[toast0]

Does Linux actually allocate buffers for each socket or does it just link to sk_buff's (which I understand are similar to FreeBSD's mbuf's) and then limit how much storage can be linked? FreeBSD has a limit on the total ram used for mbufs as well, not sure about Linux.

Otoh, FreeBSD's maximum FD limit is set as a factor of total memory pages (edit: looked it up, it's in sys/kern/subr_param.c, the limit is one FD per four pages, unless you edit kernel source) and you've got 2M pages with 8GB ram, so you would be limited to 512k FDs total, and if you're running the client on the same machine as server, that's 256k connections. But 8G is not much for a server, and some phones have more than that... so it's not super limiting.

When you're really not doing much with the connections, userland tcp as suggest in a sibling, could help you squeeze in more connections, but if you're going to actually do work, you probably need more ram.

Btw, as a former WhatsApp server engineer, WhatsApp listens on three ports; 80, 443, and 5222. Not that that makes a significant difference in the content.

[Matthias247]

I think the socket buffers (sk_buff) are actually shared. They are all packet sized, and whatever socket needs to transmit some data or receives it gets the buffers attached. So my assumption is that the amount of required socket buffers scales more with the amount of data transmission than with the number of sockets.

But independent of socket buffers, the kernel obviously needs to allocate other state per socket, which tracks the state of the TCP connection.