What are uwsgi threads used for?

Question

I see in a uwsgi.ini file there is a configuration

[uwsgi]
socket = 127.0.0.1:3031
chdir = /home/foobar/myproject/
wsgi-file = myproject/wsgi.py
processes = 4
threads = 2
stats = 127.0.0.1:9191

I understand that each request is served in a different process. Then what are threads used for ?

Answer 1

Adding an additional answer here due to a huge GOTCHA I had today.

Shared memory works across threads, but not processes. That is, if you have some module level thing like:

# mymodule
mycache = {}

mycache[key] = value

del mycache[key]

...

Deletes in one process will NOT be reflected in the cache of another process. However, deletes in one thread, if only one process is used, will persist across threads.

So if you are using shared memory like this, you have two options:

1. all caches should be "safe" and "read through" (ie on cache miss, try to load the real data)
2. always run threads=X but processes=1

Answer 2

Both processes and threads can be used for increasing concurrency. Threads are cheaper than processes and use less resources, but may not always run in parallel because of Python GIL.

Also, quoting the uWSGI documentation:

There is no magic rule for setting the number of processes or threads to use. It is very much application and system dependent. Simple math like processes = 2 * cpucores will not be enough. You need to experiment with various setups and be prepared to constantly monitor your apps. uwsgitop could be a great tool to find the best values.