What is Performance testing for software applications?

Performance testing is required in every field. Without doing some validation for performance testing, quality and success cannot be said to be achieved. Similarly in the field of computer science and engineering, performance testing in software applications is of great importance. Performance testing is done to find out the execution speed and time of the program, and to ensure its effectiveness. Software performance testing basically involves some quantitative tests that can be performed (in a computer lab for example), number of millions of instructions per second (MIPS) and measurement of response time. It also involves some tests for qualitative aspects such as scalability, interoperability and reliability.
Stress testing is carried out simultaneously with performance testing. So finally we can define software performance testing as a testing in software engineering that is done to find out the measure of some qualitative or quantitative aspect under a specific workload. Sometimes, it is also used to relate other quantitative and qualitative aspects such as usage of resources, scalability and reliability. Software performance testing is a concept of performance engineering which is very essential to build good software.
Performance testing consists of many sub testing genres. Few have been discussed below:
1. Stress testing: This testing is done to determine the limits of the capacity of the software application. Basically this is done to check the robustness of the application software. Robustness is checked against heavy loads i.e., to say above the maximum limit.
2. Load testing: This is simplest of all the testings. This testing is usually done to check the behavior of the application software or program under different amounts of load. Load can either be several users using the same application or the difficulty level or length of the task. Time is set for task completion. The response timing is recorded simultaneously. This test can also be used to test the databases and network servers.
3. Spike testing: This testing is carried out by spiking the particular and observing the behavior of the concerned application software under each case that whether it is able to take the load or whether it fails.
4. Endurance testing: As the name suggests the test determines if the application software can sustain a specific load for a certain time. This test also checks out for memory leaks which can lead to application damage. Care is taken for performance degradation. Throughput is checked in the beginning, at the end and at several points of time between the tests. This is done to see if the application continues to behave properly under sustained use or crashes down.
5. Isolation testing: This test is basically done to check for the faulty part of the program or the application software.
6. Configuration testing: This testing tests the configuration of the application software application. It also checks for the effects of changes in configuration on the software application and its performance.

Before carrying out performance testing some performance goals must be set since performance testing helps in many ways like:
1. Tells us whether the application software meets the performance criteria or not.
2. It can compare the performance of two application soft wares.
3. It can find faulty parts of the program.
There are some considerations that should be kept in mind while carrying out performance testing. They have been discussed below:
1. Server response time: This is the time taken by one part of the application software to respond to the request generated by another part of the application. The best example for this is HTTP.
2. Throughput: Can be defined as the highest number of users who use concurrent applications and that is expected to be handled properly by the application.

Good book on performance testing on Amazon (link).