src: initial enablement of IsolateGroups

[jasnell]

Enable IsolateGroups!

Every isolate in V8 is created within a group. When pointer compression is enabled, all isolates within a single group are limited to a 4 GB shared pointer cage. By default, all isolates in the process share the same group, which means that when running with pointer compression, the entire process would be limited to a single 4 GB shared pointer cage. But, we can create as many IsolateGroups as we want, limited only by the amount of virtual memory available on the machine.

In the default node.js build, this is disabled. When only pointer compression is turned on, isolate groups are used.

/cc @erikcorry @nodejs/v8

[jasnell]

This is ready for review!

I'm not yet convinced that it's (pointer compression in general) is safe to enable by default as it may be too breaking for some workloads but with isolate groups at least we have the option now of using multiple worker threads and don't have the process-wide 4 GB limit. I'm going to experiment next the v8 option to allow for an 8 GB heap limit.

There should be no real noticeable difference in performance but overall apps should be using significantly less memory with pointer compression enabled. To give a rough idea, here's two comparisons of the memory just starting the node.js repl.. first is existing node 24, the second with main with pointer compression enabled:

Welcome to Node.js v24.8.0.
Type ".help" for more information.
> process.memoryUsage()
{
  rss: 59670528,
  heapTotal: 11632640,
  heapUsed: 6980872,
  external: 2465990,
  arrayBuffers: 10541
}
> .exit
jasnell@FF4V42X6YN mine % ./node   
Welcome to Node.js v25.0.0-pre.
Type ".help" for more information.
> process.memoryUsage()
{
  rss: 50642944,
  heapTotal: 5242880,
  heapUsed: 3777440,
  external: 2503911,
  arrayBuffers: 35716
}

Note that heapUsed is cut in half.

[mcollina]

I've tested and it works pretty well.

[jasnell]

Marking this author ready as it has one sign off and has passed CI... however, before landing I'd like to see if we can get signoffs from @targos and or @joyeecheung and or @addaleax .

[joyeecheung]

LGTM % nits

[jasnell]

Added an additional compile flag to switch on shared cage.

@deepak1556 ... once this lands, you'll just need to add --experimental-pointer-compression-shared-cage to your ./configure in addition to the existing --experimental-enable-pointer-compression flag and you'll have the single cage back.

[joyeecheung]

--experimental-enable-pointer-compression: https://ci.nodejs.org/job/node-test-commit-linux-pointer-compression/952/
--experimental-enable-pointer-compression --experimental-pointer-compression-shared-cage: https://ci.nodejs.org/job/node-test-commit-linux-pointer-compression/953/

[jasnell]

Thank you for starting the additional ci jobs! Saved me a step! :-)

[Qard]

Mostly LGTM, but with a safety question.

[deepak1556]

you'll just need to add --experimental-pointer-compression-shared-cage to your ./configure in addition to the existing --experimental-enable-pointer-compression flag and you'll have the single cage back.

Perfect, thank you!

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/69771/

[nodejs-github-bot]

Landed in c2f3c21

[verwaest]

(8gib heaps aren't currently supported by v8, the flag is simply very early WIP without much progress.)

[jasnell]

Yeah, I got confirmation on that yesterday. It's unfortunate. Maybe someday. Still, this is a massive improvement over what we had before!