Testing of the app/server performance cannot be done properly without a right tools. There is many load or stress test tools so let's pick at least this two:
Apachebench is available out of the box with Apache.
ab is a tool for benchmarking your Apache Hypertext Transfer Protocol (HTTP) server. It is designed to give you an impression of how your current Apache installation performs. This especially shows you how many requests per second your Apache installation is capable of serving.
It is operated from command line. Opensource.
http://httpd.apache.org/docs/2.2/programs/ab.html
The Apache JMeter™ desktop application is open source software, a 100% pure Java application designed to load test functional behavior and measure performance. It was originally designed for testing Web Applications but has since expanded to other test functions.
On image: Examples of load test visualisation using graph in JMeter
JMeter simulates the concurrent users nicely, can export logs to CSV,...
Opensource, written in JAVA. Has UI which allows to set up load tests for non-programmers.
https://jmeter.apache.org
