Distributed load generation

A single process running Locust can simulate a reasonably high throughput. For a simple test plan it should be able to make many hundreds of requests per second, thousands if you use FastHttpUser.

But if your test plan is complex or you want to run even more load, you’ll need to scale out to multiple processes, maybe even multiple machines.

To do this, you start one instance of Locust in master mode using the --master flag and multiple worker instances using the --worker flag. If the workers are not on the same machine as the master you use --master-host to point them to the IP/hostname of the machine running the master.

The master instance runs Locust’s web interface, and tells the workers when to spawn/stop Users. The workers run your Users and send back statistics to the master. The master instance doesn’t run any Users itself.

Both the master and worker machines must have a copy of the locustfile when running Locust distributed.


Because Python cannot fully utilize more than one core per process (see GIL), you should typically run one worker instance per processor core on the worker machines in order to utilize all their computing power.


There is almost no limit to how many Users you can run per worker. Locust/gevent can run thousands or even tens of thousands of Users per process just fine, as long as their total request rate/RPS is not too high.


If Locust is getting close to running out of CPU resources, it will log a warning.


To start locust in master mode:

locust -f my_locustfile.py --master

And then on each worker (replace with the IP of the master machine, or leave out the parameter altogether if your workers are on the same machine as the master):

locust -f my_locustfile.py --worker --master-host=



Sets locust in master mode. The web interface will run on this node.


Sets locust in worker mode.


Optionally used together with --worker to set the hostname/IP of the master node (defaults to


Optionally used together with --worker to set the port number of the master node (defaults to 5557).


Optionally used together with --master. Determines which network interface the master node will bind to. Defaults to * (all available interfaces).


Optionally used together with --master. Determines what network ports that the master node will listen to. Defaults to 5557.


Used when starting the master node with --headless. The master node will then wait until X worker nodes has connected before the test is started.

Communicating across nodes

When running Locust in distributed mode, you may want to communicate between master and worker nodes in order to coordinate data. This can be easily accomplished with custom messages using the built in messaging hooks:

from locust import events
from locust.runners import MasterRunner, WorkerRunner

# Fired when the worker receives a message of type 'test_users'
def setup_test_users(environment, msg, **kwargs):
    for user in msg.data:
        print(f"User {user['name']} received")
    environment.runner.send_message('acknowledge_users', f"Thanks for the {len(msg.data)} users!")

# Fired when the master receives a message of type 'acknowledge_users'
def on_acknowledge(msg, **kwargs):

def on_locust_init(environment, **_kwargs):
    if not isinstance(environment.runner, MasterRunner):
        environment.runner.register_message('test_users', setup_test_users)
    if not isinstance(environment.runner, WorkerRunner):
        environment.runner.register_message('acknowledge_users', on_acknowledge)

def on_test_start(environment, **_kwargs):
    if not isinstance(environment.runner, WorkerRunner):
        users = [
            {"name": "User1"},
            {"name": "User2"},
            {"name": "User3"},
        environment.runner.send_message('test_users', users)

Note that when running locally (i.e. non-distributed), this functionality will be preserved; the messages will simply be handled by the runner that sends them.

For more details, see the complete example.

Running distributed with Docker

See Running in Docker

Running Locust distributed without the web UI

See Running Locust distributed without the web UI

Increase Locust’s performance

If you’re planning to run large-scale load tests, you might be interested to use the alternative HTTP client that’s shipped with Locust. You can read more about it here: Increase performance with a faster HTTP client.