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The process of simulating hundreds of virtual users and collecting their statistics requires considerable computing power. In order to simulate enough users to fully load a sophisticated web server configuration, more than one computer will be required. Web Performance TrainerTM allows multiple playback engines to cooperate to generate large amounts of load.
The Web Performance TrainerTM Playback Engine is a subset of the Web Performance TrainerTM software that runs only the components required to playback business cases. It has a limited user interface (text output only) primarily intended for reporting errors. Because there is no complex user interface running, a dedicated playback engine makes most efficient use of computing resources for generating virtual users. It also allows the engine to run on headless workstations, such as a Unix platform without X Windows. In the recommended usage scenario, one computer running Web Performance TrainerTM will be used to control multiple Web Performance TrainerTM Playback Engines. The engines will generate load against the web server and report the resulting statistics back to Web Performance TrainerTM.
Starting a playback engine
Starting a playback engine is similar to starting Web Performance TrainerTM. On Windows platforms, there is a menu item labeled "playback engine" in the same location as the item for starting Web Performance TrainerTM. This will start a console window. When the playback engine has finished initialization, a message will appear in the console window reading "Playback Engine started." Pressing Ctrl-C will stop the playback engine and close the window.
The Web Performance TrainerTM is started on Linux, Solaris and Unix platforms by passing the 'engine' parameter to the normal startup script:
Connecting to a playback engine
Once the playback engines have been started, they will automatically be discovered by Web Performance TrainerTM
and displayed in the "Playback Engines" section at the bottom of the Playback Tab. However, there are certain network configurations that prevent
WPTrainer from auto-discovering the engines. When this occurs, the following procedure can be used to manually connect the playback engines:
When the playback tab in Web Performance TrainerTM is selected, the Edit menu will show an item labeled "Add
Playback Engine". Selecting this menu item will show the "Add Playback Engine" dialog box. Enter the name or IP address of the computer on which
the playback engine is running into this dialog and press the OK button.
If Web Performance TrainerTM connects successfully to the playback engine, the engine will appear in the "Playback Engines" section at the bottom of the Playback tab:
Running a test on multiple playback engines
Running a test on multiple playback engines is similar to running a test with only the local engine. When a test is started, Web Performance TrainerTM will automatically allocate new virtual users to the playback engines and collect the statistics from those engine when the test completes. However, due to the additional work that Web Performance TrainerTM must perform to coordinate and monitor multiple playback engines, we recommend that the local playback engine ("localhost") be removed from the playback engine list. This can be done by selecting the engine and then using the Edit menu to delete it. The local engine may be re-activated by entering "localhost" into the Add Playback Engine dialog.
Runtime feedback
While the test is running, Web Performance TrainerTM will show the number of users that have been allocated to each engine as well as an estimate of how many users the engine is capable of supporting. In general, this estimate is not very accurate when there are only a few users running but becomes very accurate as the capacity of the engine is reached. The engines also monitor their memory and processor usage. These parameters are reported as a percent utilization as well all as a text description (Normal, High etc.).
More detailed statistics, such as those shown on the Statistics tab, are not gathered from remote engines until the test completes.
Load balancing
When a new virtual user is allocated, Web Performance TrainerTM will determine which engine should run that virtual user. If either the CPU or memory utilization of the engine has exceeded the 'normal' range (80%) then that playback engine will not be a candidate for receiving another virtual user until the parameter returns to the normal range. After eliminating these engines, Web Performance TrainerTM will allocate the virtual user to the remaining engine with the lowest number of virtual users.
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