Recently I wanted to simulate “slow” requests against a web server in order to investigate how high latency requests impacted the system overall. After some initial experimentation with writing my own code (in python), I started looking around for other solutions. I wanted to be able to use Multi-Mechanize, one of my favorite tools for http testing, without heavy modification. Eventually I stumbled onto netem.  From the FAQ:

Netem allows testing of protocols and applications on a local network with simulated wide area networks. This was originally done to test congestion algorithms in TCP/IP, but is equally applicable to web applications and file servers.

It’s dirt simple to use, and allows you to stick with whatever http traffic generating tools you like.  Here’s a few simple examples along with their impact.

Multi-Mechanize Configuration

[global]
run_time: 30
rampup: 0
results_ts_interval: 5
progress_bar: on
console_logging: off
xml_report: off
[user_group-1]
threads: 3
script: example_httplib.py

Multi-Mechanize Test Script

import httplib
import time

class Transaction(object):
    def __init__(self):
        self.custom_timers = {}

    def run(self):
        start_timer = time.time()
        conn = httplib.HTTPConnection('www.google.com')
        conn.request('GET', '/')
        resp = conn.getresponse()
        content = resp.read()
        latency = time.time() - start_timer</p>
        self.custom_timers['Google'] = latency
        assert (resp.status == 200), 'Bad HTTP Response'

if __name__ == '__main__':
    trans = Transaction()
    trans.run()
    print trans.custom_timers

First, the control run (no artificial latency introduced):

Next, let’s add 100ms latency:

[root@aenea multi-mechanize_1.011]# tc qdisc change dev wlan0 root netem delay 100ms

Now let’s do 100ms latency, with a 20ms variation with a normal distribution:

[root@aenea multi-mechanize_1.0.11]# tc qdisc change dev wlan0 root netem delay 100ms 20ms distribution normal

This is really just scratching the surface with netem.  You can use it for:

• Delay
• Variable Delay
• Delay Distribution
• Packet Loss
• Packet Reordering
• Rate Control

Have fun!