TCP vs. UDP socket latency benchmark

Question

I have implemented a small benchmark for socket communication via TCP and UDP in Python. Surprisingly, TCP is almost exactly double as fast as UDP.

To avoid routing effects, server and client are running on the same Unix machine, but on different threads.

Maybe the code is useful. Here is the server code:

import socket
import sys

host = 'localhost'  
port = 8888
buffersize = 8
server_address = (host, port) 

def start_UDP_server():
    socket_UDP = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    socket_UDP.bind(server_address)

    print("UDP server is running...")

    while True:
        data, from_address = socket_UDP.recvfrom(buffersize)
        if not data: break
        socket_UDP.sendto(data, from_address)
    socket_UDP.close()


def start_TCP_server():
    socket_TCP = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    socket_TCP.bind(server_address)
    socket_TCP.listen(1)

    print("TCP server is running...")

    while True:    
        client, client_address = socket_TCP.accept()

        while True:
            data = client.recv(buffersize)
            if not data: break
            client.sendall(data)

        client.close()

So you can run either start_TCP_server() or start_UDP_server().

On client side the code is:

import socket
import sys
import time

host = 'localhost'  
port = 8888
buffersize = 8
server_address = (host, port) 
client_address = (host, port+1)
N = 1000000


def benchmark_UDP():
    socket_UDP = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) 
    socket_UDP.bind(client_address)

    print("Benchmark UDP...")

    duration = 0.0
    for i in range(0, N):
        b = bytes("a"*buffersize, "utf-8")
        start = time.time()
        socket_UDP.sendto(b, server_address)
        data, from_address = socket_UDP.recvfrom(buffersize)
        duration += time.time() - start

        if data != b:
            print("Error: Sent and received data are bot the same")

    print(duration*pow(10, 6)/N, "µs for UDP") 


def benchmark_TCP():
    socket_TCP = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    socket_TCP.connect(server_address)

    print("Benchmark TCP...")

    duration = 0.0
    for i in range(0, N):
        b = bytes("a"*buffersize, "utf-8")
        start = time.time()
        socket_TCP.sendall(b)
        data = socket_TCP.recv(buffersize)
        duration += time.time() - start

        if data != b:
            print("Error: Sent and received data are bot the same")

    print(duration*pow(10, 6)/N, "µs for TCP")
    socket_TCP.close()

Like for the server you can start the benchmark by benchmark_TCP() or benchmark_UDP().

The results are about 25 µs for TCP, and about 54 µs for UDP on Unix and even worse for Windows (about 30 µs for TCP and more than 200 µs for UDP). Why? I would expect a minimal advantage for UDP.

Answer 1

Your TCP socket is connected but your UDP socket is not. This means extra processing for every send/receive on the UDP socket. Call connect on each side for the UDP socket, just like you call connect/accept on the TCP socket.

Programs like iperf do this to measure accurately.