Dynamic V8 Heap Resizing for Kubernetes In-Place Scaling (--max-old-space-size-percentage)

What is the problem this feature will solve?

Node.js supports percentage-based heap configuration via --max-old-space-size-percentage. However, this flag currently calculates V8's maximum old space size only once at process startup based on initial system or cgroup memory limits.

Kubernetes supports in-place resource resizing (InPlacePodVerticalScaling), which alters container memory limits dynamically at runtime without restarting the container. Because --max-old-space-size-percentage evaluates only at startup, it cannot react to runtime cgroup changes:

1. Memory Upscaling: If Kubernetes increases a running container's memory limit, Node.js remains restricted to the initial V8 heap limit calculated at startup, failing to utilize the newly available RAM.
2. Memory Downscaling: If Kubernetes decreases the container's memory limit below the initial V8 heap allocation, the container risks being terminated by the Linux Out-Of-Memory (OOM) killer before V8 triggers garbage collection or lowers its heap boundary.

To fully support zero-downtime Kubernetes in-place resizing, --max-old-space-size-percentage needs to dynamically recalculate and apply heap limits when cgroup memory parameters change at runtime.

What is the feature you are proposing to solve the problem?

Extend --max-old-space-size-percentage to actively monitor cgroup memory limit changes and adjust the V8 heap size dynamically during process execution.

Example Workflow

# Enable percentage-based allocation
node --max-old-space-size-percentage=80 app.js

1. Initialization: Container starts with a 2 GiB memory limit. Node.js evaluates --max-old-space-size-percentage=80 and sets V8 max old space to ~1.6 GiB.
2. Kubernetes Event: Kubernetes resizes the running pod in-place from 2 GiB to 4 GiB memory limit.
3. Event Detection: Node.js detects the cgroup change event.
4. Runtime Update: Node.js recalculates 80% of 4 GiB (~3.2 GiB) and adjusts V8's isolate limits dynamically while the application continues processing traffic.

What alternatives have you considered?

No response

首先实现 --max-old-space-size-percentage 及其读取的 cgroup 内存参数，然后检查如何在运行时更新 V8 isolate 限制。针对 Kubernetes InPlacePodVerticalScaling 以及内存扩容和缩容两种情况验证设计；完成标准是在运行时限制发生变化后，无需重启即可重新应用该百分比。