits-all answers. That's why we test. Understanding this should be requirement #1 for selecting any load tester.

    Ben Simo  

  2. # Blogger Hs

    Hi Ben,

    Thanks for the commend. I found your article relating to the same topic a good one for those who required additional reading (and confirmation). The link as follows.


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