From both an academia and industry point of view, the software aging phenomenon has increased. The main focus has been to understand its effects from a verifiable observation and theoretical understanding.
"Programs, like people, get old. We can't prevent aging, but we can understand its causes, take steps to limit its effects, temporarily reverse some of the damage it has caused, and prepare for the day when the software is no longer viable."
In software engineering, software aging is the lifespan of software. As the software gets older it becomes less immune and will eventually stop functioning as it should, therefore rebooting or reinstalling the software can be seen as a short term fix. A proactive fault management method to deal with the software aging incident is software rejuvenation. This method can be classified as an environment diversity technique that usually is implemented through software rejuvenation agents (SRA).
Memory bloating and leaking, along with data corruption and unreleased file-locks are particular causes of software aging.
Proactive management of software aging
Software failures are the more likely cause of unplanned systems outages rather than hardware failures. This is because software exhibits over time an increasing failure rate due to data corruption, numerical error accumulation and unlimited resource consumption. In widely used and specialised software, a common action to clear a problem is rebooting because aging occurs due to the complexity of software and that it is never free of errors. It is almost impossible to fully verify that a piece of software is bug-free. This can be related to consumer software such as Windows and Mac OSX softwares receiving timely updates to improve performance and bug fixes. This situation can also be seen as a drawback to software aging as software development tends to be time-to-market-driven, this means software companies are meeting the short-term goal for market needs, yet do not account well for long-term ramifications such as reliability. Designing software that can be immune to aging is unlikely if not impossible. Not all software will age at the same time as some users use the system more so then others.
To prevent crashes or degradation a software rejuvenation can be employed proactively as inevitably aging leads to failures in software systems. This happens by removing the accumulated error condition and freeing up system resources, some examples to clean the internal state of the software are to flush operating system kernel tables, garbage collection, reinitialise internal data structures and a well know example of rejuvenation is a system reboot.
Multinational telecommunications corporation, AT&T have implemented software rejuvenation in billing applications. In the real time system collecting billing data in the United states for most telephone exchanges.
Different systems have also implemented the software rejuvenation method which are:
- Transaction processing systems
- Web servers
- Spacecraft systems
A program can request temporary memory from the system when it is required to store information during execution. However, the system has a limited amount of memory in total and therefore if one application is using a large amount of free memory then other applications won't be able to obtain the memory they require. In low memory conditions the system usually functions slower however, applications become unresponsive and ones which regularly request large amount of memory unexpectedly crash. The responsibility to "free" dynamically requested memory lies with the application when they have finished using it and therefore available for another application when needed. Memory leak happens when the application has dynamically allocated memory but not freed the memory when finished using it. This then doesn't allow the system or any other application to use the memory which hasn't been re-allocated causing the system to eventually run out of memory as memory leaks add up over time. In windows, explorer plug-ins and long running processes like services can impact the reliability of the operating system and in some cases make the system unusable. This would require a reboot to put it back to a functioning state.
Software rejuvenation helps with memory leaks as it involves terminating an application and thus a clean internal state is the outcome of an immediate restart. The two methods for implementing rejuvenation are:
- Time based rejuvenation
- Prediction based rejuvenation
Memory bloating occurs due to garbage collection, which is the form of automatic memory management. For example the .NET Framework's manages the allocation and release of memory for software running on a modern OS such as Windows. .Net based web services has several types of memory such as stack, unmanaged and managed heap (free space). An example is when an operating system occurs a problem because it ran out of physical memory and the OS starts swapping unmanaged heap. The common language runtime allocates memory form the managed heap every time a new object is created, thus will continue to allocate space for new objects. On the other hand, the garbage collection must carry out a collection of data to free up some memory as memory is not infinite. As allocations are being made, the garbage collection that has an optimized engine performs a collection for objects in the managed heap as they are no longer being used by the application, therefore reclaiming their memory. As a result, OS which extensively swap heap can lead to garbage collection cycles extended from milliseconds to tens of seconds. This results to usability problems with applications with loaded caches due to memory bloating.
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- More papers on Proceedings of Workshop of Software Aging and Rejuvenation (WoSAR'08,'10, '11, '12, '13, '14) at IEEE Xplore.