Linux Udp Max Size of Receive Buffer

Linux UDP max size of receive buffer

It seems that there is a limit in linux.
I have tried setting rmem_max to 2^32-1 with success.

   root@xxx:/proc/sys/net/core# echo 2147483647 > rmem_max
   root@xxx:/proc/sys/net/core# cat rmem_max
   2147483647

2^32 was too much:

   root@xxx:/proc/sys/net/core# echo 2147483648 > rmem_max
   root@xxx:/proc/sys/net/core# cat rmem_max
   -18446744071562067968

Setting to 5000000000 yields:

   root@xxx:/proc/sys/net/core# echo 5000000000 > rmem_max
   root@xxx:/proc/sys/net/core# cat rmem_max
   705032704

I have tested in python that setting and getting socket receive buffer with

   ss.setsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF, bufferSize)
   print ss.getsockopt(socket.SOL_SOCKET, socket.SO_RCVBUF)

If 'bufferSize' is less then 1024^3 program prints doubled 'bufferSize', otherwise it falls back to 256.

The value 705032704*2 = 1410065408 is close to the 1409995136 obtained by netstat.

How to monitor Linux UDP buffer available space?

Linux provides the files /proc/net/udp and /proc/net/udp6, which lists all open UDP sockets (for IPv4 and IPv6, respectively). In both of them, the columns tx_queue and rx_queue show the outgoing and incoming queues in bytes.

If everything is working as expected, you usually will not see any value different than zero in those two columns: as soon as your application generates packets they are sent through the network, and as soon those packets arrive from the network your application will wake up and receive them (the recv call immediately returns). You may see the rx_queue go up if your application has the socket open but is not invoking recv to receive the data, or if it is not processing such data fast enough.

Specifying UDP receive buffer size at runtime in Linux

You can increase the value from the default, but you can't increase it beyond the maximum value. Use setsockopt to change the SO_RCVBUF option:

int n = 1024 * 1024;
if (setsockopt(socket, SOL_SOCKET, SO_RCVBUF, &n, sizeof(n)) == -1) {
  // deal with failure, or ignore if you can live with the default size
}

Note that this is the portable solution; it should work on any POSIX platform for increasing the receive buffer size. Linux has had autotuning for a while now (since 2.6.7, and with reasonable maximum buffer sizes since 2.6.17), which automatically adjusts the receive buffer size based on load. On kernels with autotuning, it is recommended that you not set the receive buffer size using setsockopt, as that will disable the kernel's autotuning. Using setsockopt to adjust the buffer size may still be necessary on other platforms, however.

what size of the buffer is sufficient for UDP case of recvfrom?

The maximum size of an UDP datagram is 64k. Unless you're allocating a lot of buffers, just use this size.

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