Increased memory usage in nodejs version 20.16.0 due to fetch()

[BridgeAR]

Discussed in https://github.com/orgs/nodejs/discussions/54248

^{Originally posted by SteffenHo August 7, 2024}
We saw a massive increase in memory in our next.js application after updating to version 20.16.0. We build our application with docker and the 20-alpine image. Other users were also able to confirm this problem: https://www.reddit.com/r/node/comments/1ejzn64/sudden_inexplicable_memory_leak_on_new_builds/

We then switched to version 20.15.1 and the memory returned to normal.

Maybe someone else could confirm this problem too?

@ronag could you have a look into this?

[avivkeller]

FWIW @snyamathi believes that this is caused by nodejs/undici@63b7794

CC @nodejs/undici

[trivikr]

Possible fix in nodejs/undici#3445

[ronag]

@mcollina @Uzlopak seems related to fetch and finalization.

[mcollina]

First, a memory leak happens when the memory is never released. In the diagram above, the gc is at work in claiming/releasing all the memory. Therefore, this bug is not a memory leak, but a less panicking "increased memory usage" bug.

Second, I would really love to see a reproduction that is not using Next.js, because it monkey-patches fetch()... which might interact badly with the "fire and forget" logic of undici (we need to support fetches that do not consume the request).

[mcollina]

I've done a bit of research on this problem, and I came to these conclusions:

• Next.js monkey patches global fetch() in https://github.com/vercel/next.js/blob/9a1cd356dbafbfcf23d1b9ec05f772f766d05580/packages/next/src/server/lib/patch-fetch.ts#L786-L796.
• Using vanilla undici in the provided example does not show the problem, this is also highlighted in Possible memory leak in Fetch API vercel/next.js#68636 (reply in thread).
• fix: increased memory in finalization first appearing in v6.16.0 undici#3445 improves memory consumption by roughly 10% in my tests without visible issues. I couldn't replicate whether it fixes the problem, but I'll ship a release with this anyway, as it's a good improvement. I don't expect it to fix the problem.

Something in the latest undici releases increased memory consumption for the Next.js cache. This does not affect vanilla fetch() users.

We need a reproduction without Next.js or React.cache() that clearly shows the problem. I couldn't isolate it myself, and I've exhausted the time I could dedicate so far.

I've been trying to follow what their patching does, but I were not able to identify the problem.

[mcollina]

#54286 contains the patched version of undici.

[avivkeller]

#54286 contains the patched version of undici.

That PR has landed, so I've closed this issue. Feel free to reopen if you disagree.

[mistval]

I am encountering this with an Express app (not Next.js) and it appears to be an actual leak in my case. The heap grows until the process crashes with FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory. It crashes eventually even with --max-old-space-size=1000, while on Node 20.15.1, it remains stable permanently with --max-old-space-size=50.

I'll try distilling to a minimal repro case, it sounds like that could still be useful? Or are we likely all set with that PR?

[mcollina]

Open a new issue, this case is very specific to Next.js, so it'll likely be something else.