In order to get a stable and performing production environment, you may have to tweak some of the default RoboServer parameters. We will look at the following configuration options:
Number of RoboServer instances
Number of concurrent robots
Automatic memory overload detection
RoboServer runs on Oracle's Java Virtual Machine (JVM), this in turn runs on top of an operating system (OS), which runs on top of your hardware. JVM's and OS's are patched, hardware architecture change, and each new iteration aims to bring better performance; so, although we can give some general guidelines about performance, the only way to make sure you have the optimal configuration is to test it.
As a general rule you get a little more performance by starting two instances of RoboServer. The JVM uses memory management known as garbage collection (GC). On most hardware only a single CPU core is active during GC, which leaves 75% of the CPU idle on a quad-core CPU. If you start two instances of RoboServer, one instance can still use the full CPU while the other in running GC.
The amount of concurrent robots a RoboServer can run depends on the amount of CPU available, and how fast you can get the data RoboServer needs to process. The number of concurrent robots is configured in the Management Console cluster settings. A robot running against a slow website will use a lot less CPU than a robot running against a website with a fast response time, and here is why. The amount of CPU used by a program can be described with the following formula
CPU (core)% = 1 - WaitTime/TotalTime
If a robot takes 20 seconds to execute, but 15 seconds are spent waiting for the website, it is only executing for 5 seconds, thus during the 20 seconds it is using an average of 25% (of a CPU core). The steps in a robot are executed in sequence, which means that a single executing robot will only be able to utilize one CPU core at a time. Most modern CPUs have multiple cores, so a robot that executes in 20 seconds, but waits for 15 seconds, will in fact only use about 6% of a quad-core CPU.
By default RoboServer is configured to maximally run 20 robots concurrently. The number of concurrent robots is configured in the Management Console cluster settings. If all your robots use 6% CPU, the CPU will be fully utilized when you are running 16-17 robots concurrently. If you start 33 of these 6% robots concurrently, you will overload RoboServer; because the amount of CPU available is constant, the result is that each robot will take twice as long to finish. In the real world the CPU utilization of a robot may be anywhere between 5-95% of a CPU core, depending on robot logic and the website it interacts with. As a result it is hard to guess or calculate the correct value for the max concurrent robots, the only way to be sure you have the right value is to do a load test and monitor the RoboServer CPU utilization, as well as the robot runtime as load increases.
Another parameter that may affect the number of concurrent robots each RoboServer can handle is the amount of memory. The amount of memory used by robots can vary from a few megabytes (MB) to hundreds of MB. By default RoboServer is configured with 1024MB of memory, this is often not enough if you are actually executing 20 robots concurrently; check Changing the RAM Allocation to see how to control memory allocation. If you don't provide enough memory to RoboServer, you run the risk of crashing it with an out of memory error. The easiest way to ensure proper memory allocation is to monitor memory utilization during your load tests. If you allocate 2048MB memory to a RoboServer, the JVM will not allocate all of it up front, but it will reserve it from the OS (this is why allocating more than 1200MB frequently fails on 32bit Windows). Once the JVM starts to use the memory, it will not be given back to the OS. To find the optimal memory allocation you often have to run a series of load tests that push the CPU to 100%. After each test is complete, you check how much of the reserved memory was actually used by the JVM (the java.exe process). If all 1024MB (default) were used, increase (usually double) the memory and run the test again. At some point the JVM will not have used all of the reserved memory, and whatever it did use reflects the actual memory requirement and should be used to configure RoboServer.
Since RoboServer will crash if it runs out of memory, RoboServer tries to prevent this from occurring.
Before RoboServer starts a new robot it will check the memory utilization. If it is above 80% it will queue the
instead of starting it; this greatly reduces the risk of crashing RoboServer if the memory allocation is configured
This mechanism is often referred to as the 80% memory threshold. The threshold value is configurable through the