The sb_max tunable

The sb_max tunable sets an upper limit on the number of socket buffers queued to an individual socket, which controls how much buffer space is consumed by buffers that are queued to a sender's socket or to a receiver's socket.

The system accounts for socket buffers used based on the size of the buffer, not on the contents of the buffer.

If a device driver puts 100 bytes of data into a 2048-byte buffer, the system considers 2048 bytes of socket buffer space to be used. It is common for device drivers to receive buffers into a buffer that is large enough to receive the adapters maximum size packet. This often results in wasted buffer space but it would require more CPU cycles to copy the data to smaller buffers.