Choosing ARC or ORC for Deterministic Memory Management in Nim

Deterministic Memory Management with ARC and ORC

Nim's legacy garbage collector introduces unpredictable pause times. Switching to ARC (Automatic Reference Counting) or ORC (Objective Reference Counting) makes object destruction happen immediately when the reference count reaches zero, removing stop-the-world pauses and improving C/C++ interoperability.

Choosing Between ARC and ORC

ARC reclaims memory as soon as references disappear, but it leaks if objects form a reference cycle. ORC adds a cycle collector that periodically frees such islands, making it suitable for general-purpose code.

Configuration Example

Compile your Nim source with the desired memory model using the --mm flag.

# ARC without cycle collection
nim c -d:release --mm:arc main.nim

# ORC includes cycle collection
nim c -d:release --mm:orc main.nim

Where to run: In a terminal, in the directory containing main.nim. Permissions: Standard user rights are sufficient. Risk: Using --mm:arc with circular references will cause silent memory leaks that grow over time.

Worked Code Example

The following program prints a message when an object is destroyed, demonstrating deterministic timing.

type
  Resource = ref object
    name: string

proc __destroy(r: Resource) =
  echo "Destroying resource: ", r.name

proc runTask() =
  let res = Resource(name: "TemporaryBuffer")
  echo "Task is running..."
  # res goes out of scope here

echo "Starting task..."
runTask()
echo "Task completed."

Expected output (with either --mm:arc or --mm:orc):

Starting task...
Task is running...
Destroying resource: TemporaryBuffer
Task completed.

With the legacy GC the destructor might run much later, after a heap collection or at program exit.

Limits and Common Pitfalls

• Unsafe pointers: Using ptr or manual alloc/dealloc bypasses ARC/ORC; you must manage memory yourself.
• Circular references under ARC: Without ORC, reference cycles never reach zero, causing leaks.
• Legacy GC assumptions: Some older Nim libraries rely on the GC’s non-deterministic finalization; they may need updates or explicit cycle breaking.
• Global destruction order: While per‑object destruction is deterministic, the order of global variable destructors is not guaranteed; avoid critical cleanup in them.

Verifying the Choice

Run the program under Valgrind to check for leaks.

nim c -d:release --mm:arc main.nim
valgrind --leak-check=full ./main

If Valgrind reports "definitely lost" memory, inspect the code for reference cycles. Re‑compile with --mm:orc; the leak should disappear if it was caused by a cycle.