After what moment does the garbage collector realize it needs to perform a collection?

The Garbage Collector in JVM decides to perform a collection based on various factors, including:

• Lack of free memory (Heap): When JVM tries to allocate memory for a new object, but there is not enough free space. This is the most common reason.
• Reaching memory limit: If JVM has almost exhausted the memory allocated to it.
• Configurable thresholds: Some garbage collectors can be triggered when a certain percentage of memory is occupied (e.g., -XX:GCTimeRatio).
• Explicit calls to System.gc() or Runtime.getRuntime().gc(): These methods suggest a garbage collection, but do not guarantee immediate execution. JVM may ignore this call if it deems it unnecessary at the moment.
• Low-level events: Some GCs can be triggered by a timer (in low activity mode) or in response to other internal JVM signals.

Specific algorithms and conditions for triggering depend on the type of garbage collector (Serial, Parallel, G1, Shenandoah, ZGC, etc.) and its configuration. The main principle is that the collection occurs when JVM considers it necessary or most efficient to maintain stable operation and prevent OutOfMemoryError.