What is Software reliability? Why reliability is more important than efficiency

Software reliability is the function of the number of failures experienced by a particular user of that software. A software failure occurs when the software is executing.

Reliability is more important than efficiency by the following reason:

        i.            Computers are now cheap and fast: There is little need to maximize equipment usage. Paradoxically, however, faster equipment leads to increasing expectations on the part of the user so efficiency considerations cannot be completely ignored.

     ii.            Unreliable software is liable to be discarded by use: If a company attains a reputation for unreliability because of single unreliable product, it is likely to affect future sales of all of that company’s products.

   iii.            System failure costs may be enormous: For some application , such a reactor control system or an aircraft navigation system, the cost of system failure is orders of magnitude greater than the cost of the control system.

   iv.            Unreliable systems are difficult to improve: It is usually possible to tune an inefficient system because most execution time is spent in small program sections. An unreliable system is more difficult to improve as unreliability tends to be distributed throughout the system.

      v.            Efficiency is predictable:  Program takes long time to execute and users can adjust their work to take this into account. Unreliability, by contrast, usually surprises the user.

Unreliable system may cause information loss: Information is very expensive to collect and maintains; it may sometimes be worth more than the computer system on which it is processed.