How to request the Garbage Collector in node.js to run?

Question

At startup, it seems my node.js app uses around 200MB of memory. If I leave it alone for a while, it shrinks to around 9MB.

Is it possible from within the app to:

Check how much memory the app is using ?
Request the garbage collector to run ?

The reason I ask is, I load a number of files from disk, which are processed temporarily. This probably causes the memory usage to spike. But I don't want to load more files until the GC runs, otherwise there is the risk that I will run out of memory.

Any suggestions ?

How are you loading these files? If you need to load a huge file, you must use the Stream approach and you can process the file while still reading. Have you consider it? — well
– well, Commented Oct 5, 2017 at 12:39
@well That was 3 years ago. I'm working on some other project now and can't remember much about it. — Rahul Iyer
– Rahul Iyer, Commented Oct 8, 2017 at 4:12

Darryl Noakes · Accepted Answer · 2023-10-18 02:33:19Z

205

If you launch the node process with the --expose-gc flag, you can then call global.gc() to force node to run garbage collection. Keep in mind that all other execution within your node app is paused until GC completes, so don't use it too often or it will affect performance.

You might want to include a check when making GC calls from within your code so things don't go bad if node was run without the flag:

if (global.gc) {
    global.gc();
} else {
    console.log('Garbage collection unavailable.  Pass --expose-gc '
      + 'when launching node to enable forced garbage collection.');
}

edited Oct 18, 2023 at 2:33

Darryl Noakes

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answered Jun 4, 2015 at 21:25

IgnisFatuus

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13 Comments

pea3nut Over a year ago

Where the docs for "--expose-gc"? I am not find any docs in nodejs.org.

janispritzkau Over a year ago

@pea3nut it's a option of the V8 runtime, which node uses. This and other options can be listed with node --v8-options

Piyush Balapure Over a year ago

You have to pass true to the function for full GC otherwise it will just do a minor GC

cartant Over a year ago

FWIW, passing true (these days, at least) effects a minor collection: github.com/nodejs/node/blob/…

rob Over a year ago

When would the catch block run? If --expose-gc is missing then global.gc will just be undefined

|

TmTron · Accepted Answer · 2023-01-04 15:52:40Z

17

When you cannot pass the --expose-gc flag to your node process on start for any reason, you may try this:

import { setFlagsFromString } from 'v8';
import { runInNewContext } from 'vm';

setFlagsFromString('--expose_gc');
const gc = runInNewContext('gc'); // nocommit
gc();

Notes:

This worked for me in node 16.x
You may want to check process.memoryUsage() before and after running the gc
Use with care: Quote from the node docs v8.setFlagsFromString:

This method should be used with care. Changing settings after the VM has started may result in unpredictable behavior, including crashes and data loss; or it may simply do nothing.

answered Jan 4, 2023 at 15:52

TmTron

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1 Comment

n1kk Over a year ago

Thanks, worked like a charm. Really helpful when writing unit tests that are run by IDE and you don't have control over node arguments.

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