# Environment Variables

Eventlet’s behavior can be controlled by a few environment variables. These are only for the advanced user.

EVENTLET_HUB

> Used to force Eventlet to use the specified hub instead of the optimal one. See Understanding Eventlet Hubs for the list of acceptable hubs and what they mean (note that picking a hub not on the list will silently fail). Equivalent to calling `eventlet.hubs.use_hub()` at the beginning of the program.

EVENTLET_THREADPOOL_SIZE

> The size of the threadpool in `tpool`. This is an environment variable because tpool constructs its pool on first use, so any control of the pool size needs to happen before then.