When is garbage collector invoked

It's rare for an instance object to be extremely large. You can configure the threshold size for objects to go on the large object heap.

Garbage collection primarily occurs with the reclamation of short-lived objects. To optimize the performance of the garbage collector, the managed heap is divided into three generations, 0, 1, and 2, so it can handle long-lived and short-lived objects separately. The garbage collector stores new objects in generation 0. Objects created early in the application's lifetime that survive collections are promoted and stored in generations 1 and 2.

Because it's faster to compact a portion of the managed heap than the entire heap, this scheme allows the garbage collector to release the memory in a specific generation rather than release the memory for the entire managed heap each time it performs a collection. Generation 0. This is the youngest generation and contains short-lived objects. An example of a short-lived object is a temporary variable. Garbage collection occurs most frequently in this generation.

Newly allocated objects form a new generation of objects and are implicitly generation 0 collections. However, if they are large objects, they go on the large object heap LOH , which is sometimes referred to as generation 3.

Generation 3 is a physical generation that's logically collected as part of generation 2. Most objects are reclaimed for garbage collection in generation 0 and don't survive to the next generation.

If an application attempts to create a new object when generation 0 is full, the garbage collector performs a collection in an attempt to free address space for the object. The garbage collector starts by examining the objects in generation 0 rather than all objects in the managed heap. A collection of generation 0 alone often reclaims enough memory to enable the application to continue creating new objects.

Generation 1. This generation contains short-lived objects and serves as a buffer between short-lived objects and long-lived objects. After the garbage collector performs a collection of generation 0, it compacts the memory for the reachable objects and promotes them to generation 1.

Because objects that survive collections tend to have longer lifetimes, it makes sense to promote them to a higher generation. The garbage collector doesn't have to reexamine the objects in generations 1 and 2 each time it performs a collection of generation 0.

If a collection of generation 0 does not reclaim enough memory for the application to create a new object, the garbage collector can perform a collection of generation 1, then generation 2. Objects in generation 1 that survive collections are promoted to generation 2. Generation 2. This generation contains long-lived objects. An example of a long-lived object is an object in a server application that contains static data that's live for the duration of the process.

Objects in generation 2 that survive a collection remain in generation 2 until they are determined to be unreachable in a future collection. Objects on the large object heap which is sometimes referred to as generation 3 are also collected in generation 2. Garbage collections occur on specific generations as conditions warrant. Collecting a generation means collecting objects in that generation and all its younger generations.

A generation 2 garbage collection is also known as a full garbage collection, because it reclaims objects in all generations that is, all objects in the managed heap. Objects that are not reclaimed in a garbage collection are known as survivors and are promoted to the next generation:.

When the garbage collector detects that the survival rate is high in a generation, it increases the threshold of allocations for that generation. The next collection gets a substantial size of reclaimed memory.

The CLR continually balances two priorities: not letting an application's working set get too large by delaying garbage collection and not letting the garbage collection run too frequently. Because objects in generations 0 and 1 are short-lived, these generations are known as the ephemeral generations. Ephemeral generations are allocated in the memory segment that's known as the ephemeral segment.

Each new segment acquired by the garbage collector becomes the new ephemeral segment and contains the objects that survived a generation 0 garbage collection.

The old ephemeral segment becomes the new generation 2 segment. The size of the ephemeral segment varies depending on whether a system is bit or bit and on the type of garbage collector it is running workstation or server GC. Every item that the developer uses is treated this way, including class objects, static variables, and even the code itself.

As long as an object is being referenced, the JVM considers it alive. Once an object is no longer referenced and therefore is not reachable by the application code, the garbage collector removes it and reclaims the unused memory. As simple as this sounds, it raises a question: what is the first reference in the tree? Every object tree must have one or more root objects. As long as the application can reach those roots, the whole tree is reachable.

But when are those root objects considered reachable? Special objects called garbage-collection roots GC roots; see Figure 2. Figure 2. To determine which objects are no longer in use, the JVM intermittently runs what is very aptly called a mark-and-sweep algorithm. As you might intuit, it's a straightforward, two-step process:.

Garbage collection is intended to remove the cause for classic memory leaks: unreachable-but-not-deleted objects in memory. However, this works only for memory leaks in the original sense. It's possible to have unused objects that are still reachable by an application because the developer simply forgot to dereference them. If not your whole program is halted while global cleaning up occurs.

That turns out to very bad response times, but happens rarely for modern JVM's on modern computers. If you are curious about what happens in your program and when, then investigate the "jvisualvm" tool in the recent versions of the Java 6 JDK. It is really great for peeking inside. The garbage collector runs when it needs resources and on a regular basis that you able to influence by telling when is a good time to spend CPU on collecting, using System.

You can help the garbage collector by nulling references explicitly, for instance by giving your objects init methods that allocate resources and cleanup methods that explicitly clean up those resources and nulling their references.

By nulling references yourself you prevent tye garbage collector from having to find clusters of obects that have to more paths to a root. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams?

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Asked 12 years, 1 month ago. Active 4 years ago. Viewed 56k times. Improve this question. Add a comment. Active Oldest Votes. Other JVM's are of course free to pick whichever strategy they like. See below for more details: The 1. Improve this answer. Pascal Thivent k gold badges silver badges bronze badges. See ibm. Goose no, when garbage is identified, it is freed. A generational GC means that the GC only checks for certain objects whether they are garbage. It frequently checks if any newly created objects generation 0 can be freed.

Remember, GC may be a stop-the-world event, and should be as short as possible. Finalizer methods may be slow, so they should not be called during GC. At a high level simplified , it operates as follows see JLS If object has a finalizer method, add it to the finalizer queue. The object is finalizable.

If the object is reachable from an finalizable object, leave it. The object is finalizer-reachable. On next GC cycle, objects with a finalizer method that is marked finalized is treated as unreachable , and memory will be reclaimed assuming finalizer method didn't make the object reachable again. Note that many GC cycles may occur while an object is finalizable , i. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams?

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