FAQ

WSGI Bits

How do I set SCRIPT_NAME?

By default SCRIPT_NAME is an empty string. The value could be set by setting SCRIPT_NAME in the environment or as an HTTP header.

Server Stuff

How do I reload my application in Gunicorn?

You can gracefully reload by sending HUP signal to gunicorn:

$ kill -HUP masterpid

How might I test a proxy configuration?

The Hey program is a great way to test that your proxy is correctly buffering responses for the synchronous workers:

$ hey -n 10000 -c 100 http://127.0.0.1:5000/

This runs a benchmark of 10000 requests with 100 running concurrently.

How can I name processes?

If you install the Python package setproctitle Gunicorn will set the process names to something a bit more meaningful. This will affect the output you see in tools like ps and top. This helps for distinguishing the master process as well as between masters when running more than one app on a single machine. See the proc_name setting for more information.

Why is there no HTTP Keep-Alive?

The default Sync workers are designed to run behind Nginx which only uses HTTP/1.0 with its upstream servers. If you want to deploy Gunicorn to handle unbuffered requests (ie, serving requests directly from the internet) you should use one of the async workers.

Worker Processes

How do I know which type of worker to use?

Read the Design page for help on the various worker types.

What types of workers are there?

Check out the configuration docs for worker_class.

How can I figure out the best number of worker processes?

Here is our recommendation for tuning the number of workers.

How can I change the number of workers dynamically?

TTIN and TTOU signals can be sent to the master to increase or decrease the number of workers.

To increase the worker count by one:

$ kill -TTIN $masterpid

To decrease the worker count by one:

$ kill -TTOU $masterpid

Does Gunicorn suffer from the thundering herd problem?

The thundering herd problem occurs when many sleeping request handlers, which may be either threads or processes, wake up at the same time to handle a new request. Since only one handler will receive the request, the others will have been awakened for no reason, wasting CPU cycles. At this time, Gunicorn does not implement any IPC solution for coordinating between worker processes. You may experience high load due to this problem when using many workers or threads. However a work has been started to remove this issue.

Why I don’t see any logs in the console?

In version 19.0, Gunicorn doesn’t log by default in the console. To watch the logs in the console you need to use the option --log-file=-. In version 19.2, Gunicorn logs to the console by default again.

Kernel Parameters

When dealing with large numbers of concurrent connections there are a handful of kernel parameters that you might need to adjust. Generally these should only affect sites with a very large concurrent load. These parameters are not specific to Gunicorn, they would apply to any sort of network server you may be running.

These commands are for Linux. Your particular OS may have slightly different parameters.

How can I increase the maximum number of file descriptors?

One of the first settings that usually needs to be bumped is the maximum number of open file descriptors for a given process. For the confused out there, remember that Unices treat sockets as files.

$ sudo ulimit -n 2048

How can I increase the maximum socket backlog?

Listening sockets have an associated queue of incoming connections that are waiting to be accepted. If you happen to have a stampede of clients that fill up this queue new connections will eventually start getting dropped.

$ sudo sysctl -w net.core.somaxconn="2048"

How can I disable the use of sendfile()

Disabling the use sendfile() can be done by using the --no-sendfile setting or by setting the environment variable SENDFILE to 0.

Troubleshooting

How do I fix Django reporting an ImproperlyConfigured error?

With asynchronous workers, creating URLs with the reverse function of django.core.urlresolvers may fail. Use reverse_lazy instead.

How do I avoid Gunicorn excessively blocking in os.fchmod?

The current heartbeat system involves calling os.fchmod on temporary file handlers and may block a worker for arbitrary time if the directory is on a disk-backed filesystem. For example, by default /tmp is not mounted as tmpfs in Ubuntu; in AWS an EBS root instance volume may sometimes hang for half a minute and during this time Gunicorn workers may completely block in os.fchmod. os.fchmod may introduce extra delays if the disk gets full. Also Gunicorn may refuse to start if it can’t create the files when the disk is full.

Currently to avoid these problems you can use a tmpfs mount (for a new directory or for /tmp) and pass its path to --worker-tmp-dir. First, check whether your /tmp is disk-backed or RAM-backed:

$ df /tmp
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/xvda1           ...     ...       ...  ... /

No luck. If you are using Fedora or Ubuntu, you should already have a tmpfs mount at /dev/shm:

$ df /dev/shm
Filesystem     1K-blocks     Used Available Use% Mounted on
tmpfs                 ...     ...       ...  ... /dev/shm

In this case you can set --worker-tmp-dir /dev/shm, otherwise you can create a new tmpfs mount:

sudo cp /etc/fstab /etc/fstab.orig
sudo mkdir /mem
echo 'tmpfs       /mem tmpfs defaults,size=64m,mode=1777,noatime,comment=for-gunicorn 0 0' | sudo tee -a /etc/fstab
sudo mount /mem

Check the result:

$ df /mem
Filesystem     1K-blocks  Used Available Use% Mounted on
tmpfs              65536     0     65536   0% /mem

Now you can set --worker-tmp-dir /mem.

Why are Workers Silently Killed?

A sometimes subtle problem to debug is when a worker process is killed and there is little logging information about what happened.

If you use a reverse proxy like NGINX you might see 502 returned to a client.

In the gunicorn logs you might simply see [35] [INFO] Booting worker with pid: 35

It’s completely normal for workers to be killed and startup, for example due to max-requests setting. Ordinarily gunicorn will capture any signals and log something.

This particular failure case is usually due to a SIGKILL being received, as it’s not possible to catch this signal silence is usually a common side effect! A common cause of SIGKILL is when OOM killer terminates a process due to low memory condition.

This is increasingly common in container deployments where memory limits are enforced by cgroups, you’ll usually see evidence of this from dmesg:

dmesg | grep gunicorn
Memory cgroup out of memory: Kill process 24534 (gunicorn) score 1506 or sacrifice child
Killed process 24534 (gunicorn) total-vm:1016648kB, anon-rss:550160kB, file-rss:25824kB, shmem-rss:0kB

In these instances adjusting the memory limit is usually your best bet, it’s also possible to configure OOM not to send SIGKILL by default.