Why cannot v8/nodejs allocate a max-size array if sufficient memory?

Question

I'm aware the maximum length of a JavaScript array is 2^32-1, but regardless of how much memory I permit it will still fail to allocate. What's going on? One-line repro:

node --max-old-space-size=100000 -e "console.log(v8.getHeapStatistics().total_available_size);Array.from({ length: Math.pow(2, 32)-1 })"

For clarity the above example exposes 100GB to v8, which is confirmed via total_available_size. 8GB might be sufficient for testing this operation since most people won't have 100GB, but it'll fail regardless. Is this a bug in V8?

jmrk · Accepted Answer · 2022-01-17 21:14:24Z

4

(V8 developer here.)

No, it's not a bug, just an implementation-defined limit. V8 has an internal limit of ~1GB for any individual heap object. At 4 bytes (with pointer compression) per element, that means a maximum of ~256M elements per array.

There are plenty of other implementation-defined limits, e.g. for TypedArray length, string length, BigInt size, recursion depth, number of parameters per function, number of entries in a Map, etc. Pretty much everything has a limit, even if the JavaScript spec doesn't mention it explicitly, or mentions a theoretical limit that's different from implementation limits (which are implicitly assumed to exist).

answered Jan 17, 2022 at 21:14

jmrk

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Aaron Over a year ago

That's a bummer not being able to utilize available memory, but I appreciate the detailed answer!

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