# Polling

As of release 5.0, BRLTTY has been converted from being polling-based to being event-based. This means, among other things, that it's now able to become completely idle whenever it doesn't have anything useful to do. For comparison, it formerly used a huge, central, main loop which was run frequently (about 25 times per second), and which was aware of, and had to check for, every single task that might need to be performed.

In practical terms, this change, when compared with how BRLTTY used to work, delivers at least the following significant benefits:

- Noticeably improved response time. Near instant reaction to significant user and system events rather than waiting for the next polling cycle to check for them.
- Much less system load. Just a few, short-lived bursts of activity when actually necessary rather than frequent, much longer-lived checking that, far more often than not, resulted in nothing needing to be done.
- Way less battery drain. The system is no longer forced either to remain awake or to continually wake up after a rather brief nap.

There are cases in which polling is still being used. Some of these will eventually be resolved by future code changes, while others, unfortunately, are required due to host platform limitations.

Monitoring for Screen Updates

This task involves checking for size changes, content changes, highlighting changes, cursor movement, etc. It's event-based on:

- Android
- Linux text consoles if the kernel is at least 2.6.37 (released on January 4, 2011)
- Linux graphics consoles monitored via AT-SPI2.

On other platforms, frequent polling (about every 40 milliseconds) is used.

Monitoring for Serial Input

This task is event-based on:

- Linux

On other platforms, frequent polling (about every 40 milliseconds) is used.

Monitoring for USB Input

This task is event-based on:

- Linux

On other platforms, frequent polling (about every 40 milliseconds) is used.

Monitoring for Bluetooth Input

This task is event-based on:

- Android
- Linux

On other platforms, frequent polling (about every 40 milliseconds) is used.

Starting the Braille Driver

On all platforms, periodic retrying (about every five seconds) is used.

Starting the Speech Driver

On all platforms, periodic retrying (about every five seconds) is used.

Starting the Screen Driver

On all platforms, periodic retrying (about every five seconds) is used.

Starting the Keyboard Monitor

On all platforms, periodic retrying (about every five seconds) is used.

Creating the PID File

On all platforms, periodic retrying (about every five seconds) is used.