Increase buffer sizes

[bt90]

This topic came up in the Syncthing forum: https://forum.syncthing.net/t/system-is-slow-on-windows-look-here/21925

I'm unsure if users really need to modify the registry or if the socket options are enough to tune the buffer sizes. After briefly skimming the code, I couldn't spot where we do this for Windows?

[marten-seemann]

It's done here: https://github.com/quic-go/quic-go/blob/master/sys_conn_buffers.go

Not sure how / if it works on Windows. Would be great if you could try it out.

[bt90]

wireguard-go seems to directly set the socket options:

https://github.com/WireGuard/wireguard-go/blob/master/conn/controlfns_windows.go

No idea if that's necessary, though.

Would be great if you could try it out.

Can I evaluate this using Syncthing?

[marten-seemann]

This function is used to check the size of the buffer:

quic-go/sys_conn_windows.go

Lines 16 to 25 in da410a7

	func inspectReadBuffer(c syscall.RawConn) (int, error) {
	var size int
	var serr error
	if err := c.Control(func(fd uintptr) {
	size, serr = windows.GetsockoptInt(windows.Handle(fd), windows.SOL_SOCKET, windows.SO_RCVBUF)
	}); err != nil {
	return 0, err
	}
	return size, serr
	}

You could add some logging before and after we increase the buffer, and see if the value changes.

[bt90]

Judging from the debug logging, things are working as expected:

2024/04/12 17:06:34 Increased receive buffer size to 2048 kiB
2024/04/12 17:06:34 Increased send buffer size to 2048 kiB
2024/04/12 17:06:34 Setting DF for IPv4 and IPv6.
2024/04/12 17:06:34 server Listening for udp connections on [::]:22000

Simply tested via

set QUIC_GO_LOG_LEVEL=debug
.\syncthing.exe

I suspect that the speedup observed by our forum user was due to the obscenely high default buffer size of 25MB. While this is obviously too much, should we follow wireguard-go and increase our buffer to 7MB? See this commit message for more details: WireGuard/wireguard-go@f26efb6

Edit: tested with the AFD registry parameters (25MB) in place:

2024/04/12 17:16:57 Conn has receive buffer of 24414 kiB (wanted: at least 2048 kiB)
2024/04/12 17:16:57 Conn has send buffer of 24414 kiB (wanted: at least 2048 kiB)
2024/04/12 17:16:57 Setting DF for IPv4 and IPv6.
2024/04/12 17:16:57 server Listening for udp connections on [::]:22000

[marten-seemann]

I suspect that the speedup observed by our forum user was due to the obscenely high default buffer size of 25MB. While this is obviously too much, should we follow wireguard-go and increase our buffer to 7MB? See this commit message for more details: WireGuard/wireguard-go@f26efb6

Sounds reasonable. Want to create a PR?