Which GC strategy reduces latency spikes under concurrent async workloads in Nim?

The Optimal GC Strategy

To reduce stop-the-world latency spikes in concurrent async workloads, the ARC/ORC (Automatic Reference Counting / Ownership Reference Counting) memory management strategy is the most effective choice. Unlike the default Boehm GC, which uses a mark-and-sweep approach that pauses execution to scan the heap, ARC/ORC reclaims memory incrementally as reference counts drop to zero.

Mechanism and Performance Impact

In a high-concurrency async environment, the Boehm GC's pause times typically scale with the size of the live heap, leading to the observed latency spikes. ARC/ORC shifts this overhead from a single massive event to a distributed cost integrated into the program's execution flow. For most async web servers, --mm:orc is preferred over --mm:arc because ORC includes a cycle collector capable of reclaiming memory from reference cycles, which are common in complex async state machines.

Implementation Steps

1. Compile with ORC: Use the --mm:orc flag during compilation to enable Ownership Reference Counting.
2. Verify Memory Safety: Ensure that any C-interop code explicitly manages pointer lifetimes, as ARC/ORC is more aggressive in reclaiming memory than a tracing GC.
3. Profile Latency: Compare the maximum pause times of the ORC build against a baseline build using --mm:gc under a simulated load of 1,000+ concurrent requests.

Assumptions and Trade-offs

It is assumed the project is using a recent version of Nim (1.4+), where ARC/ORC has reached a stable enough state for production consideration. While ORC significantly reduces maximum pause times, it may slightly decrease overall throughput due to the constant overhead of updating reference counts. Additionally, while --gc:arcStats:on provides internal metrics, its consistency across different operating systems is not guaranteed; system-level profiling tools (such as Valgrind or OS-specific timers) are recommended for definitive verification.

Missing Diagnostic Detail: Are you utilizing any third-party C libraries that maintain long-lived pointers to Nim-managed memory? This would necessitate specific pragmas to prevent premature reclamation under ARC/ORC.