In today's context, 100 vusers is the minimum load expectation for any web application. If you want to test for just 100 or 200 users, you just need one machine to generate the load. Each vuser will in turn be a thread or process, running in background, in the same machine where your load testing tool is installed. This machine is usually called controller. Each thread/process will occupy 1MB to 20MB memory space, based on script size and data size and each will consume some amount of cpu and disk. When I need to run 2000 vusers, one single machine is not enough to generate load. Here we need to distribute the load generation process itself.
Imagine each vuser consuming 5MB memory space; if we run 1000 vusers, we will require 5GB just for the vusers alone; over and above OS and other software will consume memory. If we have a machine with 4GB memory, we cannot run 1000 vu from that machine; because, that load generating machine itself will crash. Also, when responses for all 1000 vusers are sent to the same machine, that network port of that machine will choke. Every tool has a facility to generate load from different machines. These are called load generators or load agents. From the controller machine, these load generator machines must be accessible via LAN. A small program needs to be installed on all these machines, called remote agent process.
From controller, we must specify how many users are to be executed from each of the load generator machines. If our total user count is 1000, we can specify 300 from load-gen-1, 400 from load-gen-2 and 300 from load-gen-3. The target server for which we do the load test, must be accessible from all load gens; else scripts will fail. Once the load is distributed to all load gens, when the run starts, the tool will send the scripts to the load generators and instruct those to start the vuser thread/processes in those load gens. Hence, the memory and cpu of the controller will not be consumed. Every 5 or 10 seconds, the status will be sent back to the controller from all load gens.
This helps us in 2 ways. First, it helps us to run large number of vusers using multiple regular desktops/laptops, without a need for high end machines, for the sake of generating load. Second, we can run tests on target server, from remote machine other than where the tool license is installed. You may be in New York, USA, target server may be in Ireland and tool load generator may be in Los Angeles, USA. So, this helps us to do a load test with load being generated at a different geography, not from the same place where the server is installed.
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