Linux Parameters to Tune for Large Concurrent Connections

MigratoryData has achieved 12 million concurrent connections, that to with a JVM!! Time to rethink about JVM. Some Linux parameters that are tuned are:

  • The maximum number of open files the system can handle (by tuning /proc/sys/fs/nr_open)
  • The maximum number of open files a process can have (by tuning with “ulimit -n”)
  • Increase the number of ephemeral ports on a box (the default being from 32768 to 61000) from “500 to 65535”, with “sysctl -w net.ipv4.ip_local_port_range”. On the server side, it is the same listening address and the listening port that is used to pair with each client end point address.
    • Once a connection is established, the quadruple that is used to uniquely identify a connection is <serverIP, serverListeningPort, clientIp, clientEphemeralPort>
    • We dont need so many port numbers on the server side, we need only so many open sockets
  • Reuse TCP’s TIME_WAIT sockets with “/proc/sys/net/ipv4/tcp_tw_recycle”
  • Amount of memory allocated to tcp/udp with “sysctl -w net.ipv4.[tcp|udp]_mem”
  • Balance the interrupt processing by changing the smp affinity of each tx/rx queue (now is the time to read more about the Intel 10G ethernet adapter) to a different core (writing the bit to /proc/irq/<interruptNum>/smp_affinity

One thing to note is that with all this connections, CPU was not the bottleneck, but memory was. In fact, CPU was still at around 47% when the server reached 12M connections, but memory reached its brim.

Besides, some JVM tuning is also done, which is out of scope of my mind. :-). All in all, they broke the C10M benchmark with a stock kernel properly tuned!! Which is a magnificient achievement.

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