Garbage Collection: Young Vs Old generation

#java #garbage #interview #development

Garbage collection?? Huhh it is just a small topic. wait what???? 😮😮

Then what are these terms

Young Generation, Old Generatio, Eden, Mark and sweep etc.

1. The Beginning: Empty Memory
Initially, our program's memory (the heap) is empty. We divide it into two main sections:

Young Generation (Green): Where new objects are born. It's often smaller because most objects die quickly.

Old Generation (Blue): Where objects that have proven to live a long time go. This area is typically larger.

Young generation structure:

Eden Space: Where all new objects initially go.

Survivor Space 1 (S1): Objects surviving an Eden collection are moved here.

Survivor Space 2 (S2): Objects are moved between S1 and S2 in subsequent collections.

Collection Type: The GC event that collects this area is called a Minor GC.

Process:

When Eden fills up, a Minor GC is triggered.

Objects that are still reachable (in use) in Eden and the active Survivor space are copied to the other Survivor space.

Objects that have survived a certain number of Minor GCs (known as the age threshold or tenuring threshold) are promoted (moved) to the Old Generation.

The Eden and the initially active Survivor space are then completely cleared, making the process very fast.

Old Generation (Tenured)
This area holds objects that have survived many Minor GCs in the Young Generation and have been promoted (or tenured). These are the long-lived objects.

Collection Type: The GC event that collects this area is called a Major GC (or part of a Full GC, which usually collects the entire heap, including the Young Generation).

Frequency: It is collected much less frequently than the Young Generation because the objects here are more likely to still be in use.

Algorithm: Major GC often uses algorithms optimized for reclaiming memory with a lower proportion of dead objects, such as** Mark-Sweep-Compact** (or variations like Concurrent Mark-Sweep or Garbage-First). These algorithms are generally slower but more thorough, as running them frequently would defeat the purpose of the generational model.

🔄 Promotion/Tenuring

Promotion is the process where a long-lived object is moved from the Young Generation to the Old Generation.

Each time an object survives a Minor GC, its "age" counter is incremented.

Once the object's age reaches a specific tenuring threshold (which can be configured, often between 1 and 15), it is promoted to the Old Generation during the next Minor GC.

This generational approach significantly improves GC performance by:

Reducing overhead: Applying a fast, copy-based algorithm (like Copying Collector) frequently to the small Young Generation (where most garbage is created).

Infrequently accessing the large Old Generation: Only running the slower, more complex collection algorithms when necessary for the Old Generation.