Does the Cloudflare Workers memory limit include only the JavaScript heap or also WebAssembly memory and ArrayBuffers?

Memory Limit Scope

The Cloudflare Workers memory limit applies to the total RAM usage of the V8 isolate, not just the JavaScript heap. This means that any memory allocated within the isolate's context—including WebAssembly (Wasm) linear memory and ArrayBuffer objects—counts toward the per-isolate limit.

What is Included in the Limit?

Because the runtime enforces a hard cap on the entire isolate footprint, the following components contribute to the memory usage:

• JavaScript Heap: All standard JS objects, closures, and variable allocations.
• WebAssembly Linear Memory: The memory allocated for Wasm modules, which exists outside the JS heap but inside the isolate.
• ArrayBuffers and TypedArrays: Backing stores for binary data.
• Runtime Overhead: Internal V8 and runtime structures required to execute the worker.

Memory Instrumentation API Behavior

The memory-instrumentation API provides an estimate of the total isolate footprint. It does not provide a granular breakdown between the JS heap and Wasm memory. If you allocate a large WebAssembly.Memory instance or a massive ArrayBuffer, you will see the reported memory usage increase accordingly, even if the JS heap remains small.

Verification and Testing

To verify how specific allocations affect your limit, you can use the following scoped approach in a test worker:

1. Isolate Baseline: Log the initial memory usage from the instrumentation API upon worker start.
2. Incremental Allocation: Use a loop to allocate new ArrayBuffer(size) or increase Wasm memory pages.
3. Observation: Monitor the instrumentation API values. If the value increases linearly with the buffer size, it confirms the buffer is counted toward the limit.
4. Termination Point: Continue increasing allocations until the worker is terminated with an Out-of-Memory (OOM) error to identify the actual ceiling for your specific plan.

Assumptions and Uncertainty

This behavior is based on the standard V8 isolate model used by Cloudflare Workers. However, exact limits can vary based on your plan (e.g., Bundled vs. Unbound). Because internal runtime overhead is not explicitly detailed in public documentation, the reported memory may slightly exceed your explicit allocations.

Diagnostic Detail Needed: To provide a more precise limit threshold, please specify whether you are using the Standard or Unbound usage model, as these have different memory ceilings.