Linux UDP max size of receive buffer

Question

What's the maximum size of Linux UDP receive buffer? I thought it's limited only by available RAM, but when I set

5GB for rmem_max:

echo 5000000000 > /proc/sys/net/core/rmem_max

and 4GB for the actual socket buffer (in Erlang):

gen_udp:listen(Port, [{recbuf, 4000000000}])

When I measure the buffer utilization, it shows:

# netstat -u6anp | grep 5050
udp6  1409995136      0 :::5050  :::*       13483/beam.smp

I can't exceed this 1.4GB. For smaller buffer sizes, like e.g. 500MB, actual buffer size matched the configured value. My system is Debian 6.0, the machine has 50GB RAM available.

Answer 1

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.