Memory Management in Kotlin/Native: The New MM
Explore the new Kotlin/Native memory manager, GC, and object ownership.
Memory Management in Kotlin/Native: The New MM is a free Kotlin Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
The Old Memory Model Problem
Kotlin/Native's original memory model (pre-1.7.20) was "immutable by default": objects had to be frozen before crossing thread boundaries. This made multi-threaded code cumbersome and incompatible with Kotlin coroutines semantics.
The New Memory Manager (1.7.20+)
Since Kotlin 1.7.20, Kotlin/Native uses a new garbage-collected memory model that mirrors the JVM/JS behaviour. Object mutation across threads is allowed by default. The freeze() / isFrozen APIs are deprecated and no-ops.
Garbage Collector Design
The new MM uses a stop-the-world mark-and-sweep GC that runs on a separate GC thread. It aims to minimize pause times and works incrementally. Future releases will continue to refine the GC strategy.
Coroutines and the New MM
With the new MM, kotlinx.coroutines works identically on Kotlin/Native as on the JVM. You can share mutable state between coroutines using the same patterns (e.g., a Mutex-protected mutable field) without freezing.
val mutex = Mutex()
var counter = 0
suspend fun increment() = mutex.withLock { counter++ }Thread Safety in the New MM
The new MM does not provide automatic thread safety — you still need to synchronize shared mutable state manually using Mutex, AtomicInt, AtomicReference, or confine state to a single thread via coroutine dispatchers.
Atomics in Kotlin/Native
Use kotlin.native.concurrent.AtomicInt, AtomicLong, AtomicReference for lock-free atomic operations on primitive values and references:
val counter = AtomicInt(0)
fun increment() { counter.increment() }
fun read(): Int = counter.valueObject Lifecycle and ARC on Apple Targets
On Apple platforms (iOS/macOS), Kotlin/Native uses ARC (Automatic Reference Counting) for interoperability with Objective-C objects. Kotlin-owned objects are managed by the GC; ObjC/Swift objects passed to Kotlin are bridged via ARC-compatible wrappers.
Memory Leaks: What to Watch For
Common Native memory issues: retain cycles between Kotlin objects and ObjC delegates/closures, large object graphs that are not released after use, and long-lived background workers holding references to UI objects. Use Xcode Instruments to profile on Apple targets.
Background Worker Threads
Use kotlin.native.concurrent.Worker for platform-level background threads (or prefer coroutines with a native dispatcher). Each Worker has its own event loop; communicate via execute() which transfers or shares objects according to the new MM rules.
Migrating from the Old MM
Remove all calls to freeze(), ensureNeverFrozen(), and isFrozen. Replace AtomicReference workarounds with plain mutable properties protected by a Mutex if needed. Update kotlinx.coroutines to 1.6.4+ for new MM support.
GC Tuning
The GC can be tuned via system properties or Kotlin/Native binary options. For latency-sensitive apps, experiment with kotlin.native.gc.collectIntervalMillis. For memory-constrained devices, reduce heap size via kotlin.native.gc.heapTriggerCoefficient.
Quick Check
What key change did Kotlin/Native's new memory manager (1.7.20+) introduce?
Recap: Memory Management in Kotlin/Native
Key takeaways:
- New MM (1.7.20+): GC-managed, mutable objects sharable across threads,
freeze()is a no-op - Still need manual synchronization:
Mutex,AtomicInt,AtomicReference - Apple targets use ARC for ObjC interop; Kotlin objects managed by GC
- Coroutines work identically to JVM in new MM
- Remove old
freeze()calls when migrating to new MM
Frequently asked questions
Is the “Memory Management in Kotlin/Native: The New MM” lesson free?
Yes — the full text of “Memory Management in Kotlin/Native: The New MM” is free to read here on the web, and the Kotlin Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Kotlin Academy course, upgrade to CoddyKit PRO.
What will I learn in “Memory Management in Kotlin/Native: The New MM”?
Explore the new Kotlin/Native memory manager, GC, and object ownership. You practise Kotlin Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Kotlin Academy?
No prior experience is required. Kotlin Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Memory Management in Kotlin/Native: The New MM” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Kotlin Academy lesson?
Yes. Every Kotlin Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Kotlin/Native Overview: Targets, Toolchain & Compilation
- Memory Management in Kotlin/Native: The New MM
- C Interop with cinterop and .def Files
- Kotlin/WASM: Compiling to WebAssembly for the Browser