Generational GC

Learn generational GC and Scavenge.

Generational GC

Generational garbage collection splits objects by age into two generations: young and old. New objects start in the young generation; those that survive a few collection cycles are promoted to the old generation. The young generation is collected often, the old one rarely.

A shouldPromote(survivalCount) function models the promotion decision: it returns true when the survival count is 2 or more. Example: shouldPromote(1) is false, shouldPromote(3) is true. This explains with a simple threshold when V8 moves an object from young to old. Real V8 thresholds are more complex but the idea is the same: frequently surviving objects promote into the rarely collected old generation. This reduces garbage collection cost.

The survival count

An object's survival count tells how many garbage collection cycles it has outlived. Each cycle a surviving object's count increases; once past the threshold it promotes to the old generation. This keeps hot data out of the fast-collected young generation.

Example: shouldPromote(1) returns false; the object is still young. shouldPromote(3) returns true; the object promotes to old. This model shows the core logic of generational GC with a simple comparison: survivalCount >= 2. In the old generation objects live longer and are collected less often; this boosts overall GC efficiency. Most objects die young; that is why young collection is fast.

Exercise Write the promotion function

Write function shouldPromote(survivalCount). Return survivalCount >= 2. Print console.log(shouldPromote(1)) and console.log(shouldPromote(3)).

Show example solution

function shouldPromote(survivalCount) {
  return survivalCount >= 2;
}
console.log(shouldPromote(1));
console.log(shouldPromote(3));

Test yourself

Complete the promotion threshold

return survivalCount >= ____;

Show answer

return survivalCount >= 2;

Match each concept to its description

Young generation

New objects
Old generation

Surviving objects
Survival count

Number of survivals
shouldPromote

Promotion decision
What does shouldPromote(1) return (threshold 2)?
```
function shouldPromote(s) { return s >= 2; }
shouldPromote(1)
```
- true
- 2
- 1
- false
Show answer

false — 1 < 2 so it returns false.

Next lesson: Mark-sweep

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