uwsgi: your server socket listen backlog is limite

Question

I run a flask app on uwsgi. I use supervisor to manage uwsgi process. I find the log saying that

your server socket listen backlog is limited to 100 connections.

How to overcome 100 connections limitation? My running script is as below:

[program:myapp]
command=uwsgi --master -s /tmp/app.sock --module myapp:app --processes 2 -H /srv/sites/mysite chmod-socket 666 --enable-threads

Answer 1

As it was described in previous answers:

1. Increase connections in kernel
2. Increase connections in uWSGI too

Example. If you are using docker and docker-compose.

1. Hot to increase connections in kernel

In docker-compose.yml, in block where you describe how to run uWSGI:

uwsgi_runner:
    <<: *app-base
    command: /usr/local/bin/uwsgi --ini /app/uwsgi.ini
    # ... other settings ...
    sysctls:
        net.core.somaxconn: 1024 # set max connections to 1024 in kernel

2. How to increase connections in uWSGI

In uwsgi.ini:

[uwsgi]
# ... other settings ...
listen = 1024 # set max connections to 1024 in uWSGI

Also, you can change this parameter directly in docker-compose command (-l or --listen flag) if you are not using uwsgi.ini settings file:

uwsgi_runner:
    <<: *app-base
    command: /usr/local/bin/uwsgi -l 1024 #other-parameters-here

Answer 2

Note that a "listen backlog" of 100 connections doesn't mean that your server can only handle 100 simultaneous (or total) connections - this is instead dependent on the number of configured processes or threads. The listen backlog is a socket setting telling the kernel how to limit the number of outstanding (as yet unaccapted) connections in the listen queue of a listening socket. If the number of pending connections exceeds the specified size, new ones are automatically rejected. A functioning server regularly servicing its connections should not require a large backlog size.

According to the manual, you can change the listen backlog with the -l option:

-l|--listen <num>
       set  socket  listen queue to <n> (default 100, maximum is system
       dependent)

Answer 3

You can modify SOMAXCONN in your /proc/sys/net/core/somaxconn to increase this limit. It's simply linux tuning systems.

Answer 4

Simply changing (increasing) the uwsgi's listen backlog using the -l or --listen option (as pointed by user4815162342) while starting the server, to a value greater then 128 won't allow uwsgi to run. As there's also a system level limit on Unix socket and TCP connection listen queue - the default is 128, you can verify it (for Unix socket):

cat /proc/sys/net/core/somaxconn

uwsgi had been patched, such that if the value passed to --listen parameter while starting uwsgi is greater then the system level limit (Linux kernel limit) it'll cause uwsgi to fail hard. If you want set uwsgi's listen queue limit greater than the system level limit (i.e. 128) - you must first increase the kernel's limit. Can be done executing the following commands:

$ echo 4096 > /proc/sys/net/core/somaxconn
$ cat /proc/sys/net/core/somaxconn
4096

Or

$ sysctl -w net.core.somaxconn=4096

Or, Add net.core.somaxconn=4096 to /etc/sysctl.conf for it to become permanent (survive the reboot).