Can we document --max_old_space_size or is it deprecated?

[GeorgeBailey]

I have some Node.JS processes taking more memory than I think they require. I suspect this is due to lazy GC.

In my case, I have several Node.JS processes running, across 3 user accounts. (root and two others)

After searching, it seems that I can limit the amount of 'unused' space that is held in memory by simply using the --max_old_space_size=X option. Where X is the number of megabytes allowed.

However, this does not seem to be documented so I am a little concerned that I should not be using it. Can this be documented, or is it deprecated?

[cjihrig]

I believe it is not documented because we have tried to stay away from documenting V8 options.

[GeorgeBailey]

Oh, I see it now. node --help shows an options called --v8-options. That is where it is documented. One might be able to make that more clear, but I'm not even sure that would help, so I'll just close the issue. Thanks.

[vsemozhetbyt]

@cjihrig Here is a link to the v8-flags repo. Could it be helpfull to duplicate it here?

[cjihrig]

That's a lot of documentation that would then fall on our shoulders to keep up to date. The V8 project should manage their own documentation IMO. It also looks like that v8-flags repo hasn't been updated in over 2 years.

[vsemozhetbyt]

@cjihrig Oh, no, I mean to duplicate just the link. But if the list is outdated. then may be this is not a good idea. Sorry.

[cjihrig]

Ah, sorry, I misunderstood what you meant by "duplicate it." I wouldn't have a strong opinion on duplicating the link if the v8-flags repo was being kept up to date.

[fractalf]

+1 on at least mentioning something about max_old_space_size in the docs. If only to link to somewhere else.

• Whats the default value when running node? (says 0, but that dont make sense)
• Where to find an official V8 doc on this option? (is there one?)

I agree that it's not Nodes' job to document V8 options, but still believe this memory option is a special case that deserve mentioning in the docs.

Which would be best to use:

#!/usr/local/bin/node --max_old_space_size=4096
..

or

const v8 = require('v8');
v8.setFlagsFromString('--max_old_space_size=4096');
..

Edit:
After 1 hour of searching I'm still have no idea what's the default node value, and that using this v8 option will (officially) guarantee more memory allocated.

[vsemozhetbyt]

@fractalf A posteriori, Node.js sets memory limit near 1.5 GB by default (at least for x64 OS).

[joyeecheung]

@fractalf FWIW the definitions of default values are in v8/src/heap/heap.cc:

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

Line 82 in ec02b81

max_old_generation_size_(700ul * (kPointerSize / 4) * MB),

(max_old_space_size is actually used to configure max_old_generation_size when its value is larger than 0). So on 64-bit machines that(the old generation alone) would be 1400 MB.

[fractalf]

@vsemozhetbyt @joyeecheung Thanks!