Decrease event processing latency

Some time ago, I assembled a Retropie arcade machine, combining a Raspberry PI with this case:

The manual for the case includes instructions to clone the GPIONext repo.

We've found the arcade machine great fun to play, but some friends commented on a feeling of lag playing some of the games. After many false starts, investigation of that led to this PR.

With these changes in place, there is no perceptible lag - and I doubled my high score on an old favourite. 😁

The changes are:

Removing an extraneous sleep from device.py that can cause event processing to be significantly delayed.
Remove extra use of the button debounce delay, as the gpio library already handles that.
Add locks around access to the queue of events, to ensure no events are lost to thread concurrency.

My intuition is that the additional sleep() may have been added to try and prevent events from being lost. Given that python lists are not safe for concurrent modification, event loss was possible if pressEvents() and processQueue() both tried to modify queue at the same time. Adding locks around access to queue should prevent this from happening.

I've included below a detailed analysis of the worst-case lag and how that's addressed by these changes.

Analysis

This analysis is in two parts. The first documents current behavour of the master branch as of commit 2626e5c; the second shows how the changes in this PR reduce lag.

Assumptions

Combo delay using the default of 50ms (ref: gpionext.py L#17) Button debounce using the default of 20ms (ref: gpionext.py L#26) Event timer using the sum of combo delay and button debounce, 70ms (ref: gpionext.py L#88)

Button press events occurring every 130ms, this interval chosen to demonstrate worst case lag.

Current behaviour

Timestamp	Event	Actions	device.py
t	Button press B0	Event added to queue	L#247
		70ms event timer started	L#254
t + 70ms	Event timer triggers	Event timer triggers processQueue()	L#259
		B0 dispatched after 70ms	L#276
		Sleep of 70ms starts	L#279
t + 130ms	Button press B1	Event added to queue	L#247
		Timer already started	L#257
t + 140ms		Sleep of 70ms ends
		Timer reset	L#283
t + 260ms	Button press B2	Event added to queue	L#247
		70ms event timer started	L#254
t + 330ms	Event timer triggers	Event timer triggers processQueue()	L#259
		B1 dispatched after 200ms	L#276
		B2 dispatched after 70ms	L#276
		Sleep of 70ms starts	L#279
t + 390ms		Sleep of 70ms ends
		Timer reset	L#283

The 200ms delay in dispatching Button press B1 (1/5 of a second) is long enough to impair playability of arcade games in which players routinely use split second timing.

New behaviour

Timestamp	Event	Actions	device.py
t	Button press B0	Event added to queue	L#247
		70ms event timer started	L#254
t + 70ms	Event timer triggers	Event timer triggers processQueue()	L#259
		B0 dispatched after 70ms	L#276
		Timer reset	L#283
t + 130ms	Button press B1	Event added to queue	L#247
		70ms event timer started	L#254
t + 200ms	Event timer triggers	Event timer triggers processQueue()	L#259
		B1 dispatched after 70ms	L#276
		Timer reset	L#283
t + 260ms	Button press B2	Event added to queue	L#247
		70ms event timer started	L#254
t + 330ms	Event timer triggers	Event timer triggers processQueue()	L#259
		B2 dispatched after 70ms	L#276
		Timer reset	L#283

mholgatem / GPIOnext