Fragger is a heap fragmentation inducer, meant to induce compaction of the heap on a regular basis using a limited amount of CPU and memory resources.

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The Fragger tool aids application testers in inducing inevitable-but-rare garbage collection events, such that they would occur on a regular and more frequent and reliable basis. Doing so allows the characterization of system behavior, such as response time envelope, within practical test cycle times.

Fragger works on the simple basis of repeatedly generating large sets of objects of a given size, pruning each set down to a much smaller remaining live set, and increasing the object size between passes such that is becomes unlikely to fit in the areas freed up by objects released in a previous pass without some amount of compaction. Fragger ages object sets before pruning them down in order to bypass potential artificial early compaction by young generation collectors.

By the time enough passes are done such that the total allocated space roughly matches the heap size (although a much smaller percentage is actually alive), some level of compaction likely becomes inevitable.

Fragger’s resource consumption is completely tunable, it will throttle itself to a tunable rate of allocation, and limit it’s heap footprint to configurable level.

When run with default settings, Fragger will occupy ~25% of the total heap space, and allocate objects at a rate of 20MB/sec. At these settings compaction will usually occur within 2 minutes per GB of heap. Altering the target allocation rate, as well as the heap occupancy ratio and with the number of passes in a compaction-inducing iteration, will change the frequency with which compactions occur, and the CPU percentage consumed by Fragger.

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