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How fragmentation occurs
The pinned objects effectively deny the GC the ability to combine free space during heap compaction; this can result in a heap that contains a lot of free space but in relatively small, discrete amounts, so that an allocation that appears to be well below the total free heap space fails. When the request fails, we need to run a full GC compaction to free up memory. During the compaction, processing in the JVM comes to a halt. The more frequently we run compactions the larger the degradation of performance.