Document NODE_MANY_ACCEPTS environment variable.

[martinheidegger]

While investigating a problem with electron/dat I found the NODE_MANY_ACCEPTS environment variable exclusively used in the win32 context.

node/lib/net.js

Lines 1757 to 1760 in 02fef8a

	if (simultaneousAccepts === undefined) {
	simultaneousAccepts = (process.env.NODE_MANY_ACCEPTS &&
	process.env.NODE_MANY_ACCEPTS !== '0');
	}

However I was not able to find any documentation of why this is necessary or what it does in the environment variable doc.

[bnoordhuis]

It's explained in the libuv documentation:

.. c:function:: int uv_tcp_simultaneous_accepts(uv_tcp_t* handle, int enable)

    Enable / disable simultaneous asynchronous accept requests that are
    queued by the operating system when listening for new TCP connections.

    This setting is used to tune a TCP server for the desired performance.
    Having simultaneous accepts can significantly improve the rate of accepting
    connections (which is why it is enabled by default) but may lead to uneven
    load distribution in multi-process setups.

NODE_MANY_ACCEPTS only has an effect on the child_process and cluster modules.

[Fishrock123]

Sounds like this should be documented in https://github.com/nodejs/node/blob/master/doc/api/cli.md#environment-variables and https://github.com/nodejs/node/blob/master/doc/node.1

[bnoordhuis]

IIRC, we added it as a debugging escape hatch but it's never been used and it's probably just dead code at this point. It's better to remove it than to document it.

[juggernaut451]

I would like to take this up. @Fishrock123 @bnoordhuis So it has to be documented or to be removed?

[Fishrock123]

I'm not sure, I guess we should investigate if it still works / is necessary or useful.

[martinheidegger]

The only two places it seems to be used are:

node/lib/internal/child_process.js

Lines 575 to 579 in 02fef8a

	// Update simultaneous accepts on Windows
	if (process.platform === 'win32') {
	handle._simultaneousAccepts = false;
	net._setSimultaneousAccepts(handle);
	}

and

node/lib/internal/child_process.js

Lines 673 to 676 in 02fef8a

	// Update simultaneous accepts on Windows
	if (obj.simultaneousAccepts) {
	net._setSimultaneousAccepts(handle);
	}

the c docs mention that:

... having simultaneous accepts can significantly improve the rate of accepting connections (which is why it is enabled by default)

It seems thought that the default in Node.js is to have it disabled by default.

[bnoordhuis]

It seems thought that the default in Node.js is to have it disabled by default.

I have reply deja vu, I remember answering this... It's opt-out, i.e., on by default. The two calls you link to are the opt-outs.

We should probably just call handle.setSimultaneousAccepts(false) directly and get rid of the indirection.

@juggernaut451 Pull request welcome if you want to take this on. I'd make net._setSimultaneousAccepts print a deprecation warning but otherwise leave it unchanged for now.