Memory model

Learn the Kotlin/Native memory model.

The new memory model

Kotlin/Native initially used a strict thread isolation: each thread had its own heap and nothing was shared. The new memory model relaxed that.

In the new model there is relaxed cross-thread sharing and a global stop-the-world garbage collector. This aligns Kotlin/Native behaviour with the JVM. The new model is default in Kotlin 1.9+; old settings like binaryOption("memoryModel", "experimental") are gone. This makes mutableSharedState more natural to write; freezing is largely history.

A Counter example

In the new memory model, a Counter class can be shared across threads. The old model required freezing here; the new one does not.

Example: class Counter(private var n: Int = 0) { fun inc() { n += 1 }; fun get(): Int = n }. In the new model this class can be built in one coroutine and inc() called in another; no freeze() needed. The old model would throw InvalidMutabilityException here. In this lesson's exercise you will write a simple Counter and see the new model's relief.

Exercise Write Counter

Define class Counter(private var n: Int = 0). Method fun inc() should increase n by one; fun get(): Int should return n. In main, build val c = Counter(), call c.inc() twice, and write println(c.get()).

Show example solution

class Counter(private var n: Int = 0) {
    fun inc() { n += 1 }
    fun get(): Int = n
}
fun main() {
    val c = Counter()
    c.inc(); c.inc()
    println(c.get())
}

Test yourself

Match memory model

New model

Shared heap
Old model

Thread isolation
GC

Garbage collector
freeze

Legacy freezing
What did the new memory model remove?
- The freeze requirement
- Coroutines
- Class definitions
- Type inference
Show answer

The freeze requirement — The new model made freezing unnecessary; sharing is now free.

Complete the inc method

class Counter(private var n: Int = 0) {
    fun ____() { n += 1 }
    fun get(): Int = n
}

Show answer

class Counter(private var n: Int = 0) {
    fun inc() { n += 1 }
    fun get(): Int = n
}

Next lesson: Konan

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