The interviewer dug a hole for me: how many concurrent TCP connections can a single server have?

There was a time when we were looking for network programming C10K Solutions to problems , But now from the perspective of hardware and operating system support, it's not very challenging for a single server to support tens of thousands of concurrent connections . of C10K（ Stand alone 1 10000 concurrent connection problems ） You can refer to 《 the previous 10 year , The famous C10K Concurrent connection problems [1]》 and 《The C10K problem[2]》 These two articles .

Let's assume that a single server can only support 10000 level concurrent connections at most , In fact, it's far enough for most applications , But for some Internet companies with a large user base , The number of concurrent connections often faced is millions 、 Ten million 、 Even hundreds of millions . Although the current cluster , Distributed technology can share concurrent load on multiple servers , Then we just need to expand dozens of computers to solve the problem , But we would like to mine more resources of a single server , Try to expand vertically first , Further horizontal expansion , This can effectively save server related expenses （ Hardware resources 、 Computer room 、 Operation and maintenance manpower 、 Electricity is also a big expense ）.

How much can a server support TCP Concurrent connection ？

First of all, we need to consider the limitation of file handle . stay linux The friends who write the web server program must know every one of them tcp All connections take up a file descriptor , Once the file descriptor is used up , The error returned by the new connection is “Socket/File:Can't open so many files”. At this point, you need to understand the operating system's limit on the maximum number of files that can be opened . We can go through ​​ulimit -n​​​ command 、/etc/security/limits.conf file as well as /etc/sysctl.conf File, etc. to modify the number of file handles . More details can be found in 「 Zhu Xiaosi's blog 」 This article in 《​ ​ File handle ？ File descriptor ？ Stupid is not clear ​​》.

The second thing to consider is the limitation of port range . Port number on the operating system 1024 The following are reserved by the system , from 1024-65535 It is used by users . Because each TCP All connections need a port number , So we can have at most 60000 Multiple concurrent connections . I think there's a lot of wrong thinking friends ？ Interviewers also prefer to lead digging here , There are similar problems ： One UDP Connection can be reused TCP The port occupied by the connection ？（ You can leave your answer in the message area below .）

How to identify a TCP Connect ？ The system uses a 4 Four tuples to uniquely identify a TCP Connect ： Local port number local port、 Local IP Address local ip、 Remote port number remote port、 Distal IP Address remote ip.server Usually fixed on a local port to listen , wait for client Connection request for . Address reuse not considered （unix Of SO_REUSEADDR Options ） Under the circumstances , Even if server Multiple ends ip, The local listening port is also exclusive , therefore server End tcp Connect 4 Only tuples remote ip（ That is to say client ip） and remote port（ client port） Is variable , So the biggest tcp Connect as client ip Count × client port Count , Yes IPV4, Don't consider ip Address classification and other factors , Maximum tcp The number of connections is about 2 Of 32 Power （ip Count ）×2 Of 16 Power （port Count ）, That is to say server End stand-alone maximum tcp The number of connections is about 2 Of 48 Power .

The above conclusions are all theoretical single machines TCP Number of concurrent connections , In fact, the number of concurrent connections of a single machine must be subject to hardware resources （ Memory ）、 Network resources （ bandwidth ） The limitation of , At least we can achieve hundreds of thousands of levels of concurrency .

Reference material

[1]

the previous 10 year , The famous C10K Concurrent connection problems : http://www.52im.net/thread-566-1-1.html

[2]

The C10K problem: http://www.kegel.com/c10k.html

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