$env:NODE_OPTIONS="--max-old-space-size=SIZE" adds 48MB for no reason

[Martin-Holos]

Version

17.3.0

Platform

x64

Subsystem

No response

What steps will reproduce the bug?

PS C:> $env:NODE_OPTIONS="--max-old-space-size=8196"
PS C:> node -e 'console.log(v8.getHeapStatistics().heap_size_limit)'
8644460544

How often does it reproduce? Is there a required condition?

every time

What is the expected behavior?

heap-size should be set to 8196MB (8GB)

What do you see instead?

8644460544 bytes (8244MB)

Additional information

The LTS version has a similar problem but with 24bytes, which breaks deserialization for 4096

[nponeccop]

JFYI 8 GiB = 8192 MiB, not 8196. The observed difference is exactly 48 MiB for some reason though.

8 GiB = 8 * 2^30 = 8192 * 2^20 = 8,589,934,592 bytes
8196 MiB = 8196 * 2^20 = 8,594,128,896 bytes

8644460544 - 8594128896 = 50,331,648 bytes = 48 MiB (exactly)

[lamweili]

I have the same issue on NodeJS 14.19.1.

• node --max-old-space-size=2048 -e 'console.log(v8.getHeapStatistics().heap_size_limit - (2048 * 1024 * 1024))'
50331648

• node --max-old-space-size=4096 -e 'console.log(v8.getHeapStatistics().heap_size_limit - (4096 * 1024 * 1024))'
50331648

• node --max-old-space-size=8192 -e 'console.log(v8.getHeapStatistics().heap_size_limit - (8192 * 1024 * 1024))'
50331648

The difference, 50331648 / 1024 / 1024, is 48 MiB (exactly).

[bnoordhuis]

Converted to a discussion because this is working as intended, not a bug.

The garbage collector is a generational collector, i.e., there are two spaces: the young space (a.k.a. the nursery) and the old space. --max_semi_space_size=.. controls the former, -max-old-space-size=.. of course the latter.

--max_semi_space_size=.. defaults to 16 MB but it's tripled because conceptually there is more than one nursery. That's where the extra 48 MB comes from.

[lamweili]

Indeed, I traced through and found the same explanation in the codes and tests.

node/deps/v8/src/heap/heap.cc

Lines 220 to 225 in d65514b

	size_t Heap::MaxReserved() {
	const size_t kMaxNewLargeObjectSpaceSize = max_semi_space_size_;
	return static_cast<size_t>(2 * max_semi_space_size_ +
	kMaxNewLargeObjectSpaceSize +
	max_old_generation_size());
	}

node/deps/v8/test/unittests/heap/heap-unittest.cc

Line 82 in d65514b

// The expected value is old_generation_size + 3 * semi_space_size.

[lamweili]

Perhaps the documentation needs to be clearer that the v8.getHeapStatistics().heap_size_limit is using the formula above.
Might be closely related to the following documentation improvements as well:

• doc: clarify default heap size formula since node 12 #39107
• Increase default 'max_semi_space_size' value to reduce GC overhead in V8 #42511
  • PR doc: add new useful V8 option #42575

[bnoordhuis]

Sure, a pull request would be welcome.