JavaScript heap out of memory: read the GC line before you raise the heap size
You know the moment. You reopen a long-running session in your AI coding CLI and instead of your conversation you get a wall of garbage-collector spam that ends like this: FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory Aborted (core dumped) Exit code 134, nothing partial to salvage, session unusable. The standard internet answer is "raise…
When a long-running Node.js session encounters an out-of-memory error, the garbage collector may begin printing a series of compaction messages. These messages provide crucial diagnostic information about why the JavaScript heap has reached its limit. Instead of immediately raising the heap size, it is essential to analyze these messages to determine the root cause of the issue.
V8, the JavaScript engine used by Node.js, caps the heap size at roughly half of the available physical memory, with a minimum default of 2 GB and a maximum of 4 GB on 64-bit systems with sufficient RAM. However, this cap is not a hard limit, and issues can arise even when the heap appears to have ample space.
To diagnose the problem, inspect the last few compaction messages, which indicate how much memory was reclaimed during the process. For example, a message showing "Mark-Compact 4062.4 (4098.2) - 4061.5 ( 4097.5 ) MB" reveals that only about a megabyte of memory was reclaimed out of a total of four gigabytes, suggesting that the heap is indeed full.
Common reasons for this error include retaining large amounts of objects that are still in use, such as cached file contents or session history. These retained objects prevent the garbage collector from reclaiming enough memory to expand the heap. To address this, consider starting a new session with fresh data or pruning the stored session history to reduce the memory footprint.
In some cases, the issue may stem from native memory usage rather than the JavaScript heap. This can occur when other parts of the application, such as native extensions or large files, consume significant memory. In these instances, increasing the heap size is not an effective solution, as the underlying problem lies outside the JavaScript runtime.
When troubleshooting this error, start by examining the compaction messages and heap usage before making any changes to V8 flags. If the garbage collector is unable to reclaim memory during compaction, consider increasing the heap size with the `--max-old-space-size` flag, keeping in mind the available physical memory and the potential impact on system resources. However, always verify that the issue is not related to native memory usage or other factors before resorting to this approach.
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