How many socket connections possible?

Question

Has anyone an idea how many tcp-socket connections are possible on a modern standard root server? (There is in general less traffic on each connection, but all the connections have to be up all the time.)

EDIT: We will use a Linux Server.

Answer 1

A limit on the number of open sockets is configurable in the /proc file system

cat /proc/sys/fs/file-max

Max for incoming connections in the OS defined by integer limits.

Linux itself allows billions of open sockets.

To use the sockets you need an application listening, e.g. a web server, and that will use a certain amount of RAM per socket.

RAM and CPU will introduce the real limits. (modern 2017, think millions not billions)

1 millions is possible, not easy. Expect to use X Gigabytes of RAM to manage 1 million sockets.

Outgoing TCP connections are limited by port numbers ~65000 per IP. You can have multiple IP addresses, but not unlimited IP addresses. This is a limit in TCP not Linux.

Answer 2

On Linux you should be looking at using epoll for async I/O. It might also be worth fine-tuning socket-buffers to not waste too much kernel space per connection.

I would guess that you should be able to reach 100k connections on a reasonable machine.

Answer 3

Realistically for an application, more then 4000-5000 open sockets on a single machine becomes impractical. Just checking for activity on all the sockets and managing them starts to become a performance issue - especially in real-time environments.

Answer 4

This depends not only on the operating system in question, but also on configuration, potentially real-time configuration.

For Linux:

cat /proc/sys/fs/file-max

will show the current maximum number of file descriptors total allowed to be opened simultaneously. Check out http://www.cs.uwaterloo.ca/~brecht/servers/openfiles.html

Answer 5

I achieved 1600k concurrent idle socket connections, and at the same time 57k req/s on a Linux desktop (16G RAM, I7 2600 CPU). It's a single thread http server written in C with epoll. Source code is on github, a blog here.

Edit:

I did 600k concurrent HTTP connections (client & server) on both the same computer, with JAVA/Clojure . detail info post, HN discussion: http://news.ycombinator.com/item?id=5127251

The cost of a connection(with epoll):

• application need some RAM per connection
• TCP buffer 2 * 4k ~ 10k, or more
• epoll need some memory for a file descriptor, from epoll(7)

Each registered file descriptor costs roughly 90 bytes on a 32-bit kernel, and roughly 160 bytes on a 64-bit kernel.

Answer 6

Google around for the "C10K" problem. This is basically discussion and technology around managing 10,000 or more simultaneous connections.

I suspect this number was chosen because it's hard, but theoretically possible.